@article {3407, title = {A Case for Synthesis of Recursive Quantum Unitary Programs}, journal = {Proceedings of the ACM on Programming Languages}, volume = {8}, year = {2024}, month = {12/5/2023}, pages = {1759{\textendash}1788}, abstract = {

Quantum programs are notoriously difficult to code and verify due to unintuitive quantum knowledge associated with quantum programming. Automated tools relieving the tedium and errors associated with low-level quantum details would hence be highly desirable. In this paper, we initiate the study of program synthesis for quantum unitary programs that recursively define a family of unitary circuits for different input sizes, which are widely used in existing quantum programming languages. Specifically, we present QSynth, the first quantum program synthesis framework, including a new inductive quantum programming language, its specification, a sound logic for reasoning, and an encoding of the reasoning procedure into SMT instances. By leveraging existing SMT solvers, QSynth successfully synthesizes ten quantum unitary programs including quantum adder circuits, quantum eigenvalue inversion circuits and Quantum Fourier Transformation, which can be readily transpiled to executable programs on major quantum platforms, e.g., Q$\#$, IBM Qiskit, and AWS Braket.

}, issn = {2475-1421}, doi = {10.1145/3632901}, url = {https://arxiv.org/abs/2311.11503}, author = {Deng, Haowei and Tao, Runzhou and Peng, Yuxiang and Wu, Xiaodi} } @article {3517, title = {Expanding Hardware-Efficiently Manipulable Hilbert Space via Hamiltonian Embedding}, year = {2024}, month = {1/16/2024}, abstract = {

Many promising quantum applications depend on the efficient quantum simulation of an exponentially large sparse Hamiltonian, a task known as sparse Hamiltonian simulation, which is fundamentally important in quantum computation. Although several theoretically appealing quantum algorithms have been proposed for this task, they typically require a black-box query model of the sparse Hamiltonian, rendering them impractical for near-term implementation on quantum devices.
In this paper, we propose a technique named Hamiltonian embedding. This technique simulates a desired sparse Hamiltonian by embedding it into the evolution of a larger and more structured quantum system, allowing for more efficient simulation through hardware-efficient operations. We conduct a systematic study of this new technique and demonstrate significant savings in computational resources for implementing prominent quantum applications. As a result, we can now experimentally realize quantum walks on complicated graphs (e.g., binary trees, glued-tree graphs), quantum spatial search, and the simulation of real-space Schr{\"o}dinger equations on current trapped-ion and neutral-atom platforms. Given the fundamental role of Hamiltonian evolution in the design of quantum algorithms, our technique markedly expands the horizon of implementable quantum advantages in the NISQ era.

}, url = {https://arxiv.org/abs/2401.08550}, author = {Jiaqi Leng and Joseph Li and Yuxiang Peng and Xiaodi Wu} } @article {3482, title = {Integrability of Goldilocks quantum cellular automata}, year = {2024}, month = {4/3/2024}, abstract = {

Goldilocks quantum cellular automata (QCA) have been simulated on quantum hardware and produce emergent small-world correlation networks. In Goldilocks QCA, a single-qubit unitary is applied to each qubit in a one-dimensional chain subject to a balance constraint: a qubit is updated if its neighbors are in opposite basis states. Here, we prove that a subclass of Goldilocks QCA -- including the one implemented experimentally -- map onto free fermions and therefore can be classically simulated efficiently. We support this claim with two independent proofs, one involving a Jordan--Wigner transformation and one mapping the integrable six-vertex model to QCA. We compute local conserved quantities of these QCA and predict experimentally measurable expectation values. These calculations can be applied to test large digital quantum computers against known solutions. In contrast, typical Goldilocks QCA have equilibration properties and quasienergy-level statistics that suggest nonintegrability. Still, the latter QCA conserve one quantity useful for error mitigation. Our work provides a parametric quantum circuit with tunable integrability properties with which to test quantum hardware.

}, url = {https://arxiv.org/abs/2404.02994}, author = {Logan E. Hillberry and Lorenzo Piroli and Eric Vernier and Nicole Yunger Halpern and Toma{\v z} Prosen and Lincoln D. Carr} } @article {3507, title = {Quantum One-Wayness of the Single-Round Sponge with Invertible Permutations}, year = {2024}, month = {3/7/2024}, abstract = {

Sponge hashing is a novel class of cryptographic hash algorithms which underlies the current international hash function standard SHA-3. In a nutshell, a sponge function takes as input a bit-stream of any length and processes it via a simple iterative procedure: it repeatedly feeds each block of the input into a so-called block function, and then produces a short digest which consists of a subset of the final output bits. While much is known about the post-quantum security of the sponge construction in the case when the block function is modeled as a random function or permutation, the case of invertible permutations, which more accurately models the construction underlying SHA-3, has so far remained a fundamental open problem.
In this work, we make new progress towards overcoming this barrier and show several results. First, we prove the \"double-sided zero-search\" conjecture proposed by Unruh (eprint\&$\#$39; 2021) and show that finding zero-pairs in a random 2n-bit permutation requires at least Ω(2n/2) many queries -- and this is tight due to Grover\&$\#$39;s algorithm. At the core of our proof lies a novel \"symmetrization argument\" which uses insights from the theory of Young subgroups. Second, we consider more general variants of the double-sided search problem and show similar query lower bounds for them. As an application, we prove the quantum one-wayness of the single-round sponge with invertible permutations in the quantum random oracle model.

}, url = {https://arxiv.org/abs/2403.04740}, author = {Joseph Carolan and Alexander Poremba} } @article {3484, title = {Quantum thermodynamics of nonequilibrium processes in lattice gauge theories}, year = {2024}, month = {4/3/2024}, abstract = {

A key objective in nuclear and high-energy physics is to describe nonequilibrium dynamics of matter, e.g., in the early universe and in particle colliders, starting from the Standard Model. Classical-computing methods, via the framework of lattice gauge theory, have experienced limited success in this mission. Quantum simulation of lattice gauge theories holds promise for overcoming computational limitations. Because of local constraints (Gauss\&$\#$39;s laws), lattice gauge theories have an intricate Hilbert-space structure. This structure complicates the definition of thermodynamic properties of systems coupled to reservoirs during equilibrium and nonequilibrium processes. We show how to define thermodynamic quantities such as work and heat using strong-coupling thermodynamics, a framework that has recently burgeoned within the field of quantum thermodynamics. Our definitions suit instantaneous quenches, simple nonequilibrium processes undertaken in quantum simulators. To illustrate our framework, we compute the work and heat exchanged during a quench in a Z2 lattice gauge theory coupled to matter in 1+1 dimensions. The thermodynamic quantities, as functions of the quench parameter, evidence an expected phase transition. For general thermal states, we derive a simple relation between a quantum many-body system\&$\#$39;s entanglement Hamiltonian, measurable with quantum-information-processing tools, and the Hamiltonian of mean force, used to define strong-coupling thermodynamic quantities.

}, url = {https://arxiv.org/abs/2404.02965}, author = {Zohreh Davoudi and Christopher Jarzynski and Niklas Mueller and Greeshma Oruganti and Connor Powers and Nicole Yunger Halpern} } @article {3416, title = {SimuQ: A Framework for Programming Quantum Hamiltonian Simulation with Analog Compilation}, journal = {Proceedings of the ACM on Programming Languages}, volume = {8}, year = {2024}, month = {11/19/2023}, pages = {2425{\textendash}2455}, abstract = {

Quantum Hamiltonian simulation, which simulates the evolution of quantum systems and probes quantum phenomena, is one of the most promising applications of quantum computing. Recent experimental results suggest that Hamiltonian-oriented analog quantum simulation would be advantageous over circuit-oriented digital quantum simulation in the Noisy Intermediate-Scale Quantum (NISQ) machine era. However, programming analog quantum simulators is much more challenging due to the lack of a unified interface between hardware and software. In this paper, we design and implement SimuQ, the first framework for quantum Hamiltonian simulation that supports Hamiltonian programming and pulse-level compilation to heterogeneous analog quantum simulators. Specifically, in SimuQ, front-end users specify the target quantum system with Hamiltonian Modeling Language, and the Hamiltonian-level programmability of analog quantum simulators is specified through a new abstraction called the abstract analog instruction set (AAIS) and programmed in AAIS Specification Language by hardware providers. Through a solver-based compilation, SimuQ generates executable pulse schedules for real devices to simulate the evolution of desired quantum systems, which is demonstrated on superconducting (IBM), neutral-atom (QuEra), and trapped-ion (IonQ) quantum devices. Moreover, we demonstrate the advantages of exposing the Hamiltonian-level programmability of devices with native operations or interaction-based gates and establish a small benchmark of quantum simulation to evaluate SimuQ\&$\#$39;s compiler with the above analog quantum simulators.

}, issn = {2475-1421}, doi = {10.1145/3632923}, url = {https://arxiv.org/abs/2303.02775}, author = {Peng, Yuxiang and Young, Jacob and Liu, Pengyu and Wu, Xiaodi} } @article {3257, title = {Accelerating Progress Towards Practical Quantum Advantage: The Quantum Technology Demonstration Project Roadmap}, year = {2023}, month = {3/20/2023}, abstract = {

Quantum information science and technology (QIST) is a critical and emerging technology with the potential for enormous world impact and is currently invested in by over 40 nations. To bring these large-scale investments to fruition and bridge the lower technology readiness levels (TRLs) of fundamental research at universities to the high TRLs necessary to realize the promise of practical quantum advantage accessible to industry and the public, we present a roadmap for Quantum Technology Demonstration Projects (QTDPs). Such QTDPs, focused on intermediate TRLs, are large-scale public-private partnerships with a high probability of translation from laboratory to practice. They create technology demonstrating a clear \&$\#$39;quantum advantage\&$\#$39; for science breakthroughs that are user-motivated and will provide access to a broad and diverse community of scientific users. Successful implementation of a program of QTDPs will have large positive economic impacts.

}, url = {https://arxiv.org/abs/2210.14757}, author = {Paul Alsing and Phil Battle and Joshua C. Bienfang and Tammie Borders and Tina Brower-Thomas and Lincoln D. Carr and Fred Chong and Siamak Dadras and Brian DeMarco and Ivan Deutsch and Eden Figueroa and Danna Freedman and Henry Everitt and Daniel Gauthier and Ezekiel Johnston-Halperin and Jungsang Kim and Mackillo Kira and Prem Kumar and Paul Kwiat and John Lekki and Anjul Loiacono and Marko Lon{\v c}ar and John R. Lowell and Mikhail Lukin and Celia Merzbacher and Aaron Miller and Christopher Monroe and Johannes Pollanen and David Pappas and Michael Raymer and Ronald Reano and Brandon Rodenburg and Martin Savage and Thomas Searles and Jun Ye} } @article {3415, title = {Compressed gate characterization for quantum devices with time-correlated noise}, year = {2023}, month = {12/22/2023}, abstract = {

As quantum devices make steady progress towards intermediate scale and fault-tolerant quantum computing, it is essential to develop rigorous and efficient measurement protocols that account for known sources of noise. Most existing quantum characterization protocols such as gate set tomography and randomized benchmarking assume the noise acting on the qubits is Markovian. However, this assumption is often not valid, as for the case of 1/f charge noise or hyperfine nuclear spin noise. Here, we present a general framework for quantum process tomography (QPT) in the presence of time-correlated noise. We further introduce fidelity benchmarks that quantify the relative strength of different sources of Markovian and non-Markovian noise. As an application of our method, we perform a comparative theoretical and experimental analysis of silicon spin qubits. We first develop a detailed noise model that accounts for the dominant sources of noise and validate the model against experimental data. Applying our framework for time-correlated QPT, we find that the number of independent parameters needed to characterize one and two-qubit gates can be compressed by 10x and 100x, respectively, when compared to the fully generic case. These compressions reduce the amount of tomographic measurements needed in experiment, while also significantly speeding up numerical simulations of noisy quantum circuit dynamics compared to time-dependent Hamiltonian simulation. Using this compressed noise model, we find good agreement between our theoretically predicted process fidelities and two qubit interleaved randomized benchmarking fidelities of 99.8\% measured in recent experiments on silicon spin qubits. More broadly, our formalism can be directly extended to develop efficient and scalable tuning protocols for high-fidelity control of large-arrays of quantum devices with non-Markovian noise.

}, url = {https://arxiv.org/abs/2307.14432}, author = {Michael Gullans and M. Caranti and A. R. Mills and J. R. Petta} } @article {3367, title = {Critical phase and spin sharpening in SU(2)-symmetric monitored quantum circuits}, journal = {Physical Review B}, volume = {108}, year = {2023}, month = {8/17/2023}, abstract = {

Monitored quantum circuits exhibit entanglement transitions at certain measurement rates. Such a transition separates phases characterized by how much information an observer can learn from the measurement outcomes. We study SU(2)-symmetric monitored quantum circuits, using exact numerics and a mapping onto an effective statistical-mechanics model. Due to the symmetry\&$\#$39;s non-Abelian nature, measuring qubit pairs allows for nontrivial entanglement scaling even in the measurement-only limit. We find a transition between a volume-law entangled phase and a critical phase whose diffusive purification dynamics emerge from the non-Abelian symmetry. Additionally, we numerically identify a \"spin-sharpening transition.\" On one side is a phase in which the measurements can efficiently identify the system\&$\#$39;s total spin quantum number; on the other side is a phase in which measurements cannot.

}, doi = {10.1103/physrevb.108.054307}, url = {https://arxiv.org/abs/2305.13356}, author = {Shayan Majidy and Utkarsh Agrawal and Sarang Gopalakrishnan and Andrew C. Potter and Romain Vasseur and Nicole Yunger Halpern} } @article {3361, title = {Feasibility of a trapped atom interferometer with accelerating optical traps}, year = {2023}, month = {8/23/2023}, abstract = {

In order to increase the measured phase of an atom interferometer and improve its sensitivity, researchers attempt to increase the enclosed space-time area using two methods: creating larger separations between the interferometer arms and having longer evolution times. However, increasing the evolution time reduces the bandwidth that can be sampled, whereas decreasing the evolution time worsens the sensitivity. In this paper, we attempt to address this by proposing a setup for high-bandwidth applications, with improved overall sensitivity. This is realized by accelerating and holding the atoms using optical dipole traps. We find that accelerations of up to 103-105 m/s2 can be achieved using acousto-optic deflectors (AODs) to move the traps. By comparing the sensitivity of our approach to acceleration as a baseline to traditional atom interferometry, we find a substantial improvement to the state of the art. In the limit of appropriate beam and optics stabilization, sensitivities approaching 10\−14 (m/s2)/Hz\−\−\−\√ may be achievable at 1 Hz, while detection at 1 kHz with a sensitivity an order of magnitude better than traditional free-fall atom interferometers is possible with today\&$\#$39;s systems.

}, url = {https://arxiv.org/abs/2308.12246}, author = {Gayathrini Premawardhana and Jonathan Kunjummen and Sarthak Subhankar and Jacob M. Taylor} } @article {3422, title = {Lattice quantum chromodynamics at large isospin density: 6144 pions in a box}, year = {2023}, month = {7/27/2023}, abstract = {

We present an algorithm to compute correlation functions for systems with the quantum numbers of many identical mesons from lattice quantum chromodynamics (QCD). The algorithm is numerically stable and allows for the computation of n-pion correlation functions for n\∈{1,\…,N} using a single N\×N matrix decomposition, improving on previous algorithms. We apply the algorithm to calculations of correlation functions with up to 6144 π+s using two ensembles of gauge field configurations generated with quark masses corresponding to a pion mass mπ=170 MeV and spacetime volumes of (4.43\×8.8) fm4 and (5.83\×11.6) fm4. We also discuss statistical techniques for the analysis of such systems, in which the correlation functions vary over many orders of magnitude. In particular, we observe that the many-pion correlation functions are well approximated by log-normal distributions, allowing the extraction of the energies of these systems. Using these energies, the large-isospin-density, zero-baryon-density region of the QCD phase diagram is explored. A peak is observed in the energy density at an isospin chemical potential μI\∼1.5mπ, signalling the transition into a Bose-Einstein condensed phase. The isentropic speed of sound in the medium is seen to exceed the ideal-gas (conformal) limit (c2s\≤1/3) over a wide range of chemical potential before falling towards the asymptotic expectation at μI\∼15mπ. These, and other thermodynamic observables, indicate that the isospin chemical potential must be large for the system to be well described by an ideal gas or perturbative QCD.

}, url = {https://arxiv.org/abs/2307.15014}, author = {Ryan Abbott and William Detmold and Fernando Romero-L{\'o}pez and Zohreh Davoudi and Marc Illa and Assumpta Parre{\~n}o and Robert J. Perry and Phiala E. Shanahan and Michael L. Wagman} } @article {3395, title = {Microwave signal processing using an analog quantum reservoir computer}, year = {2023}, month = {12/26/2023}, abstract = {

Quantum reservoir computing (QRC) has been proposed as a paradigm for performing machine learning with quantum processors where the training is efficient in the number of required runs of the quantum processor and takes place in the classical domain, avoiding the issue of barren plateaus in parameterized-circuit quantum neural networks. It is natural to consider using a quantum processor based on superconducting circuits to classify microwave signals that are analog -- continuous in time. However, while theoretical proposals of analog QRC exist, to date QRC has been implemented using circuit-model quantum systems -- imposing a discretization of the incoming signal in time, with each time point input by executing a gate operation. In this paper we show how a quantum superconducting circuit comprising an oscillator coupled to a qubit can be used as an analog quantum reservoir for a variety of classification tasks, achieving high accuracy on all of them. Our quantum system was operated without artificially discretizing the input data, directly taking in microwave signals. Our work does not attempt to address the question of whether QRCs could provide a quantum computational advantage in classifying pre-recorded classical signals. However, beyond illustrating that sophisticated tasks can be performed with a modest-size quantum system and inexpensive training, our work opens up the possibility of achieving a different kind of advantage than a purely computational advantage: superconducting circuits can act as extremely sensitive detectors of microwave photons; our work demonstrates processing of ultra-low-power microwave signals in our superconducting circuit, and by combining sensitive detection with QRC processing within the same system, one could achieve a quantum sensing-computational advantage, i.e., an advantage in the overall analysis of microwave signals comprising just a few photons.

}, url = {https://arxiv.org/abs/2312.16166}, author = {Alen Senanian and Sridhar Prabhu and Vladimir Kremenetski and Saswata Roy and Yingkang Cao and Jeremy Kline and Tatsuhiro Onodera and Logan G. Wright and Xiaodi Wu and Valla Fatemi and Peter L. McMahon} } @article {3358, title = {Non-equilibrium critical scaling and universality in a quantum simulator}, year = {2023}, month = {9/19/2023}, abstract = {

Universality and scaling laws are hallmarks of equilibrium phase transitions and critical phenomena. However, extending these concepts to non-equilibrium systems is an outstanding challenge. Despite recent progress in the study of dynamical phases, the universality classes and scaling laws for non-equilibrium phenomena are far less understood than those in equilibrium. In this work, using a trapped-ion quantum simulator with single-ion resolution, we investigate the non-equilibrium nature of critical fluctuations following a quantum quench to the critical point. We probe the scaling of spin fluctuations after a series of quenches to the critical Hamiltonian of a long-range Ising model. With systems of up to 50 spins, we show that the amplitude and timescale of the post-quench fluctuations scale with system size with distinct universal critical exponents. While a generic quench can lead to thermal critical behaviour, we find that a second quench from one critical state to another (i.e. a double quench) results in critical behaviour that does not have an equilibrium counterpart. Our results demonstrate the ability of quantum simulators to explore universal scaling beyond the equilibrium paradigm.

}, url = {https://arxiv.org/abs/2309.10856}, author = {A. De and P. Cook and K. Collins and W. Morong and D. Paz and P. Titum and G. Pagano and Alexey V. Gorshkov and M. Maghrebi and C. Monroe} } @article {3397, title = {Quantum-centric Supercomputing for Materials Science: A Perspective on Challenges and Future Directions}, year = {2023}, month = {12/14/2023}, abstract = {

Computational models are an essential tool for the design, characterization, and discovery of novel materials. Hard computational tasks in materials science stretch the limits of existing high-performance supercomputing centers, consuming much of their simulation, analysis, and data resources. Quantum computing, on the other hand, is an emerging technology with the potential to accelerate many of the computational tasks needed for materials science. In order to do that, the quantum technology must interact with conventional high-performance computing in several ways: approximate results validation, identification of hard problems, and synergies in quantum-centric supercomputing. In this paper, we provide a perspective on how quantum-centric supercomputing can help address critical computational problems in materials science, the challenges to face in order to solve representative use cases, and new suggested directions.

}, url = {https://arxiv.org/abs/2312.09733}, author = {Yuri Alexeev and Maximilian Amsler and Paul Baity and Marco Antonio Barroca and Sanzio Bassini and Torey Battelle and Daan Camps and David Casanova and Young jai Choi and Frederic T. Chong and Charles Chung and Chris Codella and Antonio D. Corcoles and James Cruise and Alberto Di Meglio and Jonathan Dubois and Ivan Duran and Thomas Eckl and Sophia Economou and Stephan Eidenbenz and Bruce Elmegreen and Clyde Fare and Ismael Faro and Cristina Sanz Fern{\'a}ndez and Rodrigo Neumann Barros Ferreira and Keisuke Fuji and Bryce Fuller and Laura Gagliardi and Giulia Galli and Jennifer R. Glick and Isacco Gobbi and Pranav Gokhale and Salvador de la Puente Gonzalez and Johannes Greiner and Bill Gropp and Michele Grossi and Emmanuel Gull and Burns Healy and Benchen Huang and Travis S. Humble and Nobuyasu Ito and Artur F. Izmaylov and Ali Javadi-Abhari and Douglas Jennewein and Shantenu Jha and Liang Jiang and Barbara Jones and Wibe Albert de Jong and Petar Jurcevic and William Kirby and Stefan Kister and Masahiro Kitagawa and Joel Klassen and Katherine Klymko and Kwangwon Koh and Masaaki Kondo and Doga Murat Kurkcuoglu and Krzysztof Kurowski and Teodoro Laino and Ryan Landfield and Matt Leininger and Vicente Leyton-Ortega and Ang Li and Meifeng Lin and Junyu Liu and Nicolas Lorente and Andre Luckow and Simon Martiel and Francisco Martin-Fernandez and Margaret Martonosi and Claire Marvinney and Arcesio Castaneda Medina and Dirk Merten and Antonio Mezzacapo and Kristel Michielsen and Abhishek Mitra and Tushar Mittal and Kyungsun Moon and Joel Moore and Mario Motta and Young-Hye Na and Yunseong Nam and Prineha Narang and Yu-ya Ohnishi and Daniele Ottaviani and Matthew Otten and Scott Pakin and Vincent R. Pascuzzi and Ed Penault and Tomasz Piontek and Jed Pitera and Patrick Rall and Gokul Subramanian Ravi and Niall Robertson and Matteo Rossi and Piotr Rydlichowski and Hoon Ryu and Georgy Samsonidze and Mitsuhisa Sato and Nishant Saurabh and Vidushi Sharma and Kunal Sharma and Soyoung Shin and George Slessman and Mathias Steiner and Iskandar Sitdikov and In-Saeng Suh and Eric Switzer and Wei Tang and Joel Thompson and Synge Todo and Minh Tran and Dimitar Trenev and Christian Trott and Huan-Hsin Tseng and Esin Tureci and David Garc{\'\i}a Valinas and Sofia Vallecorsa and Christopher Wever and Konrad Wojciechowski and Xiaodi Wu and Shinjae Yoo and Nobuyuki Yoshioka and Victor Wen-zhe Yu and Seiji Yunoki and Sergiy Zhuk and Dmitry Zubarev} } @article {3249, title = {SimuQ: A Domain-Specific Language For Quantum Simulation With Analog Compilation}, year = {2023}, month = {3/5/2023}, abstract = {

Hamiltonian simulation is one of the most promising applications of quantum computing. Recent experimental results suggest that continuous-time analog quantum simulation would be advantageous over gate-based digital quantum simulation in the Noisy Intermediate-Size Quantum (NISQ) machine era. However, programming such analog quantum simulators is much more challenging due to the lack of a unified interface between hardware and software, and the only few known examples are all hardware-specific. In this paper, we design and implement SimuQ, the first domain-specific language for Hamiltonian simulation that supports pulse-level compilation to heterogeneous analog quantum simulators. Specifically, in SimuQ, front-end users will specify the target Hamiltonian evolution with a Hamiltonian modeling language, and the programmability of analog simulators is specified through a new abstraction called the abstract analog instruction set by hardware providers. Through a solver-based compilation, SimuQ will generate the pulse-level instruction schedule on the target analog simulator for the desired Hamiltonian evolution, which has been demonstrated on pulse-controlled superconducting (Qiskit Pulse) and neutral-atom (QuEra Bloqade) quantum systems, as well as on normal circuit-based digital quantum machines. Moreover, we also demonstrate the advantage of analog compilation over digital compilation on IBM machines, the use of SimuQ for resource estimation for hypothetical machines, and a scalability test of SimuQ\&$\#$39;s compilation.

}, url = {https://arxiv.org/abs/2303.02775}, author = {Yuxiang Peng and Jacob Young and Pengyu Liu and Xiaodi Wu} } @article {3206, title = {Time-energy uncertainty relation for noisy quantum metrology}, journal = {PRX Quantum}, volume = {4(4)}, year = {2023}, month = {12/5/2023}, abstract = {

Detection of weak forces and precise measurement of time are two of the many applications of quantum metrology to science and technology. We consider a quantum system initialized in a pure state and whose evolution is goverened by a Hamiltonian H; a measurement can later estimate the time t for which the system has evolved. In this work, we introduce and study a fundamental trade-off which relates the amount by which noise reduces the accuracy of a quantum clock to the amount of information about the energy of the clock that leaks to the environment. Specifically, we consider an idealized scenario in which Alice prepares an initial pure state of the clock, allows the clock to evolve for a time t that is not precisely known, and then transmits the clock through a noisy channel to Bob. The environment (Eve) receives any information that is lost. We prove that Bob\&$\#$39;s loss of quantum Fisher information (QFI) about t is equal to Eve\&$\#$39;s gain of QFI about a complementary energy parameter. We also prove a more general trade-off that applies when Bob and Eve wish to estimate the values of parameters associated with two non-commuting observables. We derive the necessary and sufficient conditions for the accuracy of the clock to be unaffected by the noise. These are a subset of the Knill-Laflamme error-correction conditions; states satisfying these conditions are said to form a metrological code. We provide a scheme to construct metrological codes in the stabilizer formalism. We show that there are metrological codes that cannot be written as a quantum error-correcting code with similar distance in which the Hamiltonian acts as a logical operator, potentially offering new schemes for constructing states that do not lose any sensitivity upon application of a noisy channel. We discuss applications of our results to sensing using a many-body state subject to erasure or amplitude-damping noise.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {https://journals.aps.org/prxquantum/pdf/10.1103/PRXQuantum.4.040336}, url = {https://arxiv.org/abs/2207.13707}, author = {Faist, Philippe and Woods, Mischa P. and Victor V. Albert and Renes, Joseph M. and Eisert, Jens and Preskill, John} } @article {3308, title = {On the Two-sided Permutation Inversion Problem}, year = {2023}, month = {6/23/2023}, abstract = {

In the permutation inversion problem, the task is to find the preimage of some challenge value, given oracle access to the permutation. This is a fundamental problem in query complexity, and appears in many contexts, particularly cryptography. In this work, we examine the setting in which the oracle allows for quantum queries to both the forward and the inverse direction of the permutation -- except that the challenge value cannot be submitted to the latter. Within that setting, we consider two options for the inversion algorithm: whether it can get quantum advice about the permutation, and whether it must produce the entire preimage (search) or only the first bit (decision). We prove several theorems connecting the hardness of the resulting variations of the inversion problem, and establish a number of lower bounds. Our results indicate that, perhaps surprisingly, the inversion problem does not become significantly easier when the adversary is granted oracle access to the inverse, provided it cannot query the challenge itself.

}, url = {https://arxiv.org/abs/2306.13729}, author = {Gorjan Alagic and Chen Bai and Alexander Poremba and Kaiyan Shi} } @article {3363, title = {Verifiable measurement-based quantum random sampling with trapped ions}, year = {2023}, month = {7/26/2023}, abstract = {

Quantum computers are now on the brink of outperforming their classical counterparts. One way to demonstrate the advantage of quantum computation is through quantum random sampling performed on quantum computing devices. However, existing tools for verifying that a quantum device indeed performed the classically intractable sampling task are either impractical or not scalable to the quantum advantage regime. The verification problem thus remains an outstanding challenge. Here, we experimentally demonstrate efficiently verifiable quantum random sampling in the measurement-based model of quantum computation on a trapped-ion quantum processor. We create random cluster states, which are at the heart of measurement-based computing, up to a size of 4 x 4 qubits. Moreover, by exploiting the structure of these states, we are able to recycle qubits during the computation to sample from entangled cluster states that are larger than the qubit register. We then efficiently estimate the fidelity to verify the prepared states--in single instances and on average--and compare our results to cross-entropy benchmarking. Finally, we study the effect of experimental noise on the certificates. Our results and techniques provide a feasible path toward a verified demonstration of a quantum advantage.

}, doi = {https://doi.org/10.48550/arXiv.2307.14424}, url = {https://arxiv.org/abs/2307.14424}, author = {Martin Ringbauer and Marcel Hinsche and Thomas Feldker and Paul K. Faehrmann and Juani Bermejo-Vega and Claire Edmunds and Lukas Postler and Roman Stricker and Christian D. Marciniak and Michael Meth and Ivan Pogorelov and Rainer Blatt and Philipp Schindler and Jens Eisert and Thomas Monz and Dominik Hangleiter} } @article {3082, title = {Closing the Locality and Detection Loopholes in Multiparticle Entanglement Self-Testing}, journal = {Physical Review Letters}, volume = {128}, year = {2022}, month = {06/23/2022}, pages = {250401}, abstract = {

First proposed by Mayers and Yao, self-testing provides a certification method to infer the underlying physics of quantum experiments in a black-box scenario. Numerous demonstrations have been reported to self-test various types of entangled states. However, all the multiparticle self-testing experiments reported so far suffer from both detection and locality loopholes. Here, we report the first experimental realization of multiparticle entanglement self-testing closing the locality loophole in a photonic system, and the detection loophole in a superconducting system, respectively. We certify three-party and four-party GHZ states with at least 0.84 (1) and 0.86 (3) fidelities in a device-independent way. These results can be viewed as a meaningful advance in multiparticle loophole-free self-testing, and also significant progress on the foundations of quantum entanglement certification.

}, doi = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.250401}, url = {https://www.researchgate.net/profile/Dian-Wu/publication/361497881_Closing_the_Locality_and_Detection_Loopholes_in_Multiparticle_Entanglement_Self-Testing/links/62b55a8c1010dc02cc57530c/Closing-the-Locality-and-Detection-Loopholes-in-Multiparticle-Entangl}, author = {Dian Wu and Qi Zhao and Can Wang and Liang Huang and Yang-Fan Jiang and Bing Bai and You Zhou and Xue-Mei Gu and Feng-Ming Liu and Ying-Qiu Mao and Qi-Chao Sun and Ming-Cheng Chen and Jun Zhang and Cheng-Zhi Peng and Xiao-Bo Zhu and Qiang Zhang and Chao-Yang Lu and Jian-Wei Pan} } @article {3077, title = {Experimentally Measuring Rolling and Sliding in Three-Dimensional Dense Granular Packings}, journal = {Phys. Rev. Lett.}, volume = {129}, year = {2022}, month = {06/18/2022}, pages = {048001}, abstract = {

We experimentally measure a three-dimensional (3D) granular system\’s reversibility under cyclic compression. We image the grains using a refractive-index-matched fluid, then analyze the images using the artificial intelligence of variational autoencoders. These techniques allow us to track all the grains\’ translations and 3D rotations with accuracy sufficient to infer sliding and rolling displacements. Our observations reveal unique roles played by 3D rotational motions in granular flows. We find that rotations and contact-point motion dominate the dynamics in the bulk, far from the perturbation\’s source. Furthermore, we determine that 3D rotations are irreversible under cyclic compression. Consequently, contact-point sliding, which is dissipative, accumulates throughout the cycle. Using numerical simulations whose accuracy our experiment supports, we discover that much of the dissipation occurs in the bulk, where grains rotate more than they translate. Our observations suggest that the analysis of 3D rotations is needed for understanding granular materials\’ unique and powerful ability to absorb and dissipate energy.

}, doi = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.048001}, url = {https://arxiv.org/abs/2108.11975}, author = {Zackery A. Benson and Anton Peshkov and Nicole Yunger Halpern and Derek C. Richardson and Wolfgang Losert} } @article {3198, title = {FIPS Compliant Quantum Secure Communication using Quantum Permutation Pad}, year = {2022}, month = {12/30/2022}, abstract = {

Quantum computing has entered fast development track since Shor\&$\#$39;s algorithm was proposed in 1994. Multi-cloud services of quantum computing farms are currently available. One of which, IBM quantum computing, presented a road map showing their Kookaburra system with over 4158 qubits will be available in 2025. For the standardization of Post-Quantum Cryptography or PQC, the National Institute of Standards and Technology or NIST recently announced the first candidates for standardization with one algorithm for key encapsulation mechanism (KEM), Kyber, and three algorithms for digital signatures. NIST has also issued a new call for quantum-safe digital signature algorithms due June 1, 2023. This timeline shows that FIPS-certified quantum-safe TLS protocol would take a predictably long time. However, \"steal now, crack later\" tactic requires protecting data against future quantum threat actors today. NIST recommended the use of a hybrid mode of TLS 1.3 with its extensions to support PQC. The hybrid mode works for certain cases but FIPS certification for the hybridized cryptomodule might still be required. This paper proposes to take a nested mode to enable TLS 1.3 protocol with quantum-safe data, which can be made available today and is FIPS compliant. We discussed the performance impacts of the handshaking phase of the nested TLS 1.3 with PQC and the symmetric encryption phase. The major impact on performance using the nested mode is in the data symmetric encryption with AES. To overcome this performance reduction, we suggest using quantum encryption with a quantum permutation pad for the data encryption with a minor performance reduction of less than 10 percent.

}, keywords = {Cryptography and Security (cs.CR), FOS: Computer and information sciences, FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2301.00062}, url = {https://arxiv.org/abs/2301.00062}, author = {He, Alex and Lou, Dafu and She, Eric and Guo, Shangjie and Watson, Hareesh and Weng, Sibyl and Perepechaenko, Maria and Kuang, Rand} } @article {3070, title = {Infinite-randomness criticality in monitored quantum dynamics with static disorder}, year = {2022}, month = {5/27/2022}, abstract = {

We consider a model of monitored quantum dynamics with quenched spatial randomness: specifically, random quantum circuits with spatially varying measurement rates. These circuits undergo a measurement-induced phase transition (MIPT) in their entanglement structure, but the nature of the critical point differs drastically from the case with constant measurement rate. In particular, at the critical measurement rate, we find that the entanglement of a subsystem of size l scales as S\∼l\√; moreover, the dynamical critical exponent z=\∞. The MIPT is flanked by Griffiths phases with continuously varying dynamical exponents. We argue for this infinite-randomness scenario on general grounds and present numerical evidence that it captures some features of the universal critical properties of MIPT using large-scale simulations of Clifford circuits. These findings demonstrate that the relevance and irrelevance of perturbations to the MIPT can naturally be interpreted using a powerful heuristic known as the Harris criterion.\ 

}, url = {https://arxiv.org/abs/2205.14002}, author = {Aidan Zabalo and Justin H. Wilson and Michael Gullans and Romain Vasseur and Sarang Gopalakrishnan and David A. Huse and J. H. Pixley} } @article {3147, title = {Multi-Angle QAOA Does Not Always Need All Its Angles}, year = {2022}, month = {9/23/2022}, abstract = {

Introducing additional tunable parameters to quantum circuits is a powerful way of improving performance without increasing hardware requirements. A recently introduced multi-angle extension of the quantum approximate optimization algorithm (ma-QAOA) significantly improves the solution from QAOA by allowing the parameters for each term in the Hamiltonian to vary independently. However, prior results suggest that there is considerable redundancy in parameters, the removal of which would reduce the cost of parameter optimization. In this work, we show numerically that problem symmetries can be used to reduce the number of parameters used by ma-QAOA without decreasing the solution quality. We study MaxCut on all 7,565 connected, non-isomorphic 8-node graphs with a non-trivial symmetry group and show numerically that in 67.4\% of these graphs, symmetry can be used to reduce the number of parameters with no decrease in the objective, with the average ratio of parameters reduced by 28.1\%. Moreover, we show that in 35.9\% of the graphs this can be achieved by simply using the largest symmetry. For the graphs where reducing the number of parameters leads to a decrease in the objective, the largest symmetry can be used to reduce the parameter count by 37.1\% at the cost of only a 6.1\% decrease in the objective.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2209.11839}, url = {https://arxiv.org/abs/2209.11839}, author = {Shi, Kaiyan and Herrman, Rebekah and Shaydulin, Ruslan and Chakrabarti, Shouvanik and Pistoia, Marco and Larson, Jeffrey} } @article {3054, title = {Negative Quasiprobabilities Enhance Phase Estimation in Quantum-Optics Experiment}, journal = {Phys. Rev. Lett.}, volume = {128}, year = {2022}, month = {6/2/2022}, pages = {220504}, abstract = {

Operator noncommutation, a hallmark of quantum theory, limits measurement precision, according to uncertainty principles. Wielded correctly, though, noncommutation can boost precision. A recent foundational result relates a metrological advantage with negative quasiprobabilities\—quantum extensions of probabilities\—engendered by noncommuting operators. We crystallize the relationship in an equation that we prove theoretically and observe experimentally. Our proof-of-principle optical experiment features a filtering technique that we term partially postselected amplification (PPA). Using PPA, we measure a wave plate\’s birefringent phase. PPA amplifies, by over two orders of magnitude, the information obtained about the phase per detected photon. In principle, PPA can boost the information obtained from the average filtered photon by an arbitrarily large factor. The filter\’s amplification of systematic errors, we find, bounds the theoretically unlimited advantage in practice. PPA can facilitate any phase measurement and mitigates challenges that scale with trial number, such as proportional noise and detector saturation. By quantifying PPA\’s metrological advantage with quasiprobabilities, we reveal deep connections between quantum foundations and precision measurement.

}, doi = {10.1103/PhysRevLett.128.220504}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.128.220504}, author = {Lupu-Gladstein, Noah and Yilmaz, Y. Batuhan and Arvidsson-Shukur, David R. M. and Brodutch, Aharon and Pang, Arthur O. T. and Steinberg, Aephraim M. and Nicole Yunger Halpern} } @article {3076, title = {NISQ algorithm for the matrix elements of a generic observable}, year = {2022}, month = {5/20/2022}, abstract = {

The calculation of off-diagonal matrix elements has various applications in fields such as nuclear physics and quantum chemistry. In this paper, we present a noisy intermediate scale quantum algorithm for estimating the diagonal and off-diagonal matrix elements of a generic observable in the energy eigenbasis of a given Hamiltonian. Several numerical simulations indicate that this approach can find many of the matrix elements even when the trial functions are randomly initialized across a wide range of parameter values without, at the same time, the need to prepare the energy eigenstates.\ 

}, url = {https://arxiv.org/abs/2205.10058}, author = {Rebecca Erbanni and Kishor Bharti and Leong-Chuan Kwek and Dario Poletti} } @article {3006, title = {Operator Scaling Dimensions and Multifractality at Measurement-Induced Transitions}, journal = {Physical Review Letters}, volume = {128}, year = {2022}, month = {2/11/2022}, abstract = {

Repeated local measurements of quantum many body systems can induce a phase transition in their entanglement structure. These measurement-induced phase transitions (MIPTs) have been studied for various types of dynamics, yet most cases yield quantitatively similar values of the critical exponents, making it unclear if there is only one underlying universality class. Here, we directly probe the properties of the conformal field theories governing these MIPTs using a numerical transfer-matrix method, which allows us to extract the effective central charge, as well as the first few low-lying scaling dimensions of operators at these critical points. Our results provide convincing evidence that the generic and Clifford MIPTs for qubits lie in different universality classes and that both are distinct from the percolation transition for qudits in the limit of large onsite Hilbert space dimension. For the generic case, we find strong evidence of multifractal scaling of correlation functions at the critical point, reflected in a continuous spectrum of scaling dimensions.

}, doi = {10.1103/physrevlett.128.050602}, url = {https://arxiv.org/abs/2107.03393}, author = {Aidan Zabalo and Michael Gullans and Justin H. Wilson and Romain Vasseur and Andreas W. W. Ludwig and Sarang Gopalakrishnan and David A. Huse and J. H. Pixley} } @article {3130, title = {Provably accurate simulation of gauge theories and bosonic systems}, journal = {Quantum}, volume = {6}, year = {2022}, month = {9/20/2022}, pages = {816}, abstract = {

Quantum many-body systems involving bosonic modes or gauge fields have infinite-dimensional local Hilbert spaces which must be truncated to perform simulations of real-time dynamics on classical or quantum computers. To analyze the truncation error, we develop methods for bounding the rate of growth of local quantum numbers such as the occupation number of a mode at a lattice site, or the electric field at a lattice link. Our approach applies to various models of bosons interacting with spins or fermions, and also to both abelian and non-abelian gauge theories. We show that if states in these models are truncated by imposing an upper limit Λ on each local quantum number, and if the initial state has low local quantum numbers, then an error at most ϵ can be achieved by choosing Λ to scale polylogarithmically with ϵ\−1, an exponential improvement over previous bounds based on energy conservation. For the Hubbard-Holstein model, we numerically compute a bound on Λ that achieves accuracy ϵ, obtaining significantly improved estimates in various parameter regimes. We also establish a criterion for truncating the Hamiltonian with a provable guarantee on the accuracy of time evolution. Building on that result, we formulate quantum algorithms for dynamical simulation of lattice gauge theories and of models with bosonic modes; the gate complexity depends almost linearly on spacetime volume in the former case, and almost quadratically on time in the latter case. We establish a lower bound showing that there are systems involving bosons for which this quadratic scaling with time cannot be improved. By applying our result on the truncation error in time evolution, we also prove that spectrally isolated energy eigenstates can be approximated with accuracy ϵ by truncating local quantum numbers at Λ=polylog(ϵ\−1).

}, doi = {https://doi.org/10.22331\%2Fq-2022-09-22-816}, url = {https://arxiv.org/abs/2110.06942}, author = {Yu Tong and Victor V. Albert and Jarrod R. McClean and John Preskill and Yuan Su} } @article {3131, title = {Provably efficient machine learning for quantum many-body problems}, journal = {Science}, volume = {377}, year = {2022}, month = {9/26/2022}, abstract = {

Classical machine learning (ML) provides a potentially powerful approach to solving challenging quantum many-body problems in physics and chemistry. However, the advantages of ML over more traditional methods have not been firmly established. In this work, we prove that classical ML algorithms can efficiently predict ground state properties of gapped Hamiltonians in finite spatial dimensions, after learning from data obtained by measuring other Hamiltonians in the same quantum phase of matter. In contrast, under widely accepted complexity theory assumptions, classical algorithms that do not learn from data cannot achieve the same guarantee. We also prove that classical ML algorithms can efficiently classify a wide range of quantum phases of matter. Our arguments are based on the concept of a classical shadow, a succinct classical description of a many-body quantum state that can be constructed in feasible quantum experiments and be used to predict many properties of the state. Extensive numerical experiments corroborate our theoretical results in a variety of scenarios, including Rydberg atom systems, 2D random Heisenberg models, symmetry-protected topological phases, and topologically ordered phases.

}, doi = {10.1126/science.abk3333}, url = {https://arxiv.org/abs/2106.12627}, author = {Hsin-Yuan Huang and Richard Kueng and Giacomo Torlai and Victor V. Albert and John Preskill} } @article {2997, title = {Quantum Simulation for High Energy Physics}, year = {2022}, month = {4/7/2022}, abstract = {

It is for the first time that Quantum Simulation for High Energy Physics (HEP) is studied in the U.S. decadal particle-physics community planning, and in fact until recently, this was not considered a mainstream topic in the community. This fact speaks of a remarkable rate of growth of this subfield over the past few years, stimulated by the impressive advancements in Quantum Information Sciences (QIS) and associated technologies over the past decade, and the significant investment in this area by the government and private sectors in the U.S. and other countries. High-energy physicists have quickly identified problems of importance to our understanding of nature at the most fundamental level, from tiniest distances to cosmological extents, that are intractable with classical computers but may benefit from quantum advantage. They have initiated, and continue to carry out, a vigorous program in theory, algorithm, and hardware co-design for simulations of relevance to the HEP mission. This community whitepaper is an attempt to bring this exciting and yet challenging area of research to the spotlight, and to elaborate on what the promises, requirements, challenges, and potential solutions are over the next decade and beyond.

}, keywords = {FOS: Physical sciences, High Energy Physics - Lattice (hep-lat), High Energy Physics - Phenomenology (hep-ph), High Energy Physics - Theory (hep-th), Nuclear Theory (nucl-th), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2204.03381}, url = {https://arxiv.org/abs/2204.03381}, author = {Bauer, Christian W. and Zohreh Davoudi and Balantekin, A. Baha and Bhattacharya, Tanmoy and Carena, Marcela and de Jong, Wibe A. and Draper, Patrick and El-Khadra, Aida and Gemelke, Nate and Hanada, Masanori and Kharzeev, Dmitri and Lamm, Henry and Li, Ying-Ying and Liu, Junyu and Lukin, Mikhail and Meurice, Yannick and Monroe, Christopher and Nachman, Benjamin and Pagano, Guido and Preskill, John and Rinaldi, Enrico and Roggero, Alessandro and Santiago, David I. and Savage, Martin J. and Siddiqi, Irfan and Siopsis, George and Van Zanten, David and Wiebe, Nathan and Yamauchi, Yukari and Yeter-Aydeniz, K{\"u}bra and Zorzetti, Silvia} } @article {3189, title = {Sample-optimal classical shadows for pure states}, year = {2022}, month = {11/21/2022}, abstract = {

We consider the classical shadows task for pure states in the setting of both joint and independent measurements. The task is to measure few copies of an unknown pure state ρ in order to learn a classical description which suffices to later estimate expectation values of observables. Specifically, the goal is to approximate Tr(Oρ) for any Hermitian observable O to within additive error ϵ provided Tr(O2)\≤B and ||O||=1. Our main result applies to the joint measurement setting, where we show Θ~(B\−\−\√ϵ\−1+ϵ\−2) samples of ρ are necessary and sufficient to succeed with high probability. The upper bound is a quadratic improvement on the previous best sample complexity known for this problem. For the lower bound, we see that the bottleneck is not how fast we can learn the state but rather how much any classical description of ρ can be compressed for observable estimation. In the independent measurement setting, we show that O(Bd\−\−\−\√ϵ\−1+ϵ\−2) samples suffice. Notably, this implies that the random Clifford measurements algorithm of Huang, Kueng, and Preskill, which is sample-optimal for mixed states, is not optimal for pure states. Interestingly, our result also uses the same random Clifford measurements but employs a different estimator.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Information Theory (cs.IT), Machine Learning (cs.LG), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2211.11810}, url = {https://arxiv.org/abs/2211.11810}, author = {Grier, Daniel and Pashayan, Hakop and Luke Schaeffer} } @article {3204, title = {A scheme to create and verify scalable entanglement in optical lattice}, journal = {npj Quantum Information}, volume = {8}, year = {2022}, month = {9/4/2022}, abstract = {

To achieve scalable quantum information processing, great efforts have been devoted to the creation of large-scale entangled states in various physical systems. Ultracold atom in optical lattice is considered as one of the promising platforms due to its feasible initialization and parallel manipulation. In this work, we propose an efficient scheme to generate and characterize global entanglement in the optical lattice. With only two-layer quantum circuits, the generation utilizes two-qubit entangling gates based on the superexchange interaction in double wells. The parallelism of these operations enables the generation to be fast and scalable. To verify the entanglement of this non-stabilizer state, we mainly design three complementary detection protocols which are less resource-consuming compared to the full tomography. In particular, one just needs two homogenous local measurement settings to identify the entanglement property. Our entanglement generation and verification protocols provide the foundation for the further quantum information processing in optical lattice.

}, doi = {10.1038/s41534-022-00609-0}, url = {https://arxiv.org/abs/2209.01531}, author = {You Zhou and Bo Xiao and Meng-Da Li and Qi Zhao and Zhen-Sheng Yuan and Xiongfeng Ma and Jian-Wei Pan} } @article {2999, title = {Snowmass 2021 White Paper: Tabletop experiments for infrared quantum gravity}, year = {2022}, month = {3/22/2022}, abstract = {

Progress in the quantum readout and control of mechanical devices from single atoms to large masses may enable a first generation of experiments probing the gravitational interaction in the quantum regime, conceivably within the next decade. In this Snowmass whitepaper, we briefly outline the possibilities and challenges facing the realization of these experiments. In particular, we emphasize the need for detailed theories of modifications to the usual effective QFT of gravitons in the infrared regime E/L3<}, keywords = {FOS: Physical sciences, General Relativity and Quantum Cosmology (gr-qc), High Energy Physics - Phenomenology (hep-ph), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2203.11846}, url = {https://arxiv.org/abs/2203.11846}, author = {Carney, Daniel and Chen, Yanbei and Geraci, Andrew and M{\"u}ller, Holger and Panda, Cristian D. and Stamp, Philip C. E. and Taylor, Jacob M.} } @article {3000, title = {Snowmass 2021 White Paper: The Windchime Project}, year = {2022}, month = {3/14/2022}, abstract = {

The absence of clear signals from particle dark matter in direct detection experiments motivates new approaches in disparate regions of viable parameter space. In this Snowmass white paper, we outline the Windchime project, a program to build a large array of quantum-enhanced mechanical sensors. The ultimate aim is to build a detector capable of searching for Planck mass-scale dark matter purely through its gravitational coupling to ordinary matter. In the shorter term, we aim to search for a number of other physics targets, especially some ultralight dark matter candidates. Here, we discuss the basic design, open R\&D challenges and opportunities, current experimental efforts, and both short- and long-term physics targets of the Windchime project.

}, keywords = {Cosmology and Nongalactic Astrophysics (astro-ph.CO), FOS: Physical sciences, High Energy Physics - Experiment (hep-ex), High Energy Physics - Phenomenology (hep-ph)}, doi = {10.48550/ARXIV.2203.07242}, url = {https://arxiv.org/abs/2203.07242}, author = {The Windchime Collaboration and Attanasio, Alaina and Bhave, Sunil A. and Blanco, Carlos and Carney, Daniel and Demarteau, Marcel and Elshimy, Bahaa and Febbraro, Michael and Feldman, Matthew A. and Ghosh, Sohitri and Hickin, Abby and Hong, Seongjin and Lang, Rafael F. and Lawrie, Benjamin and Li, Shengchao and Liu, Zhen and Maldonado, Juan P. A. and Marvinney, Claire and Oo, Hein Zay Yar and Pai, Yun-Yi and Pooser, Raphael and Qin, Juehang and Sparmann, Tobias J. and Taylor, Jacob M. and Tian, Hao and Tunnell, Christopher} } @article {3063, title = {Status Report on the Third Round of the NIST Post-Quantum Cryptography Standardization Process}, journal = {NIST}, year = {2022}, month = {7/2022}, abstract = {

The National Institute of Standards and Technology is in the process of selecting publickey cryptographic algorithms through a public, competition-like process. The new publickey cryptography standards will specify additional digital signature, public-key encryption, and key-establishment algorithms to augment Federal Information Processing Standard (FIPS) 186-4, Digital Signature Standard (DSS), as well as NIST Special Publication (SP) 800-56A Revision 3, Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography, and SP 800-56B Revision 2, Recommendation for Pair-Wise Key Establishment Using Integer Factorization Cryptography. It is intended that these algorithms will be capable of protecting sensitive information well into the foreseeable future, including after the advent of quantum computers.

This report describes the evaluation and selection process of the NIST Post-Quantum Cryptography Standardization process third-round candidates based on public feedback and internal review. The report summarizes each of the 15 third-round candidate algorithms and identifies those selected for standardization, as well as those that will continue to be evaluated in a fourth round of analysis. The public-key encryption and key-establishment algorithm that will be standardized is CRYSTALS\–KYBER. The digital signatures that will be standardized are CRYSTALS\–Dilithium, FALCON, and SPHINCS+. While there are multiple signature algorithms selected, NIST recommends CRYSTALS\–Dilithium as the primary algorithm to be implemented. In addition, four of the alternate key-establishment candidate algorithms will advance to a fourth round of evaluation: BIKE, Classic McEliece, HQC, and SIKE. These candidates are still being considered for future standardization. NIST will also issue a new Call for Proposals for public-key digital signature algorithms to augment and diversify its signature portfolio.

}, doi = {https://doi.org/10.6028/NIST.IR.8413-upd1}, author = {Gorjan Alagic and Daniel Apon and David Cooper and Quynh Dang and Thinh Dang and John Kelsey and Jacob Lichtinger and Carl Miller and Dustin Moody and Rene Peralta and Ray Perlner and Angela Robinson} } @article {3199, title = {Tailoring three-dimensional topological codes for biased noise}, year = {2022}, month = {11/3/2022}, abstract = {

Tailored topological stabilizer codes in two dimensions have been shown to exhibit high storage threshold error rates and improved subthreshold performance under biased Pauli noise. Three-dimensional (3D) topological codes can allow for several advantages including a transversal implementation of non-Clifford logical gates, single-shot decoding strategies, parallelized decoding in the case of fracton codes as well as construction of fractal lattice codes. Motivated by this, we tailor 3D topological codes for enhanced storage performance under biased Pauli noise. We present Clifford deformations of various 3D topological codes, such that they exhibit a threshold error rate of 50\% under infinitely biased Pauli noise. Our examples include the 3D surface code on the cubic lattice, the 3D surface code on a checkerboard lattice that lends itself to a subsystem code with a single-shot decoder, the 3D color code, as well as fracton models such as the X-cube model, the Sierpinski model and the Haah code. We use the belief propagation with ordered statistics decoder (BP-OSD) to study threshold error rates at finite bias. We also present a rotated layout for the 3D surface code, which uses roughly half the number of physical qubits for the same code distance under appropriate boundary conditions. Imposing coprime periodic dimensions on this rotated layout leads to logical operators of weight O(n) at infinite bias and a corresponding exp[\−O(n)] subthreshold scaling of the logical failure rate, where n is the number of physical qubits in the code. Even though this scaling is unstable due to the existence of logical representations with O(1) low-rate Pauli errors, the number of such representations scales only polynomially for the Clifford-deformed code, leading to an enhanced effective distance.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2211.02116}, url = {https://arxiv.org/abs/2211.02116}, author = {Huang, Eric and Pesah, Arthur and Chubb, Christopher T. and Vasmer, Michael and Dua, Arpit} } @article {2858, title = {Algebraic Reasoning of Quantum Programs via Non-Idempotent Kleene Algebra}, year = {2021}, month = {10/13/2021}, abstract = {

We investigate the algebraic reasoning of quantum programs inspired by the success of classical program analysis based on Kleene algebra. One prominent example of such is the famous Kleene Algebra with Tests (KAT), which has furnished both theoretical insights and practical tools. The succinctness of algebraic reasoning would be especially desirable for scalable analysis of quantum programs, given the involvement of exponential-size matrices in most of the existing methods. A few key features of KAT including the idempotent law and the nice properties of classical tests, however, fail to hold in the context of quantum programs due to their unique quantum features, especially in branching. We propose the Non-idempotent Kleena Algebra (NKA) as a natural alternative and identify complete and sound semantic models for NKA as well as their appropriate quantum interpretations. In light of applications of KAT, we are able to demonstrate algebraic proofs in NKA of quantum compiler optimization and the normal form of quantum while-programs. Moreover, we extend NKA with Tests (i.e., NKAT), where tests model quantum predicates following the rules of effect algebra, and illustrate how to encode propositional quantum Hoare logic as NKAT theorems.

}, doi = {https://doi.org/10.1145/3519939.3523713}, url = {https://arxiv.org/abs/2110.07018}, author = {Yuxiang Peng and Mingsheng Ying and Xiaodi Wu} } @article {2857, title = {Chiral transport of hot carriers in graphene in the quantum Hall regime}, year = {2021}, month = {10/3/2021}, abstract = {

Photocurrent (PC) measurements can reveal the relaxation dynamics of photo-excited hot carriers beyond the linear response of conventional transport experiments, a regime important for carrier multiplication. In graphene subject to a magnetic field, PC measurements are able to probe the existence of Landau levels with different edge chiralities which is exclusive to relativistic electron systems. Here, we report the accurate measurement of PC in graphene in the quantum Hall regime. Prominent PC oscillations as a function of gate voltage on samples\&$\#$39; edges are observed. These oscillation amplitudes form an envelope which depends on the strength of the magnetic field, as does the PCs\&$\#$39; power dependence and their saturation behavior. We explain these experimental observations through a model using optical Bloch equations, incorporating relaxations through acoustic-, optical- phonons and Coulomb interactions. The simulated PC agrees with our experimental results, leading to a unified understanding of the chiral PC in graphene at various magnetic field strengths, and providing hints for the occurrence of a sizable carrier multiplication.\ 

}, url = {https://arxiv.org/abs/2110.01079}, author = {Bin Cao and Tobias Grass and Olivier Gazzano and Kishan Ashokbhai Patel and Jiuning Hu and Markus M{\"u}ller and Tobias Huber and Luca Anzi and Kenji Watanabe and Takashi Taniguchi and David Newell and Michael Gullans and Roman Sordan and Mohammad Hafezi and Glenn Solomon} } @article {2426, title = {Efficient quantum measurement of Pauli operators}, journal = {Quantum}, volume = {5}, year = {2021}, month = {01/19/2021}, chapter = {385}, abstract = {

Estimating the expectation value of an observable is a fundamental task in quantum computation. Unfortunately, it is often impossible to obtain such estimates directly, as the computer is restricted to measuring in a fixed computational basis. One common solution splits the observable into a weighted sum of Pauli operators and measures each separately, at the cost of many measurements. An improved version first groups mutually commuting Pauli operators together and then measures all operators within each group simultaneously. The effectiveness of this depends on two factors. First, to enable simultaneous measurement, circuits are required to rotate each group to the computational basis. In our work, we present two efficient circuit constructions that suitably rotate any group of k commuting n-qubit Pauli operators using at most kn\−k(k+1)/2 and O(kn/logk) two-qubit gates respectively. Second, metrics that justifiably measure the effectiveness of a grouping are required. In our work, we propose two natural metrics that operate under the assumption that measurements are distributed optimally among groups. Motivated by our new metrics, we introduce SORTED INSERTION, a grouping strategy that is explicitly aware of the weighting of each Pauli operator in the observable. Our methods are numerically illustrated in the context of the Variational Quantum Eigensolver, where the observables in question are molecular Hamiltonians. As measured by our metrics, SORTED INSERTION outperforms four conventional greedy colouring algorithms that seek the minimum number of groups.

}, doi = {https://doi.org/10.22331/q-2021-01-20-385}, url = {https://arxiv.org/abs/1908.06942}, author = {Ophelia Crawford and Barnaby van Straaten and Daochen Wang and Thomas Parks and Earl Campbell and Stephen Brierley} } @article {2877, title = {An explicit vector algorithm for high-girth MaxCut}, year = {2021}, month = {8/27/2021}, abstract = {

We give an approximation algorithm for MaxCut and provide guarantees on the average fraction of edges cut on d-regular graphs of girth \≥2k. For every d\≥3 and k\≥4, our approximation guarantees are better than those of all other classical and quantum algorithms known to the authors. Our algorithm constructs an explicit vector solution to the standard semidefinite relaxation of MaxCut and applies hyperplane rounding. It may be viewed as a simplification of the previously best known technique, which approximates Gaussian wave processes on the infinite d-regular tree.

}, url = {https://arxiv.org/abs/2108.12477}, author = {Jessica K. Thompson and Ojas Parekh and Kunal Marwaha} } @article {2771, title = {Hyper-Invariant MERA: Approximate Holographic Error Correction Codes with Power-Law Correlations}, year = {2021}, month = {3/15/2021}, abstract = {

We consider a class of holographic tensor networks that are efficiently contractible variational ansatze, manifestly (approximate) quantum error correction codes, and can support power-law correlation functions. In the case when the network consists of a single type of tensor that also acts as an erasure correction code, we show that it cannot be both locally contractible and sustain power-law correlation functions. Motivated by this no-go theorem, and the desirability of local contractibility for an efficient variational ansatz, we provide guidelines for constructing networks consisting of multiple types of tensors that can support power-law correlation. We also provide an explicit construction of one such network, which approximates the holographic HaPPY pentagon code in the limit where variational parameters are taken to be small.

}, url = {https://arxiv.org/abs/2103.08631}, author = {ChunJun Cao and Jason Pollack and Yixu Wang} } @article {2768, title = {Observation of a prethermal discrete time crystal}, year = {2021}, month = {2/2/2021}, abstract = {

The conventional framework for defining and understanding phases of matter requires thermodynamic equilibrium. Extensions to non-equilibrium systems have led to surprising insights into the nature of many-body thermalization and the discovery of novel phases of matter, often catalyzed by driving the system periodically. The inherent heating from such Floquet drives can be tempered by including strong disorder in the system, but this can also mask the generality of non-equilibrium phases. In this work, we utilize a trapped-ion quantum simulator to observe signatures of a non-equilibrium driven phase without disorder: the prethermal discrete time crystal (PDTC). Here, many-body heating is suppressed not by disorder-induced many-body localization, but instead via high-frequency driving, leading to an expansive time window where non-equilibrium phases can emerge. We observe a number of key features that distinguish the PDTC from its many-body-localized disordered counterpart, such as the drive-frequency control of its lifetime and the dependence of time-crystalline order on the energy density of the initial state. Floquet prethermalization is thus presented as a general strategy for creating, stabilizing and studying intrinsically out-of-equilibrium phases of matter.

}, url = {https://arxiv.org/abs/2102.01695}, author = {Antonis Kyprianidis and Francisco Machado and William Morong and Patrick Becker and Kate S. Collins and Dominic V. Else and Lei Feng and Paul W. Hess and Chetan Nayak and Guido Pagano and Norman Y. Yao and Christopher Monroe} } @article {2763, title = {Observation of Stark many-body localization without disorder}, year = {2021}, month = {2/14/2021}, abstract = {

Thermalization is a ubiquitous process of statistical physics, in which details of few-body observables are washed out in favor of a featureless steady state. Even in isolated quantum many-body systems, limited to reversible dynamics, thermalization typically prevails. However, in these systems, there is another possibility: many-body localization (MBL) can result in preservation of a non-thermal state. While disorder has long been considered an essential ingredient for this phenomenon, recent theoretical work has suggested that a quantum many-body system with a uniformly increasing field -- but no disorder -- can also exhibit MBL, resulting in {\textquoteleft}Stark MBL.\&$\#$39; Here we realize Stark MBL in a trapped-ion quantum simulator and demonstrate its key properties: halting of thermalization and slow propagation of correlations. Tailoring the interactions between ionic spins in an effective field gradient, we directly observe their microscopic equilibration for a variety of initial states, and we apply single-site control to measure correlations between separate regions of the spin chain. Further, by engineering a varying gradient, we create a disorder-free system with coexisting long-lived thermalized and nonthermal regions. The results demonstrate the unexpected generality of MBL, with implications about the fundamental requirements for thermalization and with potential uses in engineering long-lived non-equilibrium quantum matter.

}, url = {https://arxiv.org/abs/2102.07250}, author = {W. Morong and F. Liu and P. Becker and K. S. Collins and L. Feng and A. Kyprianidis and G. Pagano and T. You and Alexey V. Gorshkov and C. Monroe} } @article {2875, title = {Quantum Machine Learning for Finance}, year = {2021}, month = {9/9/2021}, abstract = {

Quantum computers are expected to surpass the computational capabilities of classical computers during this decade, and achieve disruptive impact on numerous industry sectors, particularly finance. In fact, finance is estimated to be the first industry sector to benefit from Quantum Computing not only in the medium and long terms, but even in the short term. This review paper presents the state of the art of quantum algorithms for financial applications, with particular focus to those use cases that can be solved via Machine Learning.

}, url = {https://arxiv.org/abs/2109.04298}, author = {Marco Pistoia and Syed Farhan Ahmad and Akshay Ajagekar and Alexander Buts and Shouvanik Chakrabarti and Dylan Herman and Shaohan Hu and Andrew Jena and Pierre Minssen and Pradeep Niroula and Arthur Rattew and Yue Sun and Romina Yalovetzky} } @article {2818, title = {Robust Self-Testing of Multiparticle Entanglement}, journal = {Phys. Rev. Lett. }, volume = {127}, year = {2021}, month = {12/7/2021}, pages = {230503}, abstract = {

Quantum self-testing is a device-independent way to certify quantum states and measurements using only the input-output statistics, with minimal assumptions about the quantum devices. Due to the high demand on tolerable noise, however, experimental self-testing was limited to two-photon systems. Here, we demonstrate the first robust self-testing for multi-particle quantum entanglement. We prepare two examples of four-photon graph states, the Greenberger-Horne-Zeilinger (GHZ) states with a fidelity of 0.957(2) and the linear cluster states with a fidelity of 0.945(2). Based on the observed input-output statistics, we certify the genuine four-photon entanglement and further estimate their qualities with respect to realistic noise in a device-independent manner.

}, doi = {https://doi.org/10.1103/PhysRevLett.127.230503}, url = {https://arxiv.org/abs/2105.10298}, author = {Dian Wu and Qi Zhao and Xue-Mei Gu and Han-Sen Zhong and You Zhou and Li-Chao Peng and Jian Qin and Yi-Han Luo and Kai Chen and Li Li and Nai-Le Liu and Chao-Yang Lu and Jian-Wei Pan} } @article {2911, title = {Spin chains, defects, and quantum wires for the quantum-double edge}, year = {2021}, month = {11/23/2021}, abstract = {

Non-Abelian defects that bind Majorana or parafermion zero modes are prominent in several topological quantum computation schemes. Underpinning their established understanding is the quantum Ising spin chain, which can be recast as a fermionic model or viewed as a standalone effective theory for the surface-code edge -- both of which harbor non-Abelian defects. We generalize these notions by deriving an effective Ising-like spin chain describing the edge of quantum-double topological order. Relating Majorana and parafermion modes to anyonic strings, we introduce quantum-double generalizations of non-Abelian defects. We develop a way to embed finite-group valued qunits into those valued in continuous groups. Using this embedding, we provide a continuum description of the spin chain and recast its non-interacting part as a quantum wire via addition of a Wess-Zumino-Novikov-Witten term and non-Abelian bosonization.

}, url = {https://arxiv.org/abs/2111.12096}, author = {Victor V. Albert and David Aasen and Wenqing Xu and Wenjie Ji and Jason Alicea and John Preskill} } @article {2879, title = {Synchronous Values of Games}, year = {2021}, month = {9/29/2021}, abstract = {

We study synchronous values of games, especially synchronous games. It is known that a synchronous game has a perfect strategy if and only if it has a perfect synchronous strategy. However, we give examples of synchronous games, in particular graph colouring games, with synchronous value that is strictly smaller than their ordinary value. Thus, the optimal strategy for a synchronous game need not be synchronous. We derive a formula for the synchronous value of an XOR game as an optimization problem over a spectrahedron involving a matrix related to the cost matrix. We give an example of a game such that the synchronous value of repeated products of the game is strictly increasing. We show that the synchronous quantum bias of the XOR of two XOR games is not multiplicative. Finally, we derive geometric and algebraic conditions that a set of projections that yields the synchronous value of a game must satisfy.

}, url = {https://arxiv.org/abs/2109.14741}, author = {J. William Helton and Hamoon Mousavi and Seyed Sajjad Nezhadi and Vern I. Paulsen and Travis B. Russell} } @article {2682, title = {Tunable three-body loss in a nonlinear Rydberg medium}, journal = {Phys. Rev. Lett., in press }, year = {2021}, month = {9/28/2020}, abstract = {

Long-range Rydberg interactions, in combination with electromagnetically induced transparency (EIT), give rise to strongly interacting photons where the strength, sign, and form of the interactions are widely tunable and controllable. Such control can be applied to both coherent and dissipative interactions, which provides the potential to generate novel few-photon states. Recently it has been shown that Rydberg-EIT is a rare system in which three-body interactions can be as strong or stronger than two-body interactions. In this work, we study a three-body scattering loss for Rydberg-EIT in a wide regime of single and two-photon detunings. Our numerical simulations of the full three-body wavefunction and analytical estimates based on Fermi\&$\#$39;s Golden Rule strongly suggest that the observed features in the outgoing photonic correlations are caused by the resonant enhancement of the three-body losses.

}, url = {https://arxiv.org/abs/2009.13599}, author = {Dalia P. Ornelas Huerta and Przemyslaw Bienias and Alexander N. Craddock and Michael Gullans and Andrew J. Hachtel and Marcin Kalinowski and Mary E. Lyon and Alexey V. Gorshkov and Steven L. Rolston and J. V. Porto} } @article {2912, title = {Verified Compilation of Quantum Oracles}, year = {2021}, month = {12/13/2021}, abstract = {

Quantum algorithms often apply classical operations, such as arithmetic or predicate checks, over a quantum superposition of classical data; these so-called oracles are often the largest components of a quantum algorithm. To ease the construction of efficient, correct oracle functions, this paper presents VQO, a high-assurance framework implemented with the Coq proof assistant. The core of VQO is OQASM, the oracle quantum assembly language. OQASM operations move qubits among three different bases via the Quantum Fourier Transform and Hadamard operations, thus admitting important optimizations, but without inducing entanglement and the exponential blowup that comes with it. OQASM\&$\#$39;s design enabled us to prove correct VQO\&$\#$39;s compilers -- from a simple imperative language called OQIMP to OQASM, and from OQASM to SQIR, a general-purpose quantum assembly language -- and allowed us to efficiently test properties of OQASM programs using the QuickChick property-based testing framework. We have used VQO to implement oracles used in Shor\&$\#$39;s and Grover\&$\#$39;s algorithms, as well as several common arithmetic operators. VQO\&$\#$39;s oracles have performance comparable to those produced by Quipper, a state-of-the-art but unverified quantum programming platform.

}, url = {https://arxiv.org/abs/2112.06700}, author = {Liyi Li and Finnegan Voichick and Kesha Hietala and Yuxiang Peng and Xiaodi Wu and Michael Hicks} } @article {2779, title = {Coherent transport of spin by adiabatic passage in quantum dot arrays}, journal = {Physical Review B}, volume = {102}, year = {2020}, month = {9/17/2020}, abstract = {

We introduce an adiabatic transfer protocol for spin states in large quantum dot arrays that is based on time-dependent modulation of the Heisenberg exchange interaction in the presence of a magnetic field gradient. We refer to this protocol as spin-CTAP (coherent transport by adiabatic passage) in analogy to a related protocol developed for charge state transfer in quantum dot arrays. The insensitivity of this adiabatic protocol to pulse imperfections has potential advantages for reading out extended spin qubit arrays. When the static exchange interaction varies across the array, a quantum-controlled version of spin-CTAP is possible, where the transfer process is conditional on the spin states in the middle of the array. This conditional operation can be used to generate N-qubit entangled GHZ states. Using a realistic noise model, we analyze the robustness of the spin-CTAP operations and find that high-fidelity (\>95\%) spin eigenstate transfer and GHZ state preparation is feasible in current devices.

}, issn = {2469-9950}, doi = {10.1103/PhysRevB.102.155404}, url = {https://arxiv.org/abs/2007.10582}, author = {Michael Gullans and Petta, J. R.} } @article {2683, title = {Coherent transport of spin by adiabatic passage in quantum dot arrays}, journal = {Phys. Rev. B}, volume = {102}, year = {2020}, month = {9/17/2020}, abstract = {

We introduce an adiabatic transfer protocol for spin states in large quantum dot arrays that is based on time-dependent modulation of the Heisenberg exchange interaction in the presence of a magnetic field gradient. We refer to this protocol as spin-CTAP (coherent transport by adiabatic passage) in analogy to a related protocol developed for charge state transfer in quantum dot arrays. The insensitivity of this adiabatic protocol to pulse imperfections has potential advantages for reading out extended spin qubit arrays. When the static exchange interaction varies across the array, a quantum-controlled version of spin-CTAP is possible, where the transfer process is conditional on the spin states in the middle of the array. This conditional operation can be used to generate N-qubit entangled GHZ states. Using a realistic noise model, we analyze the robustness of the spin-CTAP operations and find that high-fidelity (\>95\%) spin eigenstate transfer and GHZ state preparation is feasible in current devices.

}, doi = {https://doi.org/10.1103/PhysRevB.102.155404}, url = {https://arxiv.org/abs/2007.10582}, author = {Michael Gullans and J. R. Petta} } @article {2719, title = {Continuous symmetries and approximate quantum error correction}, journal = {Phys. Rev. X}, volume = {10}, year = {2020}, month = {10/26/2020}, abstract = {

Quantum error correction and symmetry arise in many areas of physics, including many-body systems, metrology in the presence of noise, fault-tolerant computation, and holographic quantum gravity. Here we study the compatibility of these two important principles. If a logical quantum system is encoded into n physical subsystems, we say that the code is covariant with respect to a symmetry group G if a G transformation on the logical system can be realized by performing transformations on the individual subsystems. For a G-covariant code with G a continuous group, we derive a lower bound on the error correction infidelity following erasure of a subsystem. This bound approaches zero when the number of subsystems n or the dimension d of each subsystem is large. We exhibit codes achieving approximately the same scaling of infidelity with n or d as the lower bound. Leveraging tools from representation theory, we prove an approximate version of the Eastin-Knill theorem: If a code admits a universal set of transversal gates and corrects erasure with fixed accuracy, then, for each logical qubit, we need a number of physical qubits per subsystem that is inversely proportional to the error parameter. We construct codes covariant with respect to the full logical unitary group, achieving good accuracy for large d (using random codes) or n (using codes based on W-states). We systematically construct codes covariant with respect to general groups, obtaining natural generalizations of qubit codes to, for instance, oscillators and rotors. In the context of the AdS/CFT correspondence, our approach provides insight into how time evolution in the bulk corresponds to time evolution on the boundary without violating the Eastin-Knill theorem, and our five-rotor code can be stacked to form a covariant holographic code.

}, doi = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.041018}, url = {https://arxiv.org/abs/1902.07714}, author = {Philippe Faist and Sepehr Nezami and Victor V. Albert and Grant Salton and Fernando Pastawski and Patrick Hayden and John Preskill} } @article {2567, title = {Exotic photonic molecules via Lennard-Jones-like potentials}, journal = {Phys. Rev. Lett.}, volume = {125}, year = {2020}, month = {9/19/2020}, abstract = {

Ultracold systems offer an unprecedented level of control of interactions between atoms. An important challenge is to achieve a similar level of control of the interactions between photons. Towards this goal, we propose a realization of a novel Lennard-Jones-like potential between photons coupled to the Rydberg states via electromagnetically induced transparency (EIT). This potential is achieved by tuning Rydberg states to a F{{\"o}}rster resonance with other Rydberg states. We consider few-body problems in 1D and 2D geometries and show the existence of self-bound clusters (\"molecules\") of photons. We demonstrate that for a few-body problem, the multi-body interactions have a significant impact on the geometry of the molecular ground state. This leads to phenomena without counterparts in conventional systems: For example, three photons in 2D preferentially arrange themselves in a line-configuration rather than in an equilateral-triangle configuration. Our result opens a new avenue for studies of many-body phenomena with strongly interacting photons.

}, doi = {https://doi.org/10.1103/PhysRevLett.125.093601}, url = {https://arxiv.org/abs/2003.07864}, author = {Przemyslaw Bienias and Michael Gullans and Marcin Kalinowski and Alexander N. Craddock and Dalia P. Ornelas-Huerta and Steven L. Rolston and J. V. Porto and Alexey V. Gorshkov} } @article {2531, title = {Many-Body Dephasing in a Trapped-Ion Quantum Simulator}, journal = {Phys. Rev. Lett.}, volume = {125}, year = {2020}, month = {8/24/2020}, abstract = {

How a closed interacting quantum many-body system relaxes and dephases as a function of time is a fundamental question in thermodynamic and statistical physics. In this work, we observe and analyse the persistent temporal fluctuations after a quantum quench of a tunable long-range interacting transverse-field Ising Hamiltonian realized with a trapped-ion quantum simulator. We measure the temporal fluctuations in the average magnetization of a finite-size system of spin-1/2 particles and observe the experimental evidence for the theoretically predicted regime of many-body dephasing. We experiment in a regime where the properties of the system are closely related to the integrable Hamiltonian with global spin-spin coupling, which enables analytical predictions even for the long-time non-integrable dynamics. We find that the measured fluctuations are exponentially suppressed with increasing system size, consistent with theoretical predictions.\ 

}, doi = {https://doi.org/10.1103/PhysRevLett.125.120605}, url = {https://arxiv.org/abs/2001.02477}, author = {Harvey B. Kaplan and Lingzhen Guo and Wen Lin Tan and Arinjoy De and Florian Marquardt and Guido Pagano and Christopher Monroe} } @article {2687, title = {Mechanical Quantum Sensing in the Search for Dark Matter}, year = {2020}, month = {8/13/2020}, type = {FERMILAB-PUB-20-378-QIS-T}, abstract = {

Numerous astrophysical and cosmological observations are best explained by the existence of dark matter, a mass density which interacts only very weakly with visible, baryonic matter. Searching for the extremely weak signals produced by this dark matter strongly motivate the development of new, ultra-sensitive detector technologies. Paradigmatic advances in the control and readout of massive mechanical systems, in both the classical and quantum regimes, have enabled unprecedented levels of sensitivity. In this white paper, we outline recent ideas in the potential use of a range of solid-state mechanical sensing technologies to aid in the search for dark matter in a number of energy scales and with a variety of coupling mechanisms.

}, url = {https://arxiv.org/abs/2008.06074}, author = {D. Carney and G. Krnjaic and D. C. Moore and C. A. Regal and G. Afek and S. Bhave and B. Brubaker and T. Corbitt and J. Cripe and N. Crisosto and A.Geraci and S. Ghosh and J. G. E. Harris and A. Hook and E. W. Kolb and J. Kunjummen and R. F. Lang and T. Li and T. Lin and Z. Liu and J. Lykken and L. Magrini and J. Manley and N. Matsumoto and A. Monte and F. Monteiro and T. Purdy and C. J. Riedel and R. Singh and S. Singh and K. Sinha and J. M. Taylor and J. Qin and D. J. Wilson and Y. Zhao} } @article {2692, title = {More of the Bulk from Extremal Area Variations}, journal = {Classical and Quantum Gravity}, volume = {38}, year = {2020}, month = {12/24/2020}, pages = {047001}, abstract = {

It was shown recently, building on work of Alexakis, Balehowksy, and Nachman that the geometry of (some portion of) a manifold with boundary is uniquely fixed by the areas of a foliation of two-dimensional disk-shaped surfaces anchored to the boundary. In the context of AdS/CFT, this implies that (a portion of) a four-dimensional bulk geometry can be fixed uniquely from the entanglement entropies of disk-shaped boundary regions, subject to several constraints. In this Note, we loosen some of these constraints, in particular allowing for the bulk foliation of extremal surfaces to be local and removing the constraint of disk topology; these generalizations ensure uniqueness of more of the deep bulk geometry by allowing for e.g. surfaces anchored on disconnected asymptotic boundaries, or HRT surfaces past a phase transition. We also explore in more depth the generality of the local foliation requirement, showing that even in a highly dynamical geometry like AdS-Vaidya it is satisfied.

}, doi = {https://iopscience.iop.org/article/10.1088/1361-6382/abcfd0/pdf}, url = {https://arxiv.org/abs/2009.07850}, author = {Ning Bao and ChunJun Cao and Sebastian Fischetti and Jason Pollack and Yibo Zhong} } @article {2571, title = {A note on blind contact tracing at scale with applications to the COVID-19 pandemic}, year = {2020}, month = {4/10/2020}, abstract = {

The current COVID-19 pandemic highlights the utility of contact tracing, when combined with case isolation and social distancing, as an important tool for mitigating the spread of a disease [1]. Contact tracing provides a mechanism of identifying individuals with a high likelihood of previous exposure to a contagious disease, allowing additional precautions to be put in place to prevent continued transmission. Here we consider a cryptographic approach to contact tracing based on secure two-party computation (2PC). We begin by considering the problem of comparing a set of location histories held by two parties to determine whether they have come within some threshold distance while at the same time maintaining the privacy of the location histories. We propose a solution to this problem using pre-shared keys, adapted from an equality testing protocol due to Ishai et al [2]. We discuss how this protocol can be used to maintain privacy within practical contact tracing scenarios, including both app-based approaches and approaches which leverage location history held by telecoms and internet service providers. We examine the efficiency of this approach and show that existing infrastructure is sufficient to support anonymised contact tracing at a national level.

}, url = {https://arxiv.org/abs/2004.05116}, author = {Jack K. Fitzsimons and Atul Mantri and Robert Pisarczyk and Tom Rainforth and Zhikuan Zhao} } @article {2568, title = {On-demand indistinguishable single photons from an efficient and pure source based on a Rydberg ensemble}, year = {2020}, month = {3/4/2020}, abstract = {

Single photons coupled to atomic systems have shown to be a promising platform for developing quantum technologies. Yet a bright on-demand, highly pure and highly indistinguishable single-photon source compatible with atomic platforms is lacking. In this work, we demonstrate such a source based on a strongly interacting Rydberg system. The large optical nonlinearities in a blockaded Rydberg ensemble convert coherent light into a single-collective excitation that can be coherently retrieved as a quantum field. We observe a single-transverse-mode efficiency up to 0.18(2), g(2)=2.0(1.5)\×10\−4, and indistinguishability of 0.982(7), making this system promising for scalable quantum information applications. Accounting for losses, we infer a generation probability up to 0.40(4). Furthermore, we investigate the effects of contaminant Rydberg excitations on the source efficiency. Finally, we introduce metrics to benchmark the performance of on-demand single-photon sources.\ 

}, url = {https://arxiv.org/abs/2003.02202}, author = {Dalia P. Ornelas-Huerta and Alexander N. Craddock and Elizabeth A. Goldschmidt and Andrew J. Hachtel and Yidan Wang and P. Bienias and Alexey V. Gorshkov and Steve L. Rolston and James V. Porto} } @article {2586, title = {Probing XY phase transitions in a Josephson junction array with tunable frustration}, year = {2020}, month = {1/22/2020}, abstract = {

The seminal theoretical works of Berezinskii, Kosterlitz, and Thouless presented a new paradigm for phase transitions in condensed matter that are driven by topological excitations. These transitions have been extensively studied in the context of two-dimensional XY models -- coupled compasses -- and have generated interest in the context of quantum simulation. Here, we use a circuit quantum-electrodynamics architecture to study the critical behavior of engineered XY models through their dynamical response. In particular, we examine not only the unfrustrated case but also the fully-frustrated case which leads to enhanced degeneracy associated with the spin rotational [U(1)] and discrete chiral (Z2) symmetries. The nature of the transition in the frustrated case has posed a challenge for theoretical studies while direct experimental probes remain elusive. Here we identify the transition temperatures for both the unfrustrated and fully-frustrated XY models by probing a Josephson junction array close to equilibrium using weak microwave excitations and measuring the temperature dependence of the effective damping obtained from the complex reflection coefficient. We argue that our probing technique is primarily sensitive to the dynamics of the U(1) part.

}, url = {https://arxiv.org/abs/2001.07877}, author = {R. Cosmic and K. Kawabata and Y. Ashida and H. Ikegami and S. Furukawa and P. Patil and J. M. Taylor and Y. Nakamura} } @article {2626, title = {On Quantum Chosen-Ciphertext Attacks and Learning with Errors}, journal = {Cryptography}, volume = {4}, year = {2020}, month = {3/21/2020}, pages = {10}, abstract = {

Large-scale quantum computing poses a major threat to classical public-key cryptography. Recently, strong \“quantum access\” security models have shown that numerous symmetric-key cryptosystems are also vulnerable. In this paper, we consider classical encryption in a model that grants the adversary quantum oracle access to encryption and decryption, but where we restrict the latter to non-adaptive (i.e., pre-challenge) queries only. We formalize this model using appropriate notions of ciphertext indistinguishability and semantic security (which are equivalent by standard arguments) and call it QCCA1 in analogy to the classical CCA1 security model. We show that the standard pseudorandom function ( PRF )-based encryption schemes are QCCA1 -secure when instantiated with quantum-secure primitives. Our security proofs use a strong bound on quantum random-access codes with shared randomness. Revisiting plain IND\−CPA -secure Learning with Errors ( LWE ) encryption, we show that leaking only a single quantum decryption query (and no other leakage or queries of any kind) allows the adversary to recover the full secret key with constant success probability. Information-theoretically, full recovery of the key in the classical setting requires at least a linear number of decryption queries. Our results thus challenge the notion that LWE is unconditionally \“just as secure\” quantumly as it is classically. The algorithm at the core of our attack is a new variant of the well-known Bernstein\–Vazirani algorithm. Finally, we emphasize that our results should not be interpreted as a weakness of these cryptosystems in their stated security setting (i.e., post-quantum chosen-plaintext secrecy). Rather, our results mean that, if these cryptosystems are exposed to chosen-ciphertext attacks (e.g., as a result of deployment in an inappropriate real-world setting) then quantum attacks are even more devastating than classical ones.

}, doi = {https://doi.org/10.3390/cryptography4010010}, author = {Gorjan Alagic and Stacey Jeffery and Maris Ozols and Alexander Poremba} } @article {2681, title = {Resonant enhancement of three-body loss between strongly interacting photons}, year = {2020}, month = {10/19/2020}, abstract = {

Rydberg polaritons provide an example of a rare type of system where three-body interactions can be as strong or even stronger than two-body interactions. The three-body interactions can be either dispersive or dissipative, with both types possibly giving rise to exotic, strongly-interacting, and topological phases of matter. Despite past theoretical and experimental studies of the regime with dispersive interaction, the dissipative regime is still mostly unexplored. Using a renormalization group technique to solve the three-body Schr{\"o}dinger equation, we show how the shape and strength of dissipative three-body forces can be universally enhanced for Rydberg polaritons. We demonstrate how these interactions relate to the transmission through a single-mode cavity, which can be used as a probe of the three-body physics in current experiment

}, url = {https://arxiv.org/abs/2010.09772}, author = {Marcin Kalinowski and Yidan Wang and Przemyslaw Bienias and Michael Gullans and Dalia P. Ornelas-Huerta and Alexander N. Craddock and Steven L. Rolston and J. V. Porto and Hans Peter B{\"u}chler and Alexey V. Gorshkov} } @article {2718, title = {Robust Encoding of a Qubit in a Molecule}, journal = {Phys. Rev. X}, volume = {10}, year = {2020}, month = {9/1/2020}, abstract = {

We construct quantum error-correcting codes that embed a finite-dimensional code space in the infinite-dimensional Hilbert space of rotational states of a rigid body. These codes, which protect against both drift in the body\’s orientation and small changes in its angular momentum, may be well suited for robust storage and coherent processing of quantum information using rotational states of a polyatomic molecule. Extensions of such codes to rigid bodies with a symmetry axis are compatible with rotational states of diatomic molecules as well as nuclear states of molecules and atoms. We also describe codes associated with general non-Abelian groups and develop orthogonality relations for coset spaces, laying the groundwork for quantum information processing with exotic configuration spaces.

}, doi = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.031050}, url = {https://arxiv.org/abs/1911.00099}, author = {Victor V. Albert and Jacob P. Covey and John Preskill} } @article {2377, title = {Simulating large quantum circuits on a small quantum computer}, journal = {Phys. Rev. Lett.}, volume = {125}, year = {2020}, month = {12/8/2020}, abstract = {

Limited quantum memory is one of the most important constraints for near-term quantum devices. Understanding whether a small quantum computer can simulate a larger quantum system, or execute an algorithm requiring more qubits than available, is both of theoretical and practical importance. In this Letter, we introduce cluster parameters K and d of a quantum circuit. The tensor network of such a circuit can be decomposed into clusters of size at most d with at most K qubits of inter-cluster quantum communication. Our main result is a simulation scheme of any (K,d)-clustered quantum circuit on a d-qubit machine in time roughly 2O(K). An important application of our result is the simulation of clustered quantum systems---such as large molecules---that can be partitioned into multiple significantly smaller clusters with weak interactions among them. Another potential application is quantum optimization: we demonstrate numerically that variational quantum eigensolvers can still perform well when restricted to clustered circuits, thus making it feasible to study large quantum systems on small quantum devices.

}, doi = {https://doi.org/10.1103/PhysRevLett.125.150504}, url = {https://arxiv.org/abs/1904.00102}, author = {Tianyi Peng and Aram Harrow and Maris Ozols and Xiaodi Wu} } @article {2676, title = {Status Report on the Second Round of the NIST Post-Quantum Cryptography Standardization Process}, journal = {NISTIR 8309}, year = {2020}, month = {07/2020}, abstract = {

The National Institute of Standards and Technology is in the process of selecting one or more public-key cryptographic algorithms through a public, competition-like process. The new public-key cryptography standards will specify one or more additional digital signatures, public-key encryption, and key-establishment algorithms to augment Federal Information Processing Standard (FIPS) 186-4, Digital Signature Standard (DSS), as well as NIST Special Publication (SP) 800-56A Revision 3, Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography, and SP 800-56B Revision 2, Recommendation for Pair-Wise Key Establishment Using Integer Factorization Cryptography. It is intended that these algorithms will be capable of protecting sensitive information well into the foreseeable future, including after the advent of quantum computers.

The NIST Post-Quantum Cryptography Standardization Process began in 2017 with 69 candidate algorithms that met both the minimum acceptance criteria and submission requirements. The first round lasted until January 2019, during which candidate algorithms were evaluated based on their security, performance, and other characteristics. NIST selected 26 algorithms to advance to the second round for more analysis. This report describes the evaluation and selection process, based on public feedback and internal review, of the second-round candidates. The report summarizes the 26 second-round candidate algorithms and identifies those selected to move forward to the third round of the competition. The third-round finalist public-key encryption and key-establishment algorithms are Classic McEliece, CRYSTALS-KYBER, NTRU, and SABER. The third-round finalists for digital signatures are CRYSTALS-DILITHIUM, FALCON, and Rainbow. These finalists will be considered for standardization at the end of the third round. In addition, eight alternate candidate algorithms will also advance to the third round: BIKE, FrodoKEM, HQC, NTRU Prime, SIKE, GeMSS, Picnic, and SPHINCS+. These additional candidates are still being considered for standardization, although this is unlikely to occur at the end of the third round. NIST hopes that the announcement of these finalists and additional candidates will serve to focus the cryptographic community\’s attention during the next round.

}, doi = {https://doi.org/10.6028/NIST.IR.8309}, author = {Gorjan Alagic and Jacob Alperin-Sheriff and Daniel Apon and David Cooper and Quynh Dang and John Kelsey and Yi-Kai Liu and Carl Miller and Dustin Moody and Rene Peralta and Ray Perlner and Angela Robinson and Daniel Smith-Tone} } @conference {2599, title = {Symmetries, graph properties, and quantum speedups}, booktitle = {Proceedings of the 61st IEEE Symposium on Foundations of Computer Science (FOCS 2020), pp. 649{\textendash}660 (2020)}, year = {2020}, month = {6/23/2020}, abstract = {

Aaronson and Ambainis (2009) and Chailloux (2018) showed that fully symmetric (partial) functions do not admit exponential quantum query speedups. This raises a natural question: how symmetric must a function be before it cannot exhibit a large quantum speedup?
In this work, we prove that hypergraph symmetries in the adjacency matrix model allow at most a polynomial separation between randomized and quantum query complexities. We also show that, remarkably, permutation groups constructed out of these symmetries are essentially the only permutation groups that prevent super-polynomial quantum speedups. We prove this by fully characterizing the primitive permutation groups that allow super-polynomial quantum speedups.
In contrast, in the adjacency list model for bounded-degree graphs (where graph symmetry is manifested differently), we exhibit a property testing problem that shows an exponential quantum speedup. These results resolve open questions posed by Ambainis, Childs, and Liu (2010) and Montanaro and de Wolf (2013).

}, doi = {https://doi.org/10.1109/FOCS46700.2020.00066}, url = {https://arxiv.org/abs/2006.12760}, author = {Shalev Ben-David and Andrew M. Childs and Andras Gilyen and William Kretschmer and Supartha Podder and Daochen Wang} } @article {2453, title = {Towards analog quantum simulations of lattice gauge theories with trapped ions}, journal = {Physical Review Research }, volume = {2}, year = {2020}, month = {4/8/2020}, abstract = {

Gauge field theories play a central role in modern physics and are at the heart of the Standard Model of elementary particles and interactions. Despite significant progress in applying classical computational techniques to simulate gauge theories, it has remained a challenging task to compute the real-time dynamics of systems described by gauge theories. An exciting possibility that has been explored in recent years is the use of highly-controlled quantum systems to simulate, in an analog fashion, properties of a target system whose dynamics are difficult to compute. Engineered atom-laser interactions in a linear crystal of trapped ions offer a wide range of possibilities for quantum simulations of complex physical systems. Here, we devise practical proposals for analog simulation of simple lattice gauge theories whose dynamics can be mapped onto spin-spin interactions in any dimension. These include 1+1D quantum electrodynamics, 2+1D Abelian Chern-Simons theory coupled to fermions, and 2+1D pure Z2 gauge theory. The scheme proposed, along with the optimization protocol applied, will have applications beyond the examples presented in this work, and will enable scalable analog quantum simulation of Heisenberg spin models in any number of dimensions and with arbitrary interaction strengths.

}, doi = {https://doi.org/10.1103/PhysRevResearch.2.023015}, url = {https://arxiv.org/abs/1908.03210}, author = {Zohreh Davoudi and Mohammad Hafezi and Christopher Monroe and Guido Pagano and Alireza Seif and Andrew Shaw} } @article {2493, title = {An approximate description of quantum states}, year = {2019}, month = {2019/11/13}, abstract = {

We introduce an approximate description of an N-qubit state, which contains sufficient information to estimate the expectation value of any observable with precision independent of N. We show, in fact, that the error in the estimation of the observables\&$\#$39; expectation values decreases as the inverse of the square root of the number of the system\&$\#$39;s identical preparations and increases, at most, linearly in a suitably defined, N-independent, seminorm of the observables. Building the approximate description of the N-qubit state only requires repetitions of single-qubit rotations followed by single-qubit measurements and can be considered for implementation on today\&$\#$39;s Noisy Intermediate-Scale Quantum (NISQ) computers. The access to the expectation values of all observables for a given state leads to an efficient variational method for the determination of the minimum eigenvalue of an observable. The method represents one example of the practical significance of the approximate description of the N-qubit state. We conclude by briefly discussing extensions to generative modelling and with fermionic operators

}, url = {https://arxiv.org/abs/1910.10543}, author = {Marco Paini and Amir Kalev} } @article {2516, title = {Beyond Spontaneous Emission: Giant Atom Bounded in Continuum}, year = {2019}, month = {12/20/2019}, abstract = {

The quantum coupling of individual superconducting qubits to microwave photons leads to remarkable experimental opportunities. Here we consider the phononic case where the qubit is coupled to an electromagnetic surface acoustic wave antenna that enables supersonic propagation of the qubit oscillations. This can be considered as a giant atom that is many phonon wavelengths long. We study an exactly solvable toy model that captures these effects, and find that this non-Markovian giant atom has a suppressed relaxation, as well as an effective vacuum coupling between a qubit excitation and a localized wave packet of sound, even in the absence of a cavity for the sound waves. Finally, we consider practical implementations of these ideas in current surface acoustic wave devices.\ 

}, url = {https://arxiv.org/abs/1912.09980}, author = {Shangjie Guo and Yidan Wang and Thomas Purdy and J. M. Taylor} } @article {2264, title = {Confined Dynamics in Long-Range Interacting Quantum Spin Chains}, journal = {Phys. Rev. Lett.}, volume = {122 }, year = {2019}, month = {04/17/2019}, abstract = {

We study the quasiparticle excitation and quench dynamics of the one-dimensional transverse-field Ising model with power-law (1/rα) interactions. We find that long-range interactions give rise to a confining potential, which couples pairs of domain walls (kinks) into bound quasiparticles, analogous to mesonic bound states in high-energy physics. We show that these bound states have dramatic consequences for the non-equilibrium dynamics following a global quantum quench, such as suppressed spreading of quantum information and oscillations of order parameters. The masses of these bound states can be read out from the Fourier spectrum of these oscillating order parameters. We then use a two-kink model to qualitatively explain the phenomenon of long-range-interaction-induced confinement. The masses of the bound states predicted by this model are in good quantitative agreement with exact diagonalization results. Moreover, we illustrate that these bound states lead to weak thermalization of local observables for initial states with energy near the bottom of the many-body energy spectrum. Our work is readily applicable to current trapped-ion experiments.

}, doi = {https://doi.org/10.1103/PhysRevLett.122.150601}, url = {https://arxiv.org/abs/1810.02365}, author = {Fangli Liu and Rex Lundgren and Paraj Titum and Guido Pagano and Jiehang Zhang and Christopher Monroe and Alexey V. Gorshkov} } @article {2409, title = {Floquet engineering of optical lattices with spatial features and periodicity below the diffraction limit}, year = {2019}, month = {06/18/2019}, abstract = {

Floquet engineering or coherent time periodic driving of quantum systems has been successfully used to synthesize Hamiltonians with novel properties. In ultracold atomic systems, this has led to experimental realizations of artificial gauge fields, topological band structures, and observation of dynamical localization, to name just a few. Here we present a Floquet-based framework to stroboscopically engineer Hamiltonians with spatial features and periodicity below the diffraction limit of light used to create them by time-averaging over various configurations of a 1D optical Kronig-Penney (KP) lattice. The KP potential is a lattice of narrow subwavelength barriers spaced by half the optical wavelength (λ/2) and arises from the non-linear optical response of the atomic dark state. Stroboscopic control over the strength and position of this lattice requires time-dependent adiabatic manipulation of the dark state spin composition. We investigate adiabaticity requirements and shape our time-dependent light fields to respect the requirements. We apply this framework to show that a λ/4-spaced lattice can be synthesized using realistic experimental parameters as an example, discuss mechanisms that limit lifetimes in these lattices, explore candidate systems and their limitations, and treat adiabatic loading into the ground band of these lattices.

}, url = {https://arxiv.org/abs/1906.07646}, author = {S. Subhankar and P. Bienias and P. Titum and T-C. Tsui and Y. Wang and Alexey V. Gorshkov and S. L. Rolston and J. V. Porto} } @article {2369, title = {Ground-state energy estimation of the water molecule on a trapped ion quantum computer}, year = {2019}, month = {03/07/2019}, abstract = {

Quantum computing leverages the quantum resources of superposition and entanglement to efficiently solve computational problems considered intractable for classical computers. Examples include calculating molecular and nuclear structure, simulating strongly-interacting electron systems, and modeling aspects of material function. While substantial theoretical advances have been made in mapping these problems to quantum algorithms, there remains a large gap between the resource requirements for solving such problems and the capabilities of currently available quantum hardware. Bridging this gap will require a co-design approach, where the expression of algorithms is developed in conjunction with the hardware itself to optimize execution. Here, we describe a scalable co-design framework for solving chemistry problems on a trapped ion quantum computer, and apply it to compute the ground-state energy of the water molecule. The robust operation of the trapped ion quantum computer yields energy estimates with errors approaching the chemical accuracy, which is the target threshold necessary for predicting the rates of chemical reaction dynamics.

}, url = {https://arxiv.org/abs/1902.10171}, author = {Yunseong Nam and Jwo-Sy Chen and Neal C. Pisenti and Kenneth Wright and Conor Delaney and Dmitri Maslov and Kenneth R. Brown and Stewart Allen and Jason M. Amini and Joel Apisdorf and Kristin M. Beck and Aleksey Blinov and Vandiver Chaplin and Mika Chmielewski and Coleman Collins and Shantanu Debnath and Andrew M. Ducore and Kai M. Hudek and Matthew Keesan and Sarah M. Kreikemeier and Jonathan Mizrahi and Phil Solomon and Mike Williams and Jaime David Wong-Campos and Christopher Monroe and Jungsang Kim} } @article {2365, title = {Heisenberg-Scaling Measurement Protocol for Analytic Functions with Quantum Sensor Networks}, journal = {Phys. Rev. A}, volume = {100}, year = {2019}, month = {10/7/2019}, abstract = {

We generalize past work on quantum sensor networks to show that, for d input parameters, entanglement can yield a factor O(d) improvement in mean squared error when estimating an analytic function of these parameters. We show that the protocol is optimal for qubit sensors, and conjecture an optimal protocol for photons passing through interferometers. Our protocol is also applicable to continuous variable measurements, such as one quadrature of a field operator. We outline a few potential applications, including calibration of laser operations in trapped ion quantum computing.

}, doi = {https://doi.org/10.1103/PhysRevA.100.042304}, url = {https://arxiv.org/abs/1901.09042}, author = {Kevin Qian and Zachary Eldredge and Wenchao Ge and Guido Pagano and Christopher Monroe and James V. Porto and Alexey V. Gorshkov} } @article {2219, title = {On non-adaptive quantum chosen-ciphertext attacks and Learning with Errors}, journal = {14th Conference on the Theory of Quantum Computation, Communication and Cryptography, TQC 2019, June 3-5, 2019, University of Maryland, College Park, Maryland, USA}, year = {2019}, pages = {1:1-1:23 }, abstract = {

Large-scale quantum computing is a significant threat to classical public-key cryptography. In strong \"quantum access\" security models, numerous symmetric-key cryptosystems are also vulnerable. We consider classical encryption in a model which grants the adversary quantum oracle access to encryption and decryption, but where the latter is restricted to non-adaptive (i.e., pre-challenge) queries only. We define this model formally using appropriate notions of ciphertext indistinguishability and semantic security (which are equivalent by standard arguments) and call it QCCA1 in analogy to the classical CCA1 security model. Using a bound on quantum random-access codes, we show that the standard PRF- and PRP-based encryption schemes are QCCA1-secure when instantiated with quantum-secure primitives. We then revisit standard IND-CPA-secure Learning with Errors (LWE) encryption and show that leaking just one quantum decryption query (and no other queries or leakage of any kind) allows the adversary to recover the full secret key with constant success probability. In the classical setting, by contrast, recovering the key uses a linear number of decryption queries, and this is optimal. The algorithm at the core of our attack is a (large-modulus version of) the well-known Bernstein-Vazirani algorithm. We emphasize that our results should *not* be interpreted as a weakness of these cryptosystems in their stated security setting (i.e., post-quantum chosen-plaintext secrecy). Rather, our results mean that, if these cryptosystems are exposed to chosen-ciphertext attacks (e.g., as a result of deployment in an inappropriate real-world setting) then quantum attacks are even more devastating than classical ones.\ 

}, doi = {https://doi.org/10.4230/LIPIcs.TQC.2019.1}, url = {https://arxiv.org/abs/1808.09655}, author = {Gorjan Alagic and Stacey Jeffery and Maris Ozols and Alexander Poremba} } @article {2520, title = {Observation of Domain Wall Confinement and Dynamics in a Quantum Simulator}, year = {2019}, month = {12/23/2019}, abstract = {

Confinement is a ubiquitous mechanism in nature, whereby particles feel an attractive force that increases without bound as they separate. A prominent example is color confinement in particle physics, in which baryons and mesons are produced by quark confinement. Analogously, confinement can also occur in low-energy quantum many-body systems when elementary excitations are confined into bound quasiparticles. Here, we report the first observation of magnetic domain wall confinement in interacting spin chains with a trapped-ion quantum simulator. By measuring how correlations spread, we show that confinement can dramatically suppress information propagation and thermalization in such many-body systems. We are able to quantitatively determine the excitation energy of domain wall bound states from non-equilibrium quench dynamics. Furthermore, we study the number of domain wall excitations created for different quench parameters, in a regime that is difficult to model with classical computers. This work demonstrates the capability of quantum simulators for investigating exotic high-energy physics phenomena, such as quark collision and string breaking

}, url = {https://arxiv.org/abs/1912.11117}, author = {W. L. Tan and P. Becker and F. Liu and G. Pagano and K. S. Collins and A. De and L. Feng and H. B. Kaplan and A. Kyprianidis and R. Lundgren and W. Morong and S. Whitsitt and Alexey V. Gorshkov and C. Monroe} } @article {2362, title = {Opportunities for Nuclear Physics \& Quantum Information Science}, year = {2019}, month = {03/13/2019}, abstract = {

his whitepaper is an outcome of the workshop Intersections between Nuclear Physics and Quantum Information held at Argonne National Laboratory on 28-30 March 2018 [www.phy.anl.gov/npqi2018/]. The workshop brought together 116 national and international experts in nuclear physics and quantum information science to explore opportunities for the two fields to collaborate on topics of interest to the U.S. Department of Energy (DOE) Office of Science, Office of Nuclear Physics, and more broadly to U.S. society and industry. The workshop consisted of 22 invited and 10 contributed talks, as well as three panel discussion sessions. Topics discussed included quantum computation, quantum simulation, quantum sensing, nuclear physics detectors, nuclear many-body problem, entanglement at collider energies, and lattice gauge theories.

}, url = {https://arxiv.org/abs/1903.05453}, author = {I. C. Clo{\"e}t and Matthew R. Dietrich and John Arrington and Alexei Bazavov and Michael Bishof and Adam Freese and Alexey V. Gorshkov and Anna Grassellino and Kawtar Hafidi and Zubin Jacob and Michael McGuigan and Yannick Meurice and Zein-Eddine Meziani and Peter Mueller and Christine Muschik and James Osborn and Matthew Otten and Peter Petreczky and Tomas Polakovic and Alan Poon and Raphael Pooser and Alessandro Roggero and Mark Saffman and Brent VanDevender and Jiehang Zhang and Erez Zohar} } @article {2529, title = {Programmable Quantum Simulations of Spin Systems with Trapped Ions}, year = {2019}, month = {12/17/2019}, abstract = {

Laser-cooled and trapped atomic ions form an ideal standard for the simulation of interacting quantum spin models. Effective spins are represented by appropriate internal energy levels within each ion, and the spins can be measured with near-perfect efficiency using state-dependent fluorescence techniques. By applying optical fields that exert optical dipole forces on the ions, their Coulomb interaction can be modulated in ways that give rise to long-range and tunable spin-spin interactions that can be reconfigured by shaping the spectrum and pattern of the laser fields. Here we review the theoretical mapping of atomic ions to interacting spin systems, the experimental preparation of complex equilibrium states, and the study of dynamical processes of this many-body interacting quantum system. The use of such quantum simulators for studying spin models may inform our understanding of exotic quantum materials and shed light on interacting quantum systems that cannot be modeled with conventional computers.\ 

}, url = {https://arxiv.org/abs/1912.07845}, author = {C. Monroe and W. C. Campbell and L. -M. Duan and Z. -X. Gong and Alexey V. Gorshkov and P. Hess and R. Islam and K. Kim and G. Pagano and P. Richerme and C. Senko and N. Y. Yao} } @article {2410, title = {Quantum Approximate Optimization with a Trapped-Ion Quantum Simulator}, year = {2019}, month = {06/06/2019}, abstract = {

Quantum computers and simulators may offer significant advantages over their classical counterparts, providing insights into quantum many-body systems and possibly solving exponentially hard problems, such as optimization and satisfiability. Here we report the first implementation of a shallow-depth Quantum Approximate Optimization Algorithm (QAOA) using an analog quantum simulator to estimate the ground state energy of the transverse field Ising model with tunable long-range interactions. First, we exhaustively search the variational control parameters to approximate the ground state energy with up to 40 trapped-ion qubits. We then interface the quantum simulator with a classical algorithm to more efficiently find the optimal set of parameters that minimizes the resulting energy of the system. We finally sample from the full probability distribution of the QAOA output with single-shot and efficient measurements of every qubit.\ 

}, url = {https://arxiv.org/abs/1906.02700}, author = {G. Pagano and A. Bapat and P. Becker and K. S. Collins and A. De and P. W. Hess and H. B. Kaplan and A. Kyprianidis and W. L. Tan and Christopher L. Baldwin and L. T. Brady and A. Deshpande and F. Liu and S. Jordan and Alexey V. Gorshkov and C. Monroe} } @article {2530, title = {Quantum Computer Systems for Scientific Discovery}, year = {2019}, month = {12/16/2019}, abstract = {

The great promise of quantum computers comes with the dual challenges of building them and finding their useful applications. We argue that these two challenges should be considered together, by co-designing full stack quantum computer systems along with their applications in order to hasten their development and potential for scientific discovery. In this context, we identify scientific and community needs, opportunities, and significant challenges for the development of quantum computers for science over the next 2-10 years. This document is written by a community of university, national laboratory, and industrial researchers in the field of Quantum Information Science and Technology, and is based on a summary from a U.S. National Science Foundation workshop on Quantum Computing held on October 21-22, 2019 in Alexandria, VA.

}, url = {https://arxiv.org/abs/1912.07577}, author = {Yuri Alexeev and Dave Bacon and Kenneth R. Brown and Robert Calderbank and Lincoln D. Carr and Frederic T. Chong and Brian DeMarco and Dirk Englund and Edward Farhi and Bill Fefferman and Alexey V. Gorshkov and Andrew Houck and Jungsang Kim and Shelby Kimmel and Michael Lange and Seth Lloyd and Mikhail D. Lukin and Dmitri Maslov and Peter Maunz and Christopher Monroe and John Preskill and Martin Roetteler and Martin Savage and Jeff Thompson and Umesh Vazirani} } @article {2532, title = {Quantum Simulators: Architectures and Opportunities}, year = {2019}, month = {12/14/2019}, abstract = {

Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operation worldwide using a wide variety of experimental platforms. Recent advances in several physical architectures promise a golden age of quantum simulators ranging from highly optimized special purpose simulators to flexible programmable devices. These developments have enabled a convergence of ideas drawn from fundamental physics, computer science, and device engineering. They have strong potential to address problems of societal importance, ranging from understanding vital chemical processes, to enabling the design of new materials with enhanced performance, to solving complex computational problems. It is the position of the community, as represented by participants of the NSF workshop on \"Programmable Quantum Simulators,\" that investment in a national quantum simulator program is a high priority in order to accelerate the progress in this field and to result in the first practical applications of quantum machines. Such a program should address two areas of emphasis: (1) support for creating quantum simulator prototypes usable by the broader scientific community, complementary to the present universal quantum computer effort in industry; and (2) support for fundamental research carried out by a blend of multi-investigator, multi-disciplinary collaborations with resources for quantum simulator software, hardware, and education.\ 

}, url = {https://arxiv.org/abs/1912.06938}, author = {Ehud Altman and Kenneth R. Brown and Giuseppe Carleo and Lincoln D. Carr and Eugene Demler and Cheng Chin and Brian DeMarco and Sophia E. Economou and Mark A. Eriksson and Kai-Mei C. Fu and Markus Greiner and Kaden R. A. Hazzard and Randall G. Hulet and Alicia J. Koll{\'a}r and Benjamin L. Lev and Mikhail D. Lukin and Ruichao Ma and Xiao Mi and Shashank Misra and Christopher Monroe and Kater Murch and Zaira Nazario and Kang-Kuen Ni and Andrew C. Potter and Pedram Roushan} } @article {2385, title = {ReQWIRE: Reasoning about Reversible Quantum Circuits}, journal = {EPTCS }, volume = {287}, year = {2019}, type = {In Proceedings QPL 2018, arXiv:1901.09476}, chapter = {299-312}, abstract = {

Common quantum algorithms make heavy use of ancillae: scratch qubits that are initialized at some state and later returned to that state and discarded. Existing quantum circuit languages let programmers assert that a qubit has been returned to the |0\> state before it is discarded, allowing for a range of optimizations. However, existing languages do not provide the tools to verify these assertions, introducing a potential source of errors. In this paper we present methods for verifying that ancillae are discarded in the desired state, and use these methods to implement a verified compiler from classical functions to quantum oracles.

}, doi = {https://doi.org/10.4204/EPTCS.287.17}, url = {https://arxiv.org/abs/1901.10118}, author = {Robert Rand and Jennifer Paykin and Dong-Ho Lee and Steve Zdancewic} } @article {2624, title = {Status Report on the First Round of the NIST Post-Quantum Cryptography Standardization Process}, journal = {School: National Institute for Standards and Technology }, year = {2019}, type = {techreport}, abstract = {

The National Institute of Standards and Technology is in the process of selecting one or more
public-key cryptographic algorithms through a public competition-like process. The new publickey cryptography standards will specify one or more additional digital signature, public-key
encryption, and key-establishment algorithms to augment FIPS 186-4, Digital Signature Standard
(DSS), as well as special publications SP 800-56A Revision 2, Recommendation for Pair-Wise
Key Establishment Schemes Using Discrete Logarithm Cryptography, and SP 800-56B,
Recommendation for Pair-Wise Key-Establishment Schemes Using Integer Factorization. It is
intended that these algorithms will be capable of protecting sensitive information well into the
foreseeable future, including after the advent of quantum computers.
In November 2017, 82 candidate algorithms were submitted to NIST for consideration. Among
these, 69 met both the minimum acceptance criteria and our submission requirements, and were
accepted as First-Round Candidates on Dec. 20, 2017, marking the beginning of the First Round
of the NIST Post-Quantum Cryptography Standardization Process. This report describes the
evaluation criteria and selection process, based on public feedback and internal review of the
first-round candidates, and summarizes the 26 candidate algorithms announced on January 30,
2019 for moving forward to the second round of the competition. The 17 Second-Round
Candidate public-key encryption and key-establishment algorithms are BIKE, Classic McEliece,
CRYSTALS-KYBER, FrodoKEM, HQC, LAC, LEDAcrypt (merger of LEDAkem/LEDApkc),
NewHope, NTRU (merger of NTRUEncrypt/NTRU-HRSS-KEM), NTRU Prime, NTS-KEM,
ROLLO (merger of LAKE/LOCKER/Ouroboros-R), Round5 (merger of Hila5/Round2), RQC,
SABER, SIKE, and Three Bears. The 9 Second-Round Candidates for digital signatures are
CRYSTALS-DILITHIUM, FALCON, GeMSS, LUOV, MQDSS, Picnic, qTESLA, Rainbow,
and SPHINCS+.

}, url = {https://nvlpubs.nist.gov/nistpubs/ir/2019/NIST.IR.8240.pdf}, author = {Gorjan Alagic and J. Alperin-Sheriff and D. Apon and D. Cooper and Q. Dang and Carl Miller and D. Moody and R. Peralta and R. Perlner and A. Robinson and D. Smith-Tone and Yi-Kai Liu} } @article {2392, title = {Toward convergence of effective field theory simulations on digital quantum computers}, year = {2019}, month = {04/18/2019}, abstract = {

We report results for simulating an effective field theory to compute the binding energy of the deuteron nucleus using a hybrid algorithm on a trapped-ion quantum computer. Two increasingly complex unitary coupled-cluster ansaetze have been used to compute the binding energy to within a few percent for successively more complex Hamiltonians. By increasing the complexity of the Hamiltonian, allowing more terms in the effective field theory expansion and calculating their expectation values, we present a benchmark for quantum computers based on their ability to scalably calculate the effective field theory with increasing accuracy. Our result of E4=\−2.220\±0.179MeV may be compared with the exact Deuteron ground-state energy \−2.224MeV. We also demonstrate an error mitigation technique using Richardson extrapolation on ion traps for the first time. The error mitigation circuit represents a record for deepest quantum circuit on a trapped-ion quantum computer.\ 

}, url = {https://arxiv.org/abs/1904.04338}, author = {Omar Shehab and Kevin A. Landsman and Yunseong Nam and Daiwei Zhu and Norbert M. Linke and Matthew J. Keesan and Raphael C. Pooser and Christopher R. Monroe} } @article {2133, title = {Bell monogamy relations in arbitrary qubit networks}, year = {2018}, month = {2018/01/09}, abstract = {

Characterizing trade-offs between simultaneous violations of multiple Bell inequalities in a large network of qubits is computationally demanding. We propose a graph-theoretic approach to efficiently produce Bell monogamy relations in arbitrary arrangements of qubits. All the relations obtained for bipartite Bell inequalities are tight and leverage only a single Bell monogamy relation. This feature is unique to bipartite Bell inequalities, as we show that there is no finite set of such elementary monogamy relations for multipartite inequalities. Nevertheless, many tight monogamy relations for multipartite inequalities can be obtained with our method as shown in explicit examples.

}, doi = {https://doi.org/10.1103/PhysRevA.98.052325}, url = {https://arxiv.org/abs/1801.03071}, author = {Minh C. Tran and Ravishankar Ramanathan and Matthew McKague and Dagomir Kaszlikowski and Tomasz Paterek} } @article {2269, title = {Bose Condensation of Photons Thermalized via Laser Cooling of Atoms}, year = {2018}, abstract = {

A Bose-Einstein condensate (BEC) is a quantum phase of matter achieved at low temperatures. Photons, one of the most prominent species of bosons, do not typically condense due to the lack of a particle number-conservation. We recently described a photon thermalization mechanism which gives rise to a grand canonical ensemble of light with effective photon number conservation between a subsystem and a particle reservoir. This mechanism occurs during Doppler laser cooling of atoms where the atoms serve as a temperature reservoir while the cooling laser photons serve as a particle reservoir. Here we address the question of the possibility of a BEC of photons in this laser cooling photon thermalization scenario and theoretically demonstrate that a Bose condensation of photons can be realized by cooling an ensemble of two-level atoms (realizable with alkaline earth atoms) inside a Fabry-Perot cavity.

}, url = {https://arxiv.org/abs/1809.07777}, author = {Chiao-Hsuan Wang and Michael Gullans and J. V. Porto and William D. Phillips and J. M. Taylor} } @article {1949, title = {BQP-completeness of Scattering in Scalar Quantum Field Theory}, journal = {Quantum}, volume = {2}, year = {2018}, month = {2018/01/08}, pages = {44}, abstract = {

Recent work has shown that quantum computers can compute scattering probabilities in massive quantum field theories, with a run time that is polynomial in the number of particles, their energy, and the desired precision. Here we study a closely related quantum field-theoretical problem: estimating the vacuum-to-vacuum transition amplitude, in the presence of spacetime-dependent classical sources, for a massive scalar field theory in (1+1) dimensions. We show that this problem is BQP-hard; in other words, its solution enables one to solve any problem that is solvable in polynomial time by a quantum computer. Hence, the vacuum-to-vacuum amplitude cannot be accurately estimated by any efficient classical algorithm, even if the field theory is very weakly coupled, unless BQP=BPP. Furthermore, the corresponding decision problem can be solved by a quantum computer in a time scaling polynomially with the number of bits needed to specify the classical source fields, and this problem is therefore BQP-complete. Our construction can be regarded as an idealized architecture for a universal quantum computer in a laboratory system described by massive phi^4 theory coupled to classical spacetime-dependent sources.

}, doi = {10.22331/q-2018-01-08-44}, url = {https://quantum-journal.org/papers/q-2018-01-08-44/}, author = {Stephen P. Jordan and Hari Krovi and Keith S. M. Lee and John Preskill} } @article {2216, title = {Coherent optical nano-tweezers for ultra-cold atoms}, year = {2018}, abstract = {

There has been a recent surge of interest and progress in creating subwavelength free-space optical potentials for ultra-cold atoms. A key open question is whether geometric potentials, which are repulsive and ubiquitous in the creation of subwavelength free-space potentials, forbid the creation of narrow traps with long lifetimes. Here, we show that it is possible to create such traps. We propose two schemes for realizing subwavelength traps and demonstrate their superiority over existing proposals. We analyze the lifetime of atoms in such traps and show that long-lived bound states are possible. This work opens a new frontier for the subwavelength control and manipulation of ultracold matter, with applications in quantum chemistry and quantum simulation.

}, url = {https://arxiv.org/abs/1808.02487}, author = {P. Bienias and S. Subhankar and Y. Wang and T-C Tsui and F. Jendrzejewski and T. Tiecke and G. Juzeliunas and L. Jiang and S. L. Rolston and J. V. Porto and Alexey V. Gorshkov} } @article {2283, title = {A Coherent Spin-Photon Interface in Silicon}, journal = {Nature }, volume = {555}, year = {2018}, month = {2018/03/29}, pages = {599-603}, abstract = {

Electron spins in silicon quantum dots are attractive systems for quantum computing due to their long coherence times and the promise of rapid scaling using semiconductor fabrication techniques. While nearest neighbor exchange coupling of two spins has been demonstrated, the interaction of spins via microwave frequency photons could enable long distance spin-spin coupling and \"all-to-all\" qubit connectivity. Here we demonstrate strong-coupling between a single spin in silicon and a microwave frequency photon with spin-photon coupling rates g_s/(2π) \> 10 MHz. The mechanism enabling coherent spin-photon interactions is based on spin-charge hybridization in the presence of a magnetic field gradient. In addition to spin-photon coupling, we demonstrate coherent control of a single spin in the device and quantum non-demolition spin state readout using cavity photons. These results open a direct path toward entangling single spins using microwave frequency photons.

}, doi = {https://doi.org/10.1038/nature25769}, url = {https://arxiv.org/abs/1710.03265}, author = {X. Mi and M. Benito and S. Putz and D. M. Zajac and J. M. Taylor and Guido Burkard and J. R. Petta} } @article {2148, title = {A coherent spin{\textendash}photon interface in silicon}, journal = {Nature}, year = {2018}, month = {2018/02/14}, abstract = {

Electron spins in silicon quantum dots are attractive systems for quantum computing owing to their long coherence times and the promise of rapid scaling of the number of dots in a system using semiconductor fabrication techniques. Although nearest-neighbour exchange coupling of two spins has been demonstrated, the interaction of spins via microwave-frequency photons could enable long-distance spin\–spin coupling and connections between arbitrary pairs of qubits (\‘all-to-all\’ connectivity) in a spin-based quantum processor. Realizing coherent spin\–photon coupling is challenging because of the small magnetic-dipole moment of a single spin, which limits magnetic-dipole coupling rates to less than 1 kilohertz. Here we demonstrate strong coupling between a single spin in silicon and a single microwave-frequency photon, with spin\–photon coupling rates of more than 10 megahertz. The mechanism that enables the coherent spin\–photon interactions is based on spin\–charge hybridization in the presence of a magnetic-field gradient. In addition to spin\–photon coupling, we demonstrate coherent control and dispersive readout of a single spin. These results open up a direct path to entangling single spins using microwave-frequency photons.

}, doi = {10.1038/nature25769}, url = {https://www.nature.com/articles/nature25769$\#$author-information}, author = {X. Mi and M. Benito and S. Putz and D. M. Zajac and J. M. Taylor and Guido Burkard and J. R. Petta} } @article {2289, title = {Cryogenic Trapped-Ion System for Large Scale Quantum Simulation}, year = {2018}, abstract = {

We present a cryogenic ion trapping system designed for large scale quantum simulation of spin models. Our apparatus is based on a segmented-blade ion trap enclosed in a 4 K cryostat, which enables us to routinely trap over 100 171Yb+ ions in a linear configuration for hours due to a low background gas pressure from differential cryo-pumping. We characterize the cryogenic vacuum by using trapped ion crystals as a pressure gauge, measuring both inelastic and elastic collision rates with the molecular background gas. We demonstrate nearly equidistant ion spacing for chains of up to 44 ions using anharmonic axial potentials. This reliable production and lifetime enhancement of large linear ion chains will enable quantum simulation of spin models that are intractable with classical computer modelling.

}, url = {https://arxiv.org/abs/1802.03118}, author = {G. Pagano and P. W. Hess and H. B. Kaplan and W. L. Tan and P. Richerme and P. Becker and A. Kyprianidis and J. Zhang and E. Birckelbaw and M. R. Hernandez and Y. Wu and C. Monroe} } @article {2141, title = {Dark state optical lattice with sub-wavelength spatial structure}, journal = {Phys. Rev. Lett.}, volume = {120}, year = {2018}, month = {2018/02/20}, pages = {083601}, abstract = {

We report on the experimental realization of a conservative optical lattice for cold atoms with a subwavelength spatial structure. The potential is based on the nonlinear optical response of three-level atoms in laser-dressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a one-dimensional array of ultranarrow barriers with widths less than 10\ nm, well below the wavelength of the lattice light, physically realizing a Kronig-Penney potential. We study the band structure and dissipation of this lattice and find good agreement with theoretical predictions. Even on resonance, the observed lifetimes of atoms trapped in the lattice are as long as 44\ ms, nearly\ 105times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimensions and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.

}, doi = {10.1103/PhysRevLett.120.083601}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.120.083601}, author = {Yang Wang and Sarthak Subhankar and Przemyslaw Bienias and Mateusz Lacki and Tsz-Chun Tsui and Mikhail A. Baranov and Alexey V. Gorshkov and Peter Zoller and James V. Porto and Steven L. Rolston} } @article {2142, title = {Dissipation induced dipole blockade and anti-blockade in driven Rydberg systems}, journal = {Phys. Rev. A}, volume = {97}, year = {2018}, month = {2018/02/28}, pages = {023424}, abstract = {

We study theoretically and experimentally the competing blockade and antiblockade effects induced by spontaneously generated contaminant Rydberg atoms in driven Rydberg systems. These contaminant atoms provide a source of strong dipole-dipole interactions and play a crucial role in the system\&$\#$39;s behavior. We study this problem theoretically using two different approaches. The first is a cumulant expansion approximation, in which we ignore third-order and higher connected correlations. Using this approach for the case of resonant drive, a many-body blockade radius picture arises, and we find qualitative agreement with previous experimental results. We further predict that as the atomic density is increased, the Rydberg population\&$\#$39;s dependence on Rabi frequency will transition from quadratic to linear dependence at lower Rabi frequencies. We study this behavior experimentally by observing this crossover at two different atomic densities. We confirm that the larger density system has a smaller crossover Rabi frequency than the smaller density system. The second theoretical approach is a set of phenomenological inhomogeneous rate equations. We compare the results of our rate-equation model to the experimental observations [E. A. Goldschmidt\ et al.,\ Phys. Rev. Lett.116, 113001 (2016)] and find that these rate equations provide quantitatively good scaling behavior of the steady-state Rydberg population for both resonant and off-resonant drives.

}, doi = {10.1103/PhysRevA.97.023424}, url = {https://link.aps.org/doi/10.1103/PhysRevA.97.023424}, author = {Jeremy T. Young and Thomas Boulier and Eric Magnan and Elizabeth A. Goldschmidt and Ryan M. Wilson and Steven L. Rolston and James V. Porto and Alexey V. Gorshkov} } @article {2147, title = {High-fidelity quantum gates in Si/SiGe double quantum dots}, journal = {Physical Review B}, volume = {97}, year = {2018}, month = {2018/02/15}, pages = {085421}, abstract = {

Motivated by recent experiments of Zajac\ et\ al.\ [Science359, 439 (2018)], we theoretically describe high-fidelity two-qubit gates using the exchange interaction between the spins in neighboring quantum dots subject to a magnetic field gradient. We use a combination of analytical calculations and numerical simulations to provide the optimal pulse sequences and parameter settings for the gate operation. We present a synchronization method which avoids detrimental spin flips during the gate operation and provide details about phase mismatches accumulated during the two-qubit gates which occur due to residual exchange interaction, nonadiabatic pulses, and off-resonant driving. By adjusting the gate times, synchronizing the resonant and off-resonant transitions, and compensating these phase mismatches by phase control, the overall gate fidelity can be increased significantly.

}, doi = {10.1103/PhysRevB.97.085421}, url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.085421}, author = {Maximilian Russ and D. M. Zajac and A. J. Sigillito and F. Borjans and J. M. Taylor and J. R. Petta and Guido Burkard} } @article {2214, title = {More is Less: Perfectly Secure Oblivious Algorithms in the Multi-Server Setting}, year = {2018}, abstract = {

The problem of Oblivious RAM (ORAM) has traditionally been studied in a single-server setting, but more recently the multi-server setting has also been considered. Yet it is still unclear whether the multi-server setting has any inherent advantages, e.g., whether the multi-server setting can be used to achieve stronger security goals or provably better efficiency than is possible in the single-server case. In this work, we construct a perfectly secure 3-server ORAM scheme that outperforms the best known single-server scheme by a logarithmic factor. In the process, we also show, for the first time, that there exist specific algorithms for which multiple servers can overcome known lower bounds in the single-server setting.\ 

}, url = {https://arxiv.org/abs/1809.00825}, author = {Hubert Chan and Jonathan Katz and Kartik Nayak and Antigoni Polychroniadou and Elaine Shi} } @article {2209, title = {Optimization of photon storage fidelity in ordered atomic arrays}, journal = {New Journal of Physics}, volume = {20}, year = {2018}, month = {2018/08/31}, abstract = {

A major application for atomic ensembles consists of a quantum memory for light, in which an optical state can be reversibly converted to a collective atomic excitation on demand. There exists a well-known fundamental bound on the storage error, when the ensemble is describable by a continuous medium governed by the Maxwell-Bloch equations. The validity of this model can break down, however, in systems such as dense, ordered atomic arrays, where strong interference in emission can give rise to phenomena such as subradiance and \"selective\" radiance. Here, we develop a general formalism that finds the maximum storage efficiency for a collection of atoms with discrete, known positions, and a given spatial mode in which an optical field is sent. As an example, we apply this technique to study a finite two-dimensional square array of atoms. We show that such a system enables a storage error that scales with atom number Na like \∼(logNa)2/N2a, and that, remarkably, an array of just 4\×4 atoms in principle allows for an efficiency comparable to a disordered ensemble with optical depth of around 600.

}, doi = {https://doi.org/10.1088/1367-2630/aadb74}, url = {https://arxiv.org/abs/1710.06312}, author = {M. T. Manzoni and M. Moreno-Cardoner and A. Asenjo-Garcia and J. V. Porto and Alexey V. Gorshkov and D. E. Chang} } @article {2218, title = {Photon propagation through dissipative Rydberg media at large input rates}, year = {2018}, abstract = {

We study the dissipative propagation of quantized light in interacting Rydberg media under the conditions of electromagnetically induced transparency (EIT). Rydberg blockade physics in optically dense atomic media leads to strong dissipative interactions between single photons. The regime of high incoming photon flux constitutes a challenging many-body dissipative problem. We experimentally study in detail for the first time the pulse shapes and the second-order correlation function of the outgoing field and compare our data with simulations based on two novel theoretical approaches well-suited to treat this many-photon limit. At low incoming flux, we report good agreement between both theories and the experiment. For higher input flux, the intensity of the outgoing light is lower than that obtained from theoretical predictions. We explain this discrepancy using a simple phenomenological model taking into account pollutants, which are nearly-stationary Rydberg excitations coming from the reabsorption of scattered probe photons. At high incoming photon rates, the blockade physics results in unconventional shapes of measured correlation functions.\ 

}, url = {https://arxiv.org/abs/1807.07586}, author = {Przemyslaw Bienias and James Douglas and Asaf Paris-Mandoki and Paraj Titum and Ivan Mirgorodskiy and Christoph Tresp and Emil Zeuthen and Michael Gullans and Marco Manzoni and Sebastian Hofferberth and Darrick Chang and Alexey V. Gorshkov} } @article {2144, title = {Photon Subtraction by Many-Body Decoherence}, year = {2018}, month = {2018/03/13}, abstract = {

We present an experimental and theoretical investigation of the scattering-induced decoherence of multiple photons stored in a strongly interacting atomic ensemble. We derive an exact solution to this many-body problem, allowing for a rigorous understanding of the underlying dissipative quantum dynamics. Combined with our experiments, this analysis demonstrates a correlated coherence-protection process, in which the induced decoherence of one photon can preserve the spatial coherence of all others. We discuss how this effect can be used to manipulate light at the quantum level, providing a robust mechanism for single-photon subtraction, and experimentally demonstrate this capability.

}, doi = {https://doi.org/10.1103/PhysRevLett.120.113601}, url = {https://arxiv.org/abs/1710.10047}, author = {Callum R. Murray and Ivan Mirgorodskiy and Christoph Tresp and Christoph Braun and Asaf Paris-Mandoki and Alexey V. Gorshkov and Sebastian Hofferberth and Thomas Pohl} } @article {2135, title = {Photon thermalization via laser cooling of atoms}, journal = {Phys. Rev. A 98, 013834}, year = {2018}, month = {2018}, abstract = {

Laser cooling of atomic motion enables a wide variety of technological and scientific explorations using cold atoms. Here we focus on the effect of laser cooling on the photons instead of on the atoms. Specifically, we show that non-interacting photons can thermalize with the atoms to a grand canonical ensemble with a non-zero chemical potential. This thermalization is accomplished via scattering of light between different optical modes, mediated by the laser cooling process. While optically thin modes lead to traditional laser cooling of the atoms, the dynamics of multiple scattering in optically thick modes has been more challenging to describe. We find that in an appropriate set of limits, multiple scattering leads to thermalization of the light with the atomic motion in a manner that approximately conserves total photon number between the laser beams and optically thick modes. In this regime, the subsystem corresponding to the thermalized modes is describable by a grand canonical ensemble with a chemical potential set by the energy of a single laser photon. We consider realization of this regime using two-level atoms in Doppler cooling, and find physically realistic conditions for rare earth atoms. With the addition of photon-photon interactions, this system could provide a new platform for exploring many-body physics.

}, doi = {https://doi.org/10.1103/PhysRevA.98.013834}, url = {https://arxiv.org/abs/1712.08643}, author = {Chiao-Hsuan Wang and Michael Gullans and J. V. Porto and William D. Phillips and J. M. Taylor} } @article {2146, title = {Probing electron-phonon interactions in the charge-photon dynamics of cavity-coupled double quantum dots}, journal = {Physical Review B}, volume = {97}, year = {2018}, month = {2018/01/16}, pages = {035305}, abstract = {

Electron-phonon coupling is known to play an important role in the charge dynamics of semiconductor quantum dots. Here we explore its role in the combined charge-photon dynamics of cavity-coupled double quantum dots. Previous work on these systems has shown that strong electron-phonon coupling leads to a large contribution to photoemission and gain from phonon-assisted emission and absorption processes. We compare the effects of this phonon sideband in three commonly investigated gate-defined quantum dot material systems: InAs nanowires and GaAs and Si two-dimensional electron gases (2DEGs). We compare our theory with existing experimental data from cavity-coupled InAs nanowire and GaAs 2DEG double quantum dots and find quantitative agreement only when the phonon sideband and photoemission processes during lead tunneling are taken into account. Finally, we show that the phonon sideband also leads to a sizable renormalization of the cavity frequency, which allows for direct spectroscopic probes of the electron-phonon coupling in these systems.

}, doi = {10.1103/PhysRevB.97.035305}, url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.035305}, author = {Michael Gullans and J. M. Taylor and J. R. Petta} } @article {2150, title = {Resonantly driven CNOT gate for electron spins}, journal = {Science}, volume = {359}, year = {2018}, month = {2018/01/26}, pages = {439-442}, abstract = {

Single-qubit rotations and two-qubit CNOT operations are crucial ingredients for universal quantum computing. Although high-fidelity single-qubit operations have been achieved using the electron spin degree of freedom, realizing a robust CNOT gate has been challenging because of rapid nuclear spin dephasing and charge noise. We demonstrate an efficient resonantly driven CNOT gate for electron spins in silicon. Our platform achieves single-qubit rotations with fidelities greater than 99\%, as verified by randomized benchmarking. Gate control of the exchange coupling allows a quantum CNOT gate to be implemented with resonant driving in ~200 nanoseconds. We used the CNOT gate to generate a Bell state with 78\% fidelity (corrected for errors in state preparation and measurement). Our quantum dot device architecture enables multi-qubit algorithms in silicon.

}, doi = {10.1126/science.aao5965}, url = {http://science.sciencemag.org/content/359/6374/439}, author = {D. M. Zajac and A. J. Sigillito and M. Russ and F. Borjans and J. M. Taylor and Guido Burkard and J. R. Petta} } @article {2213, title = {Single-photon bound states in atomic ensembles}, year = {2018}, abstract = {

We illustrate the existence of single-excitation bound states for propagating photons interacting with N two-level atoms. These bound states can be calculated from an effective spin model, and their existence relies on dissipation in the system. The appearance of these bound states is in a one-to-one correspondence with zeros in the single-photon transmission and with divergent bunching in the second-order photon-photon correlation function. We also formulate a dissipative version of Levinson\&$\#$39;s theorem for this system by looking at the relation between the number of bound states and the winding number of the transmission phases. This theorem allows a direct experimental measurement of the number of bound states using the measured transmission phases.

}, url = {https://arxiv.org/abs/1809.01147}, author = {Yidan Wang and Michael Gullans and Antoine Browaeys and J. V. Porto and Darrick E. Chang and Alexey V. Gorshkov} } @article {2205, title = {A spinor Bose-Einstein condensate phase-sensitive amplifier for SU(1,1) interferometry}, journal = {Phys. Rev}, volume = {A 98}, year = {2018}, abstract = {

The SU(1,1) interferometer was originally conceived as a Mach-Zehnder interferometer with the beam-splitters replaced by parametric amplifiers. The parametric amplifiers produce states with correlations that result in enhanced phase sensitivity. F=1 spinor Bose-Einstein condensates (BECs) can serve as the parametric amplifiers for an atomic version of such an interferometer by collisionally producing entangled pairs of \⟨F=1,m=\±1| atoms. We simulate the effect of single and double-sided seeding of the inputs to the amplifier using the truncated-Wigner approximation. We find that single-sided seeding degrades the performance of the interferometer exactly at the phase the unseeded interferometer should operate the best. Double-sided seeding results in a phase-sensitive amplifier, where the maximal sensitivity is a function of the phase relationship between the input states of the amplifier. In both single and double-sided seeding we find there exists an optimal phase shift that achieves sensitivity beyond the standard quantum limit. Experimentally, we demonstrate a spinor phase-sensitive amplifier using a BEC of 23Na in an optical dipole trap. This configuration could be used as an input to such an interferometer. We are able to control the initial phase of the double-seeded amplifier, and demonstrate sensitivity to initial population fractions as small as 0.1\%.\ 

}, url = {https://arxiv.org/abs/1807.06676}, author = {J. P. Wrubel and A. Schwettmann and D. P. Fahey and Z. Glassman and H. K. Pechkis and P. F. Griffin and R. Barnett and E. Tiesinga and P. D. Lett} } @article {2322, title = {Study of radon reduction in gases for rare event search experiments}, year = {2018}, abstract = {

The noble elements, argon and xenon, are frequently employed as the target and event detector for weakly interacting particles such as neutrinos and Dark Matter. For such rare processes, background radiation must be carefully minimized. Radon provides one of the most significant contaminants since it is an inevitable product of trace amounts of natural uranium. To design a purification system for reducing such contamination, the adsorption characteristics of radon in nitrogen, argon, and xenon carrier gases on various types of charcoals with different adsorbing properties and intrinsic radioactive purities have been studied in the temperature range of 190-295 K at flow rates of 0.5 and 2 standard liters per minute. Essential performance parameters for the various charcoals include the average breakthrough times (τ), dynamic adsorption coefficients (ka) and the number of theoretical stages (n). It is shown that the ka-values for radon in nitrogen, argon, and xenon increase as the temperature of the charcoal traps decreases, and that they are significantly larger in nitrogen and argon than in xenon gas due to adsorption saturation effects. It is found that, unlike in xenon, the dynamic adsorption coefficients for radon in nitrogen and argon strictly obey the Arrhenius law. The experimental results strongly indicate that nitric acid etched Saratech is the best candidate among all used charcoal brands. It allows reducing total radon concentration in the LZ liquid Xe detector to meet the ultimate goal in the search for Dark Matter.

}, doi = {https://doi.org/10.1016/j.nima.2018.06.076}, url = {https://arxiv.org/abs/1805.11306}, author = {K. Pushkin and C. Akerlof and D. Anbajagane and J. Armstrong and M. Arthurs and Jacob Bringewatt and T. Edberg and C. Hall and M. Lei and R. Raymond and M. Reh and D. Saini and A. Sander and J. Schaefer and D. Seymour and N. Swanson and Y. Wang and W. Lorenzon} } @article {2320, title = {Studying community development: a network analytical approach}, year = {2018}, abstract = {

Research shows that community plays a central role in learning, and strong community engages students and aids in student persistence. Thus, understanding the function and structure of communities in learning environments is essential to education. We use social network analysis to explore the community dynamics of students in a pre-matriculation, two-week summer program. Unlike previous network analysis studies in PER, we build our networks from classroom video that has been coded for student interactions using labeled, directed ties. We define 3 types of interaction: on task interactions (regarding the assigned task), on topic interactions (having to do with science, technology, engineering, and mathematics (STEM)), and off topic interactions (unrelated to the assignment or STEM). To study the development of community in this program, we analyze the shift in conversation topicality over the course of the program. Conversations are more on-task toward the end of the program and we propose that this conversational shift represents a change in student membership within their forming community.\ 

}, url = {https://arxiv.org/abs/1808.08193}, author = {C. A. Hass and Florian Genz and Mary Bridget Kustusch and Pierre-P. A. Ouime and Katarzyna Pomian and Eleanor C. Sayre and Justyna P. Zwolak} } @article {1820, title = {Cooling a harmonic oscillator by optomechanical modification of its bath}, journal = {Physical Review Letters}, volume = {118}, year = {2017}, month = {2017/05/31}, pages = {223602}, abstract = {

Optomechanical systems show tremendous promise for high sensitivity sensing of forces and modification of mechanical properties via light. For example, similar to neutral atoms and trapped ions, laser cooling of mechanical motion by radiation pressure can take single mechanical modes to their ground state. Conventional optomechanical cooling is able to introduce additional damping channel to mechanical motion, while keeping its thermal noise at the same level, and as a consequence, the effective temperature of the mechanical mode is lowered. However, the ratio of temperature to quality factor remains roughly constant, preventing dramatic advances in quantum sensing using this approach. Here we propose an approach for simultaneously reducing the thermal load on a mechanical resonator while improving its quality factor. In essence, we use the optical interaction to dynamically modify the dominant damping mechanism, providing an optomechanically-induced effect analogous to a phononic band gap. The mechanical mode of interest is assumed to be weakly coupled to its heat bath but strongly coupled to a second mechanical mode, which is cooled by radiation pressure coupling to a red detuned cavity field. We also identify a realistic optomechanical design that has the potential to realize this novel cooling scheme.

}, doi = {doi.org/10.1103/PhysRevLett.118.223602}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.223602}, author = {Xunnong Xu and Thomas Purdy and J. M. Taylor} } @article {2301, title = {Entanglement Wedge Reconstruction via Universal Recovery Channels}, year = {2017}, abstract = {

We apply and extend the theory of universal recovery channels from quantum information theory to address the problem of entanglement wedge reconstruction in AdS/CFT. It has recently been proposed that any low-energy local bulk operators in a CFT boundary region\&$\#$39;s entanglement wedge can be reconstructed on that boundary region itself. Existing work arguing for this proposal relies on algebraic consequences of the exact equivalence between bulk and boundary relative entropies, namely the theory of operator algebra quantum error correction. However, bulk and boundary relative entropies are only approximately equal in bulk effective field theory, and in similar situations it is known that predictions from exact entropic equalities can be qualitatively incorrect. The framework of universal recovery channels provides a robust demonstration of the entanglement wedge reconstruction conjecture in addition to new physical insights. Most notably, we find that a bulk operator acting in a given boundary region\&$\#$39;s entanglement wedge can be expressed as the response of the boundary region\&$\#$39;s modular Hamiltonian to a perturbation of the bulk state in the direction of the bulk operator. This formula can be interpreted as a noncommutative version of Bayes\&$\#$39; rule that attempts to undo the noise induced by restricting to only a portion of the boundary, and has an integral representation in terms of modular flows. To reach these conclusions, we extend the theory of universal recovery channels to finite-dimensional operator algebras and demonstrate that recovery channels approximately preserve the multiplicative structure of the operator algebra

}, url = {https://arxiv.org/abs/1704.05839}, author = {Jordan Cotler and Patrick Hayden and Geoffrey Penington and Grant Salton and Brian Swingle and Michael Walter} } @article {1990, title = {Genuine N -partite entanglement without N -partite correlation functions}, journal = {Physical Review A}, volume = {95}, year = {2017}, month = {2017/06/26}, pages = {062331}, abstract = {

A genuinely\ N-partite entangled state may display vanishing\ N-partite correlations measured for arbitrary local observables. In such states the genuine entanglement is noticeable solely in correlations between subsets of particles. A straightforward way to obtain such states for odd\ N\ is to design an \“antistate\” in which all correlations between an odd number of observers are exactly opposite. Evenly mixing a state with its antistate then produces a mixed state with no\ N-partite correlations, with many of them genuinely multiparty entangled. Intriguingly, all known examples of \“entanglement without correlations\” involve an\ odd\ number of particles. Here we further develop the idea of antistates, thereby shedding light on the different properties of even and odd particle systems. We conjecture that there is no antistate to any pure even-N-party entangled state making the simple construction scheme unfeasible. However, as we prove by construction, higher-rank examples of entanglement without correlations for arbitrary even\ N\ indeed exist. These classes of states exhibit genuine entanglement and even violate an\ N-partite Bell inequality, clearly demonstrating the nonclassical features of these states as well as showing their applicability for quantum information processing.

}, doi = {doi.org/10.1103/PhysRevA.95.062331}, url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.95.062331}, author = {Minh C. Tran and Margherita Zuppardo and Anna de Rosier and Lukas Knips and Wieslaw Laskowski and Tomasz Paterek and Harald Weinfurter} } @article {2149, title = {Input-output theory for spin-photon coupling in Si double quantum dots}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/12/22}, pages = {235434}, abstract = {

The interaction of qubits via microwave frequency photons enables long-distance qubit-qubit coupling and facilitates the realization of a large-scale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this theoretical work we show that a sizable coupling for a single electron spin is possible via spin-charge hybridization using a magnetic field gradient in a silicon double quantum dot. Based on parameters already shown in recent experiments, we predict optimal working points to achieve a coherent spin-photon coupling, an essential ingredient for the generation of long-range entanglement. Furthermore, we employ input-output theory to identify observable signatures of spin-photon coupling in the cavity output field, which may provide guidance to the experimental search for strong coupling in such spin-photon systems and opens the way to cavity-based readout of the spin qubit.

}, doi = {10.1103/PhysRevB.96.235434}, url = {https://link.aps.org/doi/10.1103/PhysRevB.96.235434}, author = {Benito, M. and Mi, X. and J. M. Taylor and Petta, J. R. and Burkard, Guido} } @article {2053, title = {Observation of a Many-Body Dynamical Phase Transition with a 53-Qubit Quantum Simulator}, journal = {Nature}, volume = {551}, year = {2017}, month = {2017/11/29}, pages = {601-604}, abstract = {

A quantum simulator is a restricted class of quantum computer that controls the interactions between quantum bits in a way that can be mapped to certain difficult quantum many-body problems. As more control is exerted over larger numbers of qubits, the simulator can tackle a wider range of problems, with the ultimate limit being a universal quantum computer that can solve general classes of hard problems. We use a quantum simulator composed of up to 53 qubits to study a non-equilibrium phase transition in the transverse field Ising model of magnetism, in a regime where conventional statistical mechanics does not apply. The qubits are represented by trapped ion spins that can be prepared in a variety of initial pure states. We apply a global long-range Ising interaction with controllable strength and range, and measure each individual qubit with near 99\% efficiency. This allows the single-shot measurement of arbitrary many-body correlations for the direct probing of the dynamical phase transition and the uncovering of computationally intractable features that rely on the long-range interactions and high connectivity between the qubits.

}, doi = {10.1038/nature24654}, url = {https://www.nature.com/articles/nature24654}, author = {J. Zhang and G. Pagano and P. W. Hess and A. Kyprianidis and P. Becker and H. Kaplan and Alexey V. Gorshkov and Z. -X. Gong and C. Monroe} } @article {1958, title = {Pendular trapping conditions for ultracold polar molecules enforced by external electric fields}, journal = {Physical Review A}, volume = {95}, year = {2017}, month = {2017/06/26}, pages = {063422}, abstract = {

We theoretically investigate trapping conditions for ultracold polar molecules in optical lattices, when external magnetic and electric fields are simultaneously applied. Our results are based on an accurate electronic-structure calculation of the polar\ 23Na40K polar molecule in its absolute ground state combined with a calculation of its rovibrational-hyperfine motion. We find that an electric field strength of\ 5.26(15)\ kV/cm and an angle of\ 54.7o\ between this field and the polarization of the optical laser lead to a trapping design for\ 23Na40K molecules where decoherences due laser-intensity fluctuations and fluctuations in the direction of its polarization are kept to a minimum. One standard deviation systematic and statistical uncertainties are given in parenthesis. Under such conditions pairs of hyperfine-rotational states of\ v=0\ molecules, used to induce tunable dipole-dipole interactions between them, experience ultrastable, matching trapping forces.

}, doi = {10.1103/PhysRevA.95.063422}, url = {https://arxiv.org/abs/1703.03839}, author = {Ming Li and Alexander Petrov and Constantinos Makrides and Eite Tiesinga and Svetlanta Kotochigova} } @article {2107, title = {Provable quantum state tomography via non-convex methods}, year = {2017}, month = {2017/11/19}, abstract = {

With nowadays steadily growing quantum processors, it is required to develop new quantum tomography tools that are tailored for high-dimensional systems. In this work, we describe such a computational tool, based on recent ideas from non-convex optimization. The algorithm excels in the compressed-sensing-like setting, where only a few data points are measured from a lowrank or highly-pure quantum state of a high-dimensional system. We show that the algorithm can practically be used in quantum tomography problems that are beyond the reach of convex solvers, and, moreover, is faster than other state-of-the-art non-convex approaches. Crucially, we prove that, despite being a non-convex program, under mild conditions, the algorithm is guaranteed to converge to the global minimum of the problem; thus, it constitutes a provable quantum state tomography protocol.

}, url = {https://arxiv.org/abs/1711.02524}, author = {Anastasios Kyrillidis and Amir Kalev and Dohuyng Park and Srinadh Bhojanapalli and Constantine Caramanis and Sujay Sanghavi} } @article {2048, title = {On the readiness of quantum optimization machines for industrial applications}, year = {2017}, month = {2017/08/31}, abstract = {

There have been multiple attempts to demonstrate that quantum annealing and, in particular, quantum annealing on quantum annealing machines, has the potential to outperform current classical optimization algorithms implemented on CMOS technologies. The benchmarking of these devices has been controversial. Initially, random spin-glass problems were used, however, these were quickly shown to be not well suited to detect any quantum speedup. Subsequently, benchmarking shifted to carefully crafted synthetic problems designed to highlight the quantum nature of the hardware while (often) ensuring that classical optimization techniques do not perform well on them. Even worse, to date a true sign of improved scaling with the number problem variables remains elusive when compared to classical optimization techniques. Here, we analyze the readiness of quantum annealing machines for real-world application problems. These are typically not random and have an underlying structure that is hard to capture in synthetic benchmarks, thus posing unexpected challenges for optimization techniques, both classical and quantum alike. We present a comprehensive computational scaling analysis of fault diagnosis in digital circuits, considering architectures beyond D-wave quantum annealers. We find that the instances generated from real data in multiplier circuits are harder than other representative random spin-glass benchmarks with a comparable number of variables. Although our results show that transverse-field quantum annealing is outperformed by state-of-the-art classical optimization algorithms, these benchmark instances are hard and small in the size of the input, therefore representing the first industrial application ideally suited for near-term quantum annealers.

}, url = {https://arxiv.org/abs/1708.09780}, author = {Alejandro Perdomo-Ortiz and Alexander Feldman and Asier Ozaeta and Sergei V. Isakov and Zheng Zhu and Bryan O{\textquoteright}Gorman and Helmut G. Katzgraber and Alexander Diedrich and Hartmut Neven and Johan de Kleer and Brad Lackey and Rupak Biswas} } @article {2136, title = {Simultaneous, Full Characterization of a Single-Photon State}, journal = {Physical Review X}, volume = {7}, year = {2017}, month = {2017/11/15}, pages = {041036}, abstract = {

As single-photon sources become more mature and are used more often in quantum information, communications, and measurement applications, their characterization becomes more important. Singlephoton-like light is often characterized by its brightness, as well as two quantum properties: the suppression of multiphoton content and the photon indistinguishability. While it is desirable to obtain these quantities from a single measurement, currently two or more measurements are required. Here, we show that using two-photon (n \¼ 2) number-resolving detectors, one can completely characterize single-photon-like states in a single measurement, where previously two or more measurements were necessary. We simultaneously determine the brightness, the suppression of multiphoton states, the indistinguishability, and the statistical distribution of Fock states to third order for a quantum light source. We find n \≥ 3 number-resolving detectors provide no additional advantage in the single-photon characterization. The new method extracts more information per experimental trial than a conventional measurement for all input states and is particularly more efficient for statistical mixtures of photon states. Thus, using this n \¼ 2, number-resolving detector scheme will provide advantages in a variety of quantum optics measurements and systems.

}, doi = {10.1103/PhysRevX.7.041036}, url = {https://link.aps.org/doi/10.1103/PhysRevX.7.041036}, author = {Thomay, Tim and Polyakov, Sergey V. and Gazzano, Olivier and Goldschmidt, Elizabeth and Eldredge, Zachary D. and Huber, Tobias and Loo, Vivien and Solomon, Glenn S.} } @article {2100, title = {Thermodynamic Analysis of Classical and Quantum Search Algorithms}, year = {2017}, month = {2017/09/29}, abstract = {

We analyze the performance of classical and quantum search algorithms from a thermodynamic perspective, focusing on resources such as time, energy, and memory size. We consider two examples that are relevant to post-quantum cryptography: Grover\’s search algorithm, and the quantum algorithm for collisionfinding. Using Bennett\’s \“Brownian\” model of low-power reversible computation, we show classical algorithms that have the same asymptotic energy consumption as these quantum algorithms. Thus, the quantum advantage in query complexity does not imply a reduction in these thermodynamic resource costs. In addition, we present realistic estimates of the resource costs of quantum and classical search, for near-future computing technologies. We find that, if memory is cheap, classical exhaustive search can be surprisingly competitive with Grover\’s algorithm.

}, url = {https://arxiv.org/abs/1709.10510}, author = {Ray Perlner and Yi-Kai Liu} } @article {2152, title = {Threshold Dynamics of a Semiconductor Single Atom Maser}, journal = {Physical Review Letters}, volume = {119}, year = {2017}, month = {2017/08/31}, pages = {097702}, abstract = {

We demonstrate a single atom maser consisting of a semiconductor double quantum dot (DQD) that is embedded in a high-quality-factor microwave cavity. A finite bias drives the DQD out of equilibrium, resulting in sequential single electron tunneling and masing. We develop a dynamic tuning protocol that allows us to controllably increase the time-averaged repumping rate of the DQD at a fixed level detuning, and quantitatively study the transition through the masing threshold. We further examine the crossover from incoherent to coherent emission by measuring the photon statistics across the masing transition. The observed threshold behavior is in agreement with an existing single atom maser theory when small corrections from lead emission are taken into account.

}, doi = {10.1103/PhysRevLett.119.097702}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.119.097702}, author = {Liu, Y.-Y. and Stehlik, J. and Eichler, C. and Mi, X. and Hartke, T. R. and Michael Gullans and J. M. Taylor and Petta, J. R.} } @article {1816, title = {Valley Blockade in a Silicon Double Quantum Dot}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/11/13}, pages = {205302}, abstract = {

Electrical transport in double quantum dots (DQDs) illuminates many interesting features of the dots\&$\#$39; carrier states. Recent advances in silicon quantum information technologies have renewed interest in the valley states of electrons confined in silicon. Here we show measurements of DC transport through a mesa-etched silicon double quantum dot. Comparing bias triangles (i.e., regions of allowed current in DQDs) at positive and negative bias voltages we find a systematic asymmetry in the size of the bias triangles at the two bias polarities. Asymmetries of this nature are associated with blocking of tunneling events due to the occupation of a metastable state. Several features of our data lead us to conclude that the states involved are not simple spin states. Rather, we develop a model based on selective filling of valley states in the DQD that is consistent with all of the qualitative features of our data.

}, doi = {10.1103/PhysRevB.96.205302}, url = {https://arxiv.org/abs/1607.06107}, author = {Justin K. Perron and Michael Gullans and J. M. Taylor and M. D. Stewart, Jr. and Neil M. Zimmerman} } @article {1678, title = {Anomalous broadening in driven dissipative Rydberg systems}, journal = {Physical Review Letters}, volume = {116}, year = {2016}, month = {2016/03/16}, pages = {113001}, abstract = {We observe interaction-induced broadening of the two-photon 5s-18s transition in 87Rb atoms trapped in a 3D optical lattice. The measured linewidth increases by nearly two orders of magnitude with increasing atomic density and excitation strength, with corresponding suppression of resonant scattering and enhancement of off-resonant scattering. We attribute the increased linewidth to resonant dipole-dipole interactions of 18s atoms with spontaneously created populations of nearby np states. Over a range of initial atomic densities and excitation strengths, the transition width is described by a single function of the steady-state density of Rydberg atoms, and the observed resonant excitation rate corresponds to that of a two-level system with the measured, rather than natural, linewidth. The broadening mechanism observed here is likely to have negative implications for many proposals with coherently interacting Rydberg atoms.}, doi = {10.1103/PhysRevLett.116.113001}, url = {http://arxiv.org/abs/1510.08710}, author = {E. A. Goldschmidt and T. Boulier and R. C. Brown and S. B. Koller and J. T. Young and Alexey V. Gorshkov and S. L. Rolston and J. V. Porto} } @article {1815, title = {Double Quantum Dot Floquet Gain Medium}, journal = {Physical Review X}, volume = {6}, year = {2016}, month = {2016/11/07}, pages = {041027}, abstract = {

Strongly driving a two-level quantum system with light leads to a ladder of Floquet states separated by the photon energy. Nanoscale quantum devices allow the interplay of confined electrons, phonons, and photons to be studied under strong driving conditions. Here we show that a single electron in a periodically driven DQD functions as a \"Floquet gain medium,\" where population imbalances in the DQD Floquet quasi-energy levels lead to an intricate pattern of gain and loss features in the cavity response. We further measure a large intra-cavity photon number n_c in the absence of a cavity drive field, due to equilibration in the Floquet picture. Our device operates in the absence of a dc current -- one and the same electron is repeatedly driven to the excited state to generate population inversion. These results pave the way to future studies of non-classical light and thermalization of driven quantum systems.

}, doi = {10.1103/PhysRevX.6.041027}, url = {http://journals.aps.org/prx/abstract/10.1103/PhysRevX.6.041027}, author = {J. Stehlik and Y.-Y. Liu and C. Eichler and T. R. Hartke and X. Mi and Michael Gullans and J. M. Taylor and J. R. Petta} } @article {1781, title = {A Hubbard model for ultracold bosonic atoms interacting via zero-point-energy induced three-body interactions}, journal = {Physical Review A}, volume = {93}, year = {2016}, month = {2016/04/19}, pages = {043616}, abstract = {

We show that for ultra-cold neutral bosonic atoms held in a three-dimensional periodic potential or optical lattice, a Hubbard model with dominant, attractive three-body interactions can be generated. In fact, we derive that the effect of pair-wise interactions can be made small or zero starting from the realization that collisions occur at the zero-point energy of an optical lattice site and the strength of the interactions is energy dependent from effective-range contributions. We determine the strength of the two- and three-body interactions for scattering from van-der-Waals potentials and near Fano-Feshbach resonances. For van-der-Waals potentials, which for example describe scattering of alkaline-earth atoms, we find that the pair-wise interaction can only be turned off for species with a small negative scattering length, leaving the 88Sr isotope a possible candidate. Interestingly, for collisional magnetic Feshbach resonances this restriction does not apply and there often exist magnetic fields where the two-body interaction is small. We illustrate this result for several known narrow resonances between alkali-metal atoms as well as chromium atoms. Finally, we compare the size of the three-body interaction with hopping rates and describe limits due to three-body recombination.

}, doi = {10.1103/PhysRevA.93.043616}, url = {http://journals.aps.org/pra/abstract/10.1103/PhysRevA.93.043616}, author = {Saurabh Paul and P. R. Johnson and Eite Tiesinga} } @article {1783, title = {Joint product numerical range and geometry of reduced density matrices}, year = {2016}, month = {2016/06/23}, abstract = {The reduced density matrices of a many-body quantum system form a convex set, whose three-dimensional projection Θ is convex in R3. The boundary ∂Θ of Θ may exhibit nontrivial geometry, in particular ruled surfaces. Two physical mechanisms are known for the origins of ruled surfaces: symmetry breaking and gapless. In this work, we study the emergence of ruled surfaces for systems with local Hamiltonians in infinite spatial dimension, where the reduced density matrices are known to be separable as a consequence of the quantum de Finetti{\textquoteright}s theorem. This allows us to identify the reduced density matrix geometry with joint product numerical range Π of the Hamiltonian interaction terms. We focus on the case where the interaction terms have certain structures, such that ruled surface emerge naturally when taking a convex hull of Π. We show that, a ruled surface on ∂Θ sitting in Π has a gapless origin, otherwise it has a symmetry breaking origin. As an example, we demonstrate that a famous ruled surface, known as the oloid, is a possible shape of Θ, with two boundary pieces of symmetry breaking origin separated by two gapless lines.}, url = {http://arxiv.org/abs/1606.07422}, author = {Jianxin Chen and Cheng Guo and Zhengfeng Ji and Yiu-Tung Poon and Nengkun Yu and Bei Zeng and Jie Zhou} } @article {1834, title = {Many-body decoherence dynamics and optimised operation of a single-photon switch}, journal = {New Journal of Physics}, volume = {18}, year = {2016}, month = {2016/09/13}, pages = {092001}, abstract = {

We develop a theoretical framework to characterize the decoherence dynamics due to multi-photon scattering in an all-optical switch based on Rydberg atom induced nonlinearities. By incorporating the knowledge of this decoherence process into optimal photon storage and retrieval strategies, we establish optimised switching protocols for experimentally relevant conditions, and evaluate the corresponding limits in the achievable fidelities. Based on these results we work out a simplified description that reproduces recent experiments [arXiv:1511.09445] and provides a new interpretation in terms of many-body decoherence involving multiple incident photons and multiple gate excitations forming the switch. Aside from offering insights into the operational capacity of realistic photon switching capabilities, our work provides a complete description of spin wave decoherence in a Rydberg quantum optics setting, and has immediate relevance to a number of further applications employing photon storage in Rydberg media.\ 

}, doi = {10.1088/1367-2630/18/9/092001}, url = {http://iopscience.iop.org/article/10.1088/1367-2630/18/9/092001}, author = {Callum R. Murray and Alexey V. Gorshkov and Thomas Pohl} } @article {2508, title = {Measurement Protocol for the Entanglement Spectrum of Cold Atoms}, journal = {Phys. Rev. X }, volume = {6(4)}, year = {2016}, month = {2016/11/22}, abstract = {

Entanglement, and, in particular the entanglement spectrum, plays a major role in characterizing many-body quantum systems. While there has been a surge of theoretical works on the subject, no experimental measurement has been performed to date because of the lack of an implementable measurement scheme. Here, we propose a measurement protocol to access the entanglement spectrum of many-body states in experiments with cold atoms in optical lattices. Our scheme effectively performs a Ramsey spectroscopy of the entanglement Hamiltonian and is based on the ability to produce several copies of the state under investigation together with the possibility to perform a global swap gate between two copies conditioned on the state of an auxiliary qubit. We show how the required conditional swap gate can be implemented with cold atoms, either by using Rydberg interactions or coupling the atoms to a cavity mode. We illustrate these ideas on a simple (extended) Bose-Hubbard model where such a measurement protocol reveals topological features of the Haldane phase.\ 

}, doi = {https://doi.org/10.1103/PhysRevX.6.041033}, url = {https://arxiv.org/abs/1605.08624}, author = {Hannes Pichler and Guanyu Zhu and Alireza Seif and Peter Zoller and Mohammad Hafezi} } @article {1822, title = {Observation of Optomechanical Quantum Correlations at Room Temperature}, year = {2016}, month = {2016/05/18}, abstract = {

By shining laser light through a nanomechanical beam, we measure the beam\&$\#$39;s thermally driven vibrations and perturb its motion with optical forces at a level dictated by the Heisenberg measurement-disturbance uncertainty relation. Such quantum backaction is typically difficult to observe at room temperature where the motion driven by optical quantum intensity fluctuations is many orders of magnitude smaller than the thermal motion. We demonstrate a cross-correlation technique to distinguish optically driven motion from thermally driven motion, observing this quantum backaction signature up to room temperature. While it is often difficult to absolutely calibrate optical detection, we use the scale of the quantum correlations, which is determined by fundamental constants, to gauge the size of thermal motion, demonstrating a path towards absolute thermometry with quantum mechanically calibrated ticks.

}, url = {http://arxiv.org/abs/1605.05664}, author = {T. P. Purdy and K. E. Grutter and K. Srinivasan and J. M. Taylor} } @article {1697, title = {Photoassociation of spin polarized Chromium}, journal = {Physical Review A}, volume = {93}, year = {2016}, month = {2016/02/29}, pages = {021406}, abstract = {We report the homonuclear photoassociation (PA) of ultracold 52Cr atoms in an optical dipole trap. This constitutes the first measurement of PA in an element with total electron spin S~>1. Although Cr, with its 7S3 ground and 7P4,3,2 excited states, is expected to have a complicated PA spectrum we show that a spin polarized cloud exhibits a remarkably simple PA spectrum when circularly polarized light is applied. Over a scan range of 20 GHz below the 7P3 asymptote we observe two distinct vibrational series each following a LeRoy-Bernstein law for a C3/R3 potential with excellent agreement. We determine the C3 coefficients of the Hund{\textquoteright}s case c) relativistic adiabatic potentials to be -1.83{\textpm}0.02 a.u. and -1.46{\textpm}0.01a.u.. Theoretical non-rotating Movre-Pichler calculations enable a first assignment of the series to Ω=6u and 5g potential energy curves. In a different set of experiments we disturb the selection rules by a transverse magnetic field which leads to additional PA series.}, doi = {10.1103/PhysRevA.93.021406}, url = {http://arxiv.org/abs/1512.04378}, author = {Jahn R{\"u}hrig and Tobias B{\"a}uerle and Paul S. Julienne and Eite Tiesinga and Tilman Pfau} } @article {1706, title = {Pure-state tomography with the expectation value of Pauli operators}, journal = {Physical Review A}, volume = {93}, year = {2016}, month = {2016/03/31}, pages = {032140}, abstract = {

We examine the problem of finding the minimum number of Pauli measurements needed to uniquely determine an arbitrary n-qubit pure state among all quantum states. We show that only 11 Pauli measurements are needed to determine an arbitrary two-qubit pure state compared to the full quantum state tomography with 16 measurements, and only 31 Pauli measurements are needed to determine an arbitrary three-qubit pure state compared to the full quantum state tomography with 64 measurements. We demonstrate that our protocol is robust under depolarizing error with simulated random pure states. We experimentally test the protocol on two- and three-qubit systems with nuclear magnetic resonance techniques. We show that the pure state tomography protocol saves us a number of measurements without considerable loss of fidelity. We compare our protocol with same-size sets of randomly selected Pauli operators and find that our selected set of Pauli measurements significantly outperforms those random sampling sets. As a direct application, our scheme can also be used to reduce the number of settings needed for pure-state tomography in quantum optical systems.

}, doi = {http://dx.doi.org/10.1103/PhysRevA.93.032140}, url = {http://arxiv.org/abs/1601.05379}, author = {Xian Ma and Tyler Jackson and Hui Zhou and Jianxin Chen and Dawei Lu and Michael D. Mazurek and Kent A.G. Fisher and Xinhua Peng and David Kribs and Kevin J. Resch and Zhengfeng Ji and Bei Zeng and Raymond Laflamme} } @article {1707, title = {Quantifying the coherence of pure quantum states}, journal = {Physical Review A}, volume = {94}, year = {2016}, month = {2016/10/07}, pages = {042313}, abstract = {

In recent years, several measures have been proposed for characterizing the coherence of a given quantum state. We derive several results that illuminate how these measures behave when restricted to pure states. Notably, we present an explicit characterization of the closest incoherent state to a given pure state under the trace distance measure of coherence, and we affirm a recent conjecture that the l1 measure of coherence of a pure state is never smaller than its relative entropy of coherence. We then use our result to show that the states maximizing the trace distance of coherence are exactly the maximally coherent states, and we derive a new inequality relating the negativity and distillable entanglement of pure states.

}, doi = {10.1103/PhysRevA.94.042313}, url = {https://doi.org/10.1103/PhysRevA.94.042313}, author = {Jianxin Chen and Nathaniel Johnston and Chi-Kwong Li and Sarah Plosker} } @article {1692, title = {Sisyphus Thermalization of Photons in a Cavity-Coupled Double Quantum Dot}, journal = {Physical Review Letters}, volume = {117}, year = {2016}, month = {2016/07/25}, pages = {056801}, abstract = {

A strongly driven quantum system, coupled to a thermalizing bath, generically evolves into a highly non-thermal state as the external drive competes with the equilibrating force of the bath. We demonstrate a notable exception to this picture for a microwave resonator interacting with a periodically driven double quantum dot (DQD). In the limit of strong driving and long times, we show that the resonator field can be driven into a thermal state with a chemical potential given by a harmonic of the drive frequency. Such tunable chemical potentials are achievable with current devices and would have broad utility for quantum simulation in circuit quantum electrodynamics. As an example, we show how several DQDs embedded in an array of microwave resonators can induce a phase transition to a Bose-Einstein condensate of light.

}, doi = {http://dx.doi.org/10.1103/PhysRevLett.117.056801}, url = {http://arxiv.org/abs/1512.01248}, author = {Michael Gullans and J. Stehlik and Y. -Y. Liu and J. R. Petta and J. M. Taylor} } @article {1689, title = {Tomography is necessary for universal entanglement detection with single-copy observables}, journal = {Physical Review Letters}, volume = {116}, year = {2016}, month = {2016/06/07}, pages = {230501}, abstract = {Entanglement, one of the central mysteries of quantum mechanics, plays an essential role in numerous applications of quantum information theory. A natural question of both theoretical and experimental importance is whether universal entanglement detection is possible without full state tomography. In this work, we prove a no-go theorem that rules out this possibility for any non-adaptive schemes that employ single-copy measurements only. We also examine in detail a previously implemented experiment, which claimed to detect entanglement of two-qubit states via adaptive single-copy measurements without full state tomography. By performing the experiment and analyzing the data, we demonstrate that the information gathered is indeed sufficient to reconstruct the state. These results reveal a fundamental limit for single-copy measurements in entanglement detection, and provides a general framework to study the detection of other interesting properties of quantum states, such as the positivity of partial transpose and the k-symmetric extendibility.}, doi = {10.1103/PhysRevLett.116.230501}, url = {http://arxiv.org/abs/1511.00581}, author = {Dawei Lu and Tao Xin and Nengkun Yu and Zhengfeng Ji and Jianxin Chen and Guilu Long and Jonathan Baugh and Xinhua Peng and Bei Zeng and Raymond Laflamme} } @article {1823, title = {Wannier functions using a discrete variable representation for optical lattices}, journal = {Physical Review A}, volume = {94}, year = {2016}, month = {2016/09/07}, pages = {033606}, abstract = {

We propose a numerical method using the discrete variable representation (DVR) for constructing real-valued Wannier functions localized in a unit cell for both symmetric and asymmetric periodic potentials. We apply these results to finding Wannier functions for ultracold atoms trapped in laser-generated optical lattices. Following S. Kivelson [Phys. Rev. B26, 4269 (1982)], for a symmetric lattice with inversion symmetry, we construct Wannier functions as eigenstates of the position operators\ x,y, and\ z\ restricted to single-particle Bloch functions belonging to one or more bands. To ensure that the Wannier functions are real-valued, we numerically obtain the band structure and real-valued eigenstates using a uniform Fourier grid DVR. We then show, by a comparison of tunneling energies, that the Wannier functions are accurate for both inversion-symmetric and asymmetric potentials to better than 10 significant digits when using double-precision arithmetic. The calculations are performed for an optical lattice with double-wells per unit cell with tunable asymmetry along the\ x\ axis and a single sinusoidal potential along the perpendicular directions. Localized functions at the two potential minima within each unit cell are similarly constructed, but using a superposition of single-particle solutions from the two lowest bands. We finally use these localized basis functions to determine the two-body interaction energies in the Bose-Hubbard model and show the dependence of these energies on lattice asymmetry.

}, doi = {http://dx.doi.org/10.1103/PhysRevA.94.033606}, url = {http://journals.aps.org/pra/abstract/10.1103/PhysRevA.94.033606}, author = {Saurabh Paul and Eite Tiesinga} } @article {1472, title = {2D Superexchange mediated magnetization dynamics in an optical lattice}, journal = {Science}, volume = {348}, year = {2015}, month = {2015/04/30}, pages = {540 - 544}, abstract = { The competition of magnetic exchange interactions and tunneling underlies many complex quantum phenomena observed in real materials. We study non-equilibrium magnetization dynamics in an extended 2D system by loading effective spin-1/2 bosons into a spin-dependent optical lattice, and we use the lattice to separately control the resonance conditions for tunneling and superexchange. After preparing a non-equilibrium anti-ferromagnetically ordered state, we observe relaxation dynamics governed by two well-separated rates, which scale with the underlying Hamiltonian parameters associated with superexchange and tunneling. Remarkably, with tunneling off-resonantly suppressed, we are able to observe superexchange dominated dynamics over two orders of magnitude in magnetic coupling strength, despite the presence of vacancies. In this regime, the measured timescales are in agreement with simple theoretical estimates, but the detailed dynamics of this 2D, strongly correlated, and far-from-equilibrium quantum system remain out of reach of current computational techniques. }, doi = {10.1126/science.aaa1385}, url = {http://arxiv.org/abs/1411.7036v1}, author = {R. C. Brown and R. Wyllie and S. B. Koller and E. A. Goldschmidt and Michael Foss-Feig and J. V. Porto} } @article {1462, title = {Discontinuity of Maximum Entropy Inference and Quantum Phase Transitions}, journal = {New Journal of Physics}, volume = {17}, year = {2015}, month = {2015/08/10}, pages = {083019}, abstract = { In this paper, we discuss the connection between two genuinely quantum phenomena --- the discontinuity of quantum maximum entropy inference and quantum phase transitions at zero temperature. It is shown that the discontinuity of the maximum entropy inference of local observable measurements signals the non-local type of transitions, where local density matrices of the ground state change smoothly at the transition point. We then propose to use the quantum conditional mutual information of the ground state as an indicator to detect the discontinuity and the non-local type of quantum phase transitions in the thermodynamic limit. }, doi = {10.1088/1367-2630/17/8/083019}, url = {http://arxiv.org/abs/1406.5046v2}, author = {Jianxin Chen and Zhengfeng Ji and Chi-Kwong Li and Yiu-Tung Poon and Yi Shen and Nengkun Yu and Bei Zeng and Duanlu Zhou} } @article {2561, title = {Driving Rabi oscillations at the giant dipole resonance in xenon}, journal = {Phys. Rev. A }, volume = {92}, year = {2015}, month = {11/23/2015}, abstract = {

Free-electron lasers (FELs) produce short and very intense light pulses in the XUV and x-ray regimes. We investigate the possibility to drive Rabi oscillations in xenon with an intense FEL pulse by using the unusually large dipole strength of the giant-dipole resonance (GDR). The GDR decays within less than 30 as due to its position, which is above the 4d ionization threshold. We find that intensities around 1018 W/cm2 are required to induce Rabi oscillations with a period comparable to the lifetime. The pulse duration should not exceed 100 as because xenon will be fully ionized within a few lifetimes. Rabi oscillations reveal themselves also in the photoelectron spectrum in form of Autler-Townes splittings extending over several tens of electronvolt.

}, doi = {https://doi.org/10.1103/PhysRevA.92.053424}, url = {https://arxiv.org/abs/1511.00058}, author = {Stefan Pabst and Daochen Wang and Robin Santra} } @article {1188, title = {Fractional Quantum Hall States of Rydberg Polaritons}, journal = {Physical Review A}, volume = {91}, year = {2015}, month = {2015/03/31}, pages = {033838}, abstract = { We propose a scheme for realizing fractional quantum Hall states of light. In our scheme, photons of two polarizations are coupled to different atomic Rydberg states to form two flavors of Rydberg polaritons that behave as an effective spin. An array of optical cavity modes overlapping with the atomic cloud enables the realization of an effective spin-1/2 lattice. We show that the dipolar interaction between such polaritons, inherited from the Rydberg states, can be exploited to create a flat, topological band for a single spin-flip excitation. At half filling, this gives rise to a photonic (or polaritonic) fractional Chern insulator -- a lattice-based, fractional quantum Hall state of light. }, doi = {10.1103/PhysRevA.91.033838}, url = {http://arxiv.org/abs/1411.6624v1}, author = {Mohammad F. Maghrebi and Norman Y. Yao and Mohammad Hafezi and Thomas Pohl and Ofer Firstenberg and Alexey V. Gorshkov} } @article {1500, title = {Injection Locking of a Semiconductor Double Quantum Dot Micromaser}, journal = {Physical Review A}, volume = {92}, year = {2015}, month = {2015/11/02}, pages = {053802}, abstract = { Emission linewidth is an important figure of merit for masers and lasers. We recently demonstrated a semiconductor double quantum dot (DQD) micromaser where photons are generated through single electron tunneling events. Charge noise directly couples to the DQD energy levels, resulting in a maser linewidth that is more than 100 times larger than the Schawlow-Townes prediction. Here we demonstrate a linewidth narrowing of more than a factor 10 by locking the DQD emission to a coherent tone that is injected to the input port of the cavity. We measure the injection locking range as a function of cavity input power and show that it is in agreement with the Adler equation. The position and amplitude of distortion sidebands that appear outside of the injection locking range are quantitatively examined. Our results show that this unconventional maser, which is impacted by strong charge noise and electron-phonon coupling, is well described by standard laser models. }, doi = {10.1103/PhysRevA.92.053802}, url = {http://arxiv.org/abs/1508.04147}, author = {Y. -Y. Liu and J. Stehlik and Michael Gullans and J. M. Taylor and J. R. Petta} } @article {1274, title = {Large effective three-body interaction in a double-well optical lattice}, journal = {Phys. Rev. A 92, 023602}, volume = {92}, year = {2015}, month = {2015/08/03}, pages = {023602}, abstract = { We study ultracold atoms in an optical lattice with two local minima per unit cell and show that the low energy states of a multi-band Bose-Hubbard (BH) Hamiltonian with only pair-wise interactions is equivalent to an effective single-band Hamiltonian with strong three-body interactions. We focus on a double-well optical lattice with a symmetric double well along the $x$ axis and single well structure along the perpendicular directions. Tunneling and two-body interaction energies are obtained from an exact band-structure calculation and numerically-constructed Wannier functions in order to construct a BH Hamiltonian spanning the lowest two bands. Our effective Hamiltonian is constructed from the ground state of the $N$-atom Hamiltonian for each unit cell obtained within the subspace spanned by the Wannier functions of two lowest bands. The model includes hopping between ground states of neighboring unit cells. We show that such an effective Hamiltonian has strong three-body interactions that can be easily tuned by changing the lattice parameters. Finally, relying on numerical mean-field simulations, we show that the effective Hamiltonian is an excellent approximation of the two-band BH Hamiltonian over a wide range of lattice parameters, both in the superfluid and Mott insulator regions. }, url = {http://journals.aps.org/pra/abstract/10.1103/PhysRevA.92.023602}, author = {Saurabh Paul and Eite Tiesinga} } @article {1333, title = {Optomechanical reference accelerometer}, journal = {Metrologia}, volume = {52}, year = {2015}, month = {2015/09/08}, pages = {654}, abstract = {

We present an optomechanical accelerometer with high dynamic range, high bandwidth and read-out noise levels below 8 ${\mu}$g/$\sqrt{\mathrm{Hz}}$. The straightforward assembly and low cost of our device make it a prime candidate for on-site reference calibrations and autonomous navigation. We present experimental data taken with a vacuum sealed, portable prototype and deduce the achieved bias stability and scale factor accuracy. Additionally, we present a comprehensive model of the device physics that we use to analyze the fundamental noise sources and accuracy limitations of such devices.

}, doi = {10.1088/0026-1394/52/5/654}, url = {http://iopscience.iop.org/article/10.1088/0026-1394/52/5/654/meta;jsessionid=C2B417A5CD50B9B57EE14C78E1783802.ip-10-40-1-105}, author = {Oliver Gerberding and Felipe Guzman Cervantes and John Melcher and Jon R. Pratt and J. M. Taylor} } @article {1332, title = {Phonon-Assisted Gain in a Semiconductor Double Quantum Dot Maser}, journal = {Physical Review Letters}, volume = {114}, year = {2015}, month = {2015/05/13}, pages = {196802}, abstract = {We develop a microscopic model for the recently demonstrated double quantum dot (DQD) maser. In characterizing the gain of this device we find that, in addition to the direct stimulated emission of photons, there is a large contribution from the simultaneous emission of a photon and a phonon, i.e., the phonon sideband. We show that this phonon-assisted gain typically dominates the overall gain which leads to masing. Recent experimental data are well fit with our model. }, doi = {10.1103/PhysRevLett.114.196802}, url = {http://arxiv.org/abs/1501.03499v3}, author = {Michael Gullans and Y. -Y. Liu and J. Stehlik and J. R. Petta and J. M. Taylor} } @article {1499, title = {Semiconductor double quantum dot micromaser}, journal = {Science}, volume = {347}, year = {2015}, month = {2015/01/15}, pages = {285 - 287}, abstract = { The coherent generation of light, from masers to lasers, relies upon the specific structure of the individual emitters that lead to gain. Devices operating as lasers in the few-emitter limit provide opportunities for understanding quantum coherent phenomena, from THz sources to quantum communication. Here we demonstrate a maser that is driven by single electron tunneling events. Semiconductor double quantum dots (DQDs) serve as a gain medium and are placed inside of a high quality factor microwave cavity. We verify maser action by comparing the statistics of the emitted microwave field above and below the maser threshold. }, doi = {10.1126/science.aaa2501}, url = {http://arxiv.org/abs/1507.06359v1}, author = {Y. -Y. Liu and J. Stehlik and C. Eichler and Michael Gullans and J. M. Taylor and J. R. Petta} } @article {1177, title = {Kitaev chains with long-range pairing}, journal = {Physical Review Letters}, volume = {113}, year = {2014}, month = {2014/10/9}, abstract = { We propose and analyze a generalization of the Kitaev chain for fermions with long-range $p$-wave pairing, which decays with distance as a power-law with exponent $\alpha$. Using the integrability of the model, we demonstrate the existence of two types of gapped regimes, where correlation functions decay exponentially at short range and algebraically at long range ($\alpha > 1$) or purely algebraically ($\alpha < 1$). Most interestingly, along the critical lines, long-range pairing is found to break conformal symmetry for sufficiently small $\alpha$. This is accompanied by a violation of the area law for the entanglement entropy in large parts of the phase diagram in the presence of a gap, and can be detected via the dynamics of entanglement following a quench. Some of these features may be relevant for current experiments with cold atomic ions. }, doi = {10.1103/PhysRevLett.113.156402}, url = {http://arxiv.org/abs/1405.5440v2}, author = {Davide Vodola and Luca Lepori and Elisa Ercolessi and Alexey V. Gorshkov and Guido Pupillo} } @article {1342, title = {Optical detection of radio waves through a nanomechanical transducer}, journal = {Nature}, volume = {507}, year = {2014}, month = {2014/3/5}, pages = {81 - 85}, abstract = {Low-loss transmission and sensitive recovery of weak radio-frequency (rf) and microwave signals is an ubiquitous technological challenge, crucial in fields as diverse as radio astronomy, medical imaging, navigation and communication, including those of quantum states. Efficient upconversion of rf-signals to an optical carrier would allow transmitting them via optical fibers dramatically reducing losses, and give access to the mature toolbox of quantum optical techniques, routinely enabling quantum-limited signal detection. Research in the field of cavity optomechanics has shown that nanomechanical oscillators can couple very strongly to either microwave or optical fields. An oscillator accommodating both functionalities would bear great promise as the intermediate platform in a radio-to-optical transduction cascade. Here, we demonstrate such an opto-electro-mechanical transducer utilizing a high-Q nanomembrane. A moderate voltage bias (<10V) is sufficient to induce strong coupling between the voltage fluctuations in a rf resonance circuit and the membrane{\textquoteright}s displacement, which is simultaneously coupled to light reflected off its metallized surface. The circuit acts as an antenna; the voltage signals it induces are detected as an optical phase shift with quantum-limited sensitivity. The half-wave voltage is in the microvolt range, orders of magnitude below that of standard optical modulators. The noise added by the membrane is suppressed by the electro-mechanical cooperativity C~6800 and has a temperature of 40mK, far below 300K where the entire device is operated. This corresponds to a sensitivity limit as low as 5 pV/Hz^1/2, or -210dBm/Hz in a narrow band around 1 MHz. Our work introduces an entirely new approach to all-optical, ultralow-noise detection of classical electronic signals, and sets the stage for coherent upconversion of low-frequency quantum signals to the optical domain. }, doi = {10.1038/nature13029}, url = {http://arxiv.org/abs/1307.3467v2}, author = {T. Bagci and A. Simonsen and S. Schmid and L. G. Villanueva and E. Zeuthen and J. Appel and J. M. Taylor and A. S{\o}rensen and K. Usami and A. Schliesser and E. S. Polzik} } @article {1403, title = {Quantum Algorithms for Fermionic Quantum Field Theories}, year = {2014}, month = {2014/04/28}, abstract = { Extending previous work on scalar field theories, we develop a quantum algorithm to compute relativistic scattering amplitudes in fermionic field theories, exemplified by the massive Gross-Neveu model, a theory in two spacetime dimensions with quartic interactions. The algorithm introduces new techniques to meet the additional challenges posed by the characteristics of fermionic fields, and its run time is polynomial in the desired precision and the energy. Thus, it constitutes further progress towards an efficient quantum algorithm for simulating the Standard Model of particle physics. }, url = {http://arxiv.org/abs/1404.7115v1}, author = {Stephen P. Jordan and Keith S. M. Lee and John Preskill} } @article {1395, title = {Quantum Computation of Scattering in Scalar Quantum Field Theories}, journal = {Quantum Information and Computation}, volume = {14}, year = {2014}, month = {2014/09/01}, pages = {1014-1080}, abstract = { Quantum field theory provides the framework for the most fundamental physical theories to be confirmed experimentally, and has enabled predictions of unprecedented precision. However, calculations of physical observables often require great computational complexity and can generally be performed only when the interaction strength is weak. A full understanding of the foundations and rich consequences of quantum field theory remains an outstanding challenge. We develop a quantum algorithm to compute relativistic scattering amplitudes in massive phi-fourth theory in spacetime of four and fewer dimensions. The algorithm runs in a time that is polynomial in the number of particles, their energy, and the desired precision, and applies at both weak and strong coupling. Thus, it offers exponential speedup over existing classical methods at high precision or strong coupling. }, url = {http://arxiv.org/abs/1112.4833v1}, author = {Stephen P. Jordan and Keith S. M. Lee and John Preskill} } @article {1478, title = {Quantum correlations and entanglement in far-from-equilibrium spin systems }, journal = {Physical Review A}, volume = {90}, year = {2014}, month = {2014/12/15}, abstract = { By applying complementary analytic and numerical methods, we investigate the dynamics of spin-$1/2$ XXZ models with variable-range interactions in arbitrary dimensions. The dynamics we consider is initiated from uncorrelated states that are easily prepared in experiments, and can be equivalently viewed as either Ramsey spectroscopy or a quantum quench. Our primary focus is the dynamical emergence of correlations and entanglement in these far-from-equilibrium interacting quantum systems: we characterize these correlations by the entanglement entropy, concurrence, and squeezing, which are inequivalent measures of entanglement corresponding to different quantum resources. In one spatial dimension, we show that the time evolution of correlation functions manifests a non-perturbative dynamic singularity. This singularity is characterized by a universal power-law exponent that is insensitive to small perturbations. Explicit realizations of these models in current experiments using polar molecules, trapped ions, Rydberg atoms, magnetic atoms, and alkaline-earth and alkali atoms in optical lattices, along with the relative merits and limitations of these different systems, are discussed. }, doi = {10.1103/PhysRevA.90.063622}, url = {http://arxiv.org/abs/1406.0937v1}, author = {Kaden R. A. Hazzard and Mauritz van den Worm and Michael Foss-Feig and Salvatore R. Manmana and Emanuele Dalla Torre and Tilman Pfau and Michael Kastner and Ana Maria Rey} } @article {1295, title = {Spin-orbit-coupled topological Fulde-Ferrell states of fermions in a harmonic trap }, journal = {Physical Review A}, volume = {90}, year = {2014}, month = {2014/11/7}, abstract = { Motivated by recent experimental breakthroughs in generating spin-orbit coupling in ultracold Fermi gases using Raman laser beams, we present a systematic study of spin-orbit-coupled Fermi gases confined in a quasi-one-dimensional trap in the presence of an in-plane Zeeman field (which can be realized using a finite two-photon Raman detuning). We find that a topological Fulde-Ferrell state will emerge, featuring finite-momentum Cooper pairing and zero-energy Majorana excitations localized near the edge of the trap based on the self-consistent Bogoliubov-de Genes (BdG) equations. We find analytically the wavefunctions of the Majorana modes. Finally using the time-dependent BdG we show how the finite-momentum pairing field manifests itself in the expansion dynamics of the atomic cloud. }, doi = {10.1103/PhysRevA.90.053606}, url = {http://arxiv.org/abs/1404.6211v1}, author = {Lei Jiang and Eite Tiesinga and Xia-Ji Liu and Hui Hu and Han Pu} } @article {1841, title = {Attractive Photons in a Quantum Nonlinear Medium}, journal = {Nature (London)}, volume = {502}, year = {2013}, pages = {71}, url = {http://dx.doi.org/10.1038/nature12512}, author = {Ofer Firstenberg and Thibault Peyronel and Qi-Yu Liang and Alexey V. Gorshkov and Mikhail D. Lukin and Vladan Vuletic} } @article {1161, title = {Dissipative Many-body Quantum Optics in Rydberg Media}, journal = {Physical Review Letters}, volume = {110}, year = {2013}, month = {2013/4/9}, abstract = { We develop a theoretical framework for the dissipative propagation of quantized light in interacting optical media under conditions of electromagnetically induced transparency (EIT). The theory allows us to determine the peculiar spatiotemporal structure of the output of two complementary Rydberg-EIT-based light-processing modules: the recently demonstrated single-photon filter and the recently proposed single-photon subtractor, which, respectively, let through and absorb a single photon. In addition to being crucial for applications of these and other optical quantum devices, the theory opens the door to the study of exotic dissipative many-body dynamics of strongly interacting photons in nonlinear nonlocal media. }, doi = {10.1103/PhysRevLett.110.153601}, url = {http://arxiv.org/abs/1211.7060v1}, author = {Alexey V. Gorshkov and Rejish Nath and Thomas Pohl} } @article {1273, title = {Formation and decay of Bose-Einstein condensates in an excited band of a double-well optical lattice }, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/9/12}, abstract = { We study the formation and collision-aided decay of an ultra-cold atomic Bose-Einstein condensate in the first excited band of a double-well 2D-optical lattice with weak harmonic confinement in the perpendicular $z$ direction. This lattice geometry is based on an experiment by Wirth et al. The double well is asymmetric, with the local ground state in the shallow well nearly degenerate with the first excited state of the adjacent deep well. We compare the band structure obtained from a tight-binding (TB) model with that obtained numerically using a plane wave basis. We find the TB model to be in quantitative agreement for the lowest two bands, qualitative for next two bands, and inadequate for even higher bands. The band widths of the excited bands are much larger than the harmonic oscillator energy spacing in the $z$ direction. We then study the thermodynamics of a non-interacting Bose gas in the first excited band. We estimate the condensate fraction and critical temperature, $T_c$, as functions of lattice parameters. For typical atom numbers, the critical energy $k_BT_c$, with $k_B$ the Boltzmann constant, is larger than the excited band widths and harmonic oscillator energy. Using conservation of total energy and atom number, we show that the temperature increases after the lattice transformation. Finally, we estimate the time scale for a two-body collision-aided decay of the condensate as a function of lattice parameters. The decay involves two processes, the dominant one in which both colliding atoms decay to the ground band, and the second involving excitation of one atom to a higher band. For this estimate, we have used TB wave functions for the lowest four bands, and numerical estimates for higher bands. The decay rate rapidly increases with lattice depth, but stays smaller than the tunneling rate between the $s$ and $p$ orbitals in adjacent wells. }, doi = {10.1103/PhysRevA.88.033615}, url = {http://arxiv.org/abs/1308.4449v1}, author = {Saurabh Paul and Eite Tiesinga} } @article {1843, title = {Quantum Nonlinear Optics: Strongly Interacting Photons}, journal = {Opt. Photonics News}, volume = {24}, year = {2013}, pages = {48}, url = {http://www.osa-opn.org/abstract.cfm?URI=opn-24-12-48}, author = {Firstenberg, O and Lukin, M D and Peyronel, T and Liang, Q -Y and Vuletic, V and Alexey V. Gorshkov and Hofferberth, S and Pohl, T} } @article {1287, title = {Soliton dynamics of an atomic spinor condensate on a Ring Lattice}, journal = {Physical Review A}, volume = {87}, year = {2013}, month = {2013/3/6}, abstract = { We study the dynamics of macroscopically-coherent matter waves of an ultra-cold atomic spin-one or spinor condensate on a ring lattice of six sites and demonstrate a novel type of spatio-temporal internal Josephson effect. Using a discrete solitary mode of uncoupled spin components as an initial condition, the time evolution of this many-body system is found to be characterized by two dominant frequencies leading to quasiperiodic dynamics at various sites. The dynamics of spatially-averaged and spin-averaged degrees of freedom, however, is periodic enabling an unique identification of the two frequencies. By increasing the spin-dependent atom-atom interaction strength we observe a resonance state, where the ratio of the two frequencies is a characteristic integer multiple and the spin-and-spatial degrees of freedom oscillate in "unison". Crucially, this resonant state is found to signal the onset to chaotic dynamics characterized by a broad band spectrum. In a ferromagnetic spinor condensate with attractive spin-dependent interactions, the resonance is accompanied by a transition from oscillatory- to rotational-type dynamics as the time evolution of the relative phase of the matter wave of the individual spin projections changes from bounded to unbounded. }, doi = {10.1103/PhysRevA.87.033608}, url = {http://arxiv.org/abs/1301.5851v1}, author = {Indubala I Satija and Carlos L. Pando and Eite Tiesinga} } @article {1306, title = {Spinor dynamics in an antiferromagnetic spin-1 thermal Bose gas}, journal = {Physical Review Letters}, volume = {111}, year = {2013}, month = {2013/7/9}, abstract = { We present experimental observations of coherent spin-population oscillations in a cold thermal, Bose gas of spin-1 sodium-23 atoms. The population oscillations in a multi-spatial-mode thermal gas have the same behavior as those observed in a single-spatial-mode antiferromagnetic spinor Bose Einstein condensate. We demonstrate this by showing that the two situations are described by the same dynamical equations, with a factor of two change in the spin-dependent interaction coefficient, which results from the change to particles with distinguishable momentum states in the thermal gas. We compare this theory to the measured spin population evolution after times up to a few hundreds of ms, finding quantitative agreement with the amplitude and period. We also measure the damping time of the oscillations as a function of magnetic field. }, doi = {10.1103/PhysRevLett.111.025301}, url = {http://arxiv.org/abs/1306.4255v1}, author = {Hyewon K. Pechkis and Jonathan P. Wrubel and Arne Schwettmann and Paul F. Griffin and Ryan Barnett and Eite Tiesinga and Paul D. Lett} } @article {1279, title = {Anisotropy induced Feshbach resonances in a quantum dipolar gas of magnetic atoms }, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/9/7}, abstract = { We explore the anisotropic nature of Feshbach resonances in the collision between ultracold magnetic submerged-shell dysprosium atoms, which can only occur due to couplings to rotating bound states. This is in contrast to well-studied alkali-metal atom collisions, where most Feshbach resonances are hyperfine induced and due to rotation-less bound states. Our novel first-principle coupled-channel calculation of the collisions between open-4f-shell spin-polarized bosonic dysprosium reveals a striking correlation between the anisotropy due to magnetic dipole-dipole and electrostatic interactions and the Feshbach spectrum as a function of an external magnetic field. Over a 20 mT magnetic field range we predict about a dozen Feshbach resonances and show that the resonance locations are exquisitely sensitive to the dysprosium isotope. }, doi = {10.1103/PhysRevLett.109.103002}, url = {http://arxiv.org/abs/1203.4172v1}, author = {Alexander Petrov and Eite Tiesinga and Svetlana Kotochigova} } @article {1793, title = {Non-Recursively Constructible Recursive Families of Graphs}, journal = {The Electronic Journal of Combinatorics}, volume = {19}, year = {2012}, month = {2012/04/16}, abstract = {In a publication by Noy and Rib{\'o}, it was shown that recursively constructible families of graphs are recursive. The authors also conjecture that the converse holds; that is, recursive families are also recursively constructible. In this paper, we provide two specific counterexamples to this conjecture, which we then extend to an infinite family of counterexamples.}, url = {http://www.combinatorics.org/ojs/index.php/eljc/article/view/2211}, author = {Colleen Bouey and Christina Graves and Aaron Ostrander and Gregory Palma} } @article {1397, title = {Quantum Algorithms for Quantum Field Theories}, journal = {Science}, volume = {336}, year = {2012}, month = {2012/05/31}, pages = {1130 - 1133}, abstract = { Quantum field theory reconciles quantum mechanics and special relativity, and plays a central role in many areas of physics. We develop a quantum algorithm to compute relativistic scattering probabilities in a massive quantum field theory with quartic self-interactions (phi-fourth theory) in spacetime of four and fewer dimensions. Its run time is polynomial in the number of particles, their energy, and the desired precision, and applies at both weak and strong coupling. In the strong-coupling and high-precision regimes, our quantum algorithm achieves exponential speedup over the fastest known classical algorithm. }, doi = {10.1126/science.1217069}, url = {http://arxiv.org/abs/1111.3633v2}, author = {Stephen P. Jordan and Keith S. M. Lee and John Preskill} } @article {1845, title = {Quantum nonlinear optics with single photons enabled by strongly interacting atoms}, journal = {Nature (London)}, volume = {488}, year = {2012}, pages = {57}, url = {http://www.nature.com/nature/journal/v488/n7409/full/nature11361.html}, author = {Peyronel, Thibault and Firstenberg, Ofer and Liang, Qi-Yu and Hofferberth, Sebastian and Alexey V. Gorshkov and Pohl, Thomas and Lukin, Mikhail D. and Vuletic, Vladan} } @article {1349, title = {Laser cooling and optical detection of excitations in a LC electrical circuit}, journal = {Physical Review Letters}, volume = {107}, year = {2011}, month = {2011/12/27}, abstract = {We explore a method for laser cooling and optical detection of excitations in a LC electrical circuit. Our approach uses a nanomechanical oscillator as a transducer between optical and electronic excitations. An experimentally feasible system with the oscillator capacitively coupled to the LC and at the same time interacting with light via an optomechanical force is shown to provide strong electro-mechanical coupling. Conditions for improved sensitivity and quantum limited readout of electrical signals with such an "optical loud speaker" are outlined. }, doi = {10.1103/PhysRevLett.107.273601}, url = {http://arxiv.org/abs/1108.2035v1}, author = {J. M. Taylor and A. S. S{\o}rensen and C. M. Marcus and E. S. Polzik} } @article {1165, title = {Light storage in an optically thick atomic ensemble under conditions of electromagnetically induced transparency and four-wave mixing }, journal = {Physical Review A}, volume = {83}, year = {2011}, month = {2011/6/20}, abstract = { We study the modification of a traditional electromagnetically induced transparency (EIT) stored light technique that includes both EIT and four-wave mixing (FWM) in an ensemble of hot Rb atoms. The standard treatment of light storage involves the coherent and reversible mapping of one photonic mode onto a collective spin coherence. It has been shown that unwanted, competing processes such as four-wave mixing are enhanced by EIT and can significantly modify the signal optical pulse propagation. We present theoretical and experimental evidence to indicate that while a Stokes field is indeed detected upon retrieval of the signal field, any information originally encoded in a seeded Stokes field is not independently preserved during the storage process. We present a simple model that describes the propagation dynamics of the fields and the impact of FWM on the spin wave. }, doi = {10.1103/PhysRevA.83.063823}, url = {http://arxiv.org/abs/1103.2131v1}, author = {Nathaniel B. Phillips and Alexey V. Gorshkov and Irina Novikova} } @article {1174, title = {Photon-Photon Interactions via Rydberg Blockade}, journal = {Physical Review Letters}, volume = {107}, year = {2011}, month = {2011/9/22}, abstract = { We develop the theory of light propagation under the conditions of electromagnetically induced transparency (EIT) in systems involving strongly interacting Rydberg states. Taking into account the quantum nature and the spatial propagation of light, we analyze interactions involving few-photon pulses. We demonstrate that this system can be used for the generation of nonclassical states of light including trains of single photons with an avoided volume between them, for implementing photon-photon quantum gates, as well as for studying many-body phenomena with strongly correlated photons. }, doi = {10.1103/PhysRevLett.107.133602}, url = {http://arxiv.org/abs/1103.3700v1}, author = {Alexey V. Gorshkov and Johannes Otterbach and Michael Fleischhauer and Thomas Pohl and Mikhail D. Lukin} } @article {1501, title = {Adiabatic preparation of many-body states in optical lattices}, journal = {Physical Review A}, volume = {81}, year = {2010}, month = {2010/6/22}, abstract = { We analyze a technique for the preparation of low entropy many body states of atoms in optical lattices based on adiabatic passage. In particular, we show that this method allows preparation of strongly correlated states as stable highest energy states of Hamiltonians that have trivial ground states. As an example, we analyze the generation of antiferromagnetically ordered states by adiabatic change of a staggered field acting on the spins of bosonic atoms with ferromagnetic interactions. }, doi = {10.1103/PhysRevA.81.061603}, url = {http://arxiv.org/abs/0906.2567v3}, author = {Anders S. Sorensen and Ehud Altman and Michael Gullans and J. V. Porto and Mikhail D. Lukin and Eugene Demler} } @article {1431, title = {Efficient Direct Tomography for Matrix Product States}, year = {2010}, month = {2010/02/24}, abstract = { In this note, we describe a method for reconstructing matrix product states from a small number of efficiently-implementable measurements. Our method is exponentially faster than standard tomography, and it can also be used to certify that the unknown state is an MPS. The basic idea is to use local unitary operations to measure in the Schmidt basis, giving direct access to the MPS representation. This compares favorably with recently and independently proposed methods that recover the MPS tensors by performing a variational minimization, which is computationally intractable in certain cases. Our method also has the advantage of recovering any MPS, while other approaches were limited to special classes of states that exclude important examples such as GHZ and W states. }, url = {http://arxiv.org/abs/1002.4632v1}, author = {Olivier Landon-Cardinal and Yi-Kai Liu and David Poulin} } @article {1436, title = {Efficient quantum state tomography}, journal = {Nature Communications}, volume = {1}, year = {2010}, month = {2010/12/21}, pages = {149}, abstract = { Quantum state tomography, the ability to deduce the state of a quantum system from measured data, is the gold standard for verification and benchmarking of quantum devices. It has been realized in systems with few components, but for larger systems it becomes infeasible because the number of quantum measurements and the amount of computation required to process them grows exponentially in the system size. Here we show that we can do exponentially better than direct state tomography for a wide range of quantum states, in particular those that are well approximated by a matrix product state ansatz. We present two schemes for tomography in 1-D quantum systems and touch on generalizations. One scheme requires unitary operations on a constant number of subsystems, while the other requires only local measurements together with more elaborate post-processing. Both schemes rely only on a linear number of experimental operations and classical postprocessing that is polynomial in the system size. A further strength of the methods is that the accuracy of the reconstructed states can be rigorously certified without any a priori assumptions. }, doi = {10.1038/ncomms1147}, url = {http://arxiv.org/abs/1101.4366v1}, author = {Marcus Cramer and Martin B. Plenio and Steven T. Flammia and David Gross and Stephen D. Bartlett and Rolando Somma and Olivier Landon-Cardinal and Yi-Kai Liu and David Poulin} } @article {1195, title = {Realization of Coherent Optically Dense Media via Buffer-Gas Cooling}, journal = {Physical Review A}, volume = {79}, year = {2009}, month = {2009/1/6}, abstract = { We demonstrate that buffer-gas cooling combined with laser ablation can be used to create coherent optical media with high optical depth and low Doppler broadening that offers metastable states with low collisional and motional decoherence. Demonstration of this generic technique opens pathways to coherent optics with a large variety of atoms and molecules. We use helium buffer gas to cool 87Rb atoms to below 7 K and slow atom diffusion to the walls. Electromagnetically induced transparency (EIT) in this medium allows for 50\% transmission in a medium with initial OD >70 and for slow pulse propagation with large delay-bandwidth products. In the high-OD regime, we observe high-contrast spectrum oscillations due to efficient four-wave mixing. }, doi = {10.1103/PhysRevA.79.013806}, url = {http://arxiv.org/abs/0805.1416v2}, author = {Tao Hong and Alexey V. Gorshkov and David Patterson and Alexander S. Zibrov and John M. Doyle and Mikhail D. Lukin and Mara G. Prentiss} } @article {1852, title = {Slow light propagation and amplification via electromagnetically induced transparency and four-wave mixing in an optically dense atomic vapor}, journal = {J. Mod. Opt.}, volume = {56}, year = {2009}, pages = {1916}, url = {http://www.informaworld.com/smpp/content~db=all~content=a913545405}, author = {Phillips, N B and Alexey V. Gorshkov and Novikova, I} } @article {1305, title = {Multilevel effects in the Rabi oscillations of a Josephson phase qubit}, journal = {Physical Review B}, volume = {78}, year = {2008}, month = {2008/9/15}, abstract = { We present Rabi oscillation measurements of a Nb/AlOx/Nb dc superconducting quantum interference device (SQUID) phase qubit with a 100 um^2 area junction acquired over a range of microwave drive power and frequency detuning. Given the slightly anharmonic level structure of the device, several excited states play an important role in the qubit dynamics, particularly at high power. To investigate the effects of these levels, multiphoton Rabi oscillations were monitored by measuring the tunneling escape rate of the device to the voltage state, which is particularly sensitive to excited state population. We compare the observed oscillation frequencies with a simplified model constructed from the full phase qubit Hamiltonian and also compare time-dependent escape rate measurements with a more complete density-matrix simulation. Good quantitative agreement is found between the data and simulations, allowing us to identify a shift in resonance (analogous to the ac Stark effect), a suppression of the Rabi frequency, and leakage to the higher excited states. }, doi = {10.1103/PhysRevB.78.104510}, url = {http://arxiv.org/abs/0806.4711v2}, author = {S. K. Dutta and Frederick W. Strauch and R. M. Lewis and Kaushik Mitra and Hanhee Paik and T. A. Palomaki and Eite Tiesinga and J. R. Anderson and Alex J. Dragt and C. J. Lobb and F. C. Wellstood} } @article {1164, title = {Optimal light storage in atomic vapor}, journal = {Physical Review A}, volume = {78}, year = {2008}, month = {2008/8/1}, abstract = { We study procedures for the optimization of efficiency of light storage and retrieval based on the dynamic form of electromagnetically induced transparency (EIT) in warm Rb vapor. We present a detailed analysis of two recently demonstrated optimization protocols: a time-reversal-based iteration procedure, which finds the optimal input signal pulse shape for any given control field, and a procedure based on the calculation of an optimal control field for any given signal pulse shape. We verify that the two procedures are consistent with each other, and that they both independently achieve the maximum memory efficiency for any given optical depth. We observe good agreement with theoretical predictions for moderate optical depths (<25), while at higher optical depths the experimental efficiency falls below the theoretically predicted values. We identify possible effects responsible for this reduction in memory efficiency. }, doi = {10.1103/PhysRevA.78.023801}, url = {http://arxiv.org/abs/0805.3348v1}, author = {Nathaniel B. Phillips and Alexey V. Gorshkov and Irina Novikova} } @article {1163, title = {Optimal light storage with full pulse shape control}, journal = {Physical Review A}, volume = {78}, year = {2008}, month = {2008/8/20}, abstract = { We experimentally demonstrate optimal storage and retrieval of light pulses of arbitrary shape in atomic ensembles. By shaping auxiliary control pulses, we attain efficiencies approaching the fundamental limit and achieve precise retrieval into any predetermined temporal profile. Our techniques, demonstrated in warm Rb vapor, are applicable to a wide range of systems and protocols. As an example, we present their potential application to the creation of optical time-bin qubits and to controlled partial retrieval. }, doi = {10.1103/PhysRevA.78.021802}, url = {http://arxiv.org/abs/0805.1927v1}, author = {Irina Novikova and Nathaniel B. Phillips and Alexey V. Gorshkov} } @article {1853, title = {Optimal light storage with full pulse-shape control}, journal = {Phys. Rev. A}, volume = {78}, year = {2008}, pages = {021802(R)}, url = {http://link.aps.org/abstract/PRA/v78/e021802/}, author = {Novikova, I and Phillips, N B and Alexey V. Gorshkov} } @article {1854, title = {Optimizing Slow and Stored Light for Multidisciplinary Applications}, journal = {Proc. SPIE}, volume = {6904}, year = {2008}, pages = {69040C}, url = {http://spie.org/x648.xml?product_id=772216\&Search_Origin=QuickSearch\&Search_Results_URL=http://spie.org/x1636.xml\&Alternate_URL=http://spie.org/x18509.xml\&Alternate_URL_Name=timeframe\&Alternate_URL_Value=Exhibitors\&UseJavascript=1\&Please_Wait_URL=http://s}, author = {Klein, M and Xiao, Y and Alexey V. Gorshkov and M Hohensee and C D Leung and M R Browning and Phillips, D F and Novikova, I and Walsworth, R L} } @article {1855, title = {Suppression of Inelastic Collisions Between Polar Molecules With a Repulsive Shield}, journal = {Phys. Rev. Lett.}, volume = {101}, year = {2008}, pages = {073201}, url = {http://link.aps.org/abstract/PRL/v101/e073201/}, author = {Alexey V. Gorshkov and Rabl, P and Pupillo, G and Micheli, A and Zoller, P and Lukin, M D and B{\"u}chler, H P} } @article {1253, title = {Improved quantum algorithms for the ordered search problem via semidefinite programming }, journal = {Physical Review A}, volume = {75}, year = {2007}, month = {2007/3/26}, abstract = { One of the most basic computational problems is the task of finding a desired item in an ordered list of N items. While the best classical algorithm for this problem uses log_2 N queries to the list, a quantum computer can solve the problem using a constant factor fewer queries. However, the precise value of this constant is unknown. By characterizing a class of quantum query algorithms for ordered search in terms of a semidefinite program, we find new quantum algorithms for small instances of the ordered search problem. Extending these algorithms to arbitrarily large instances using recursion, we show that there is an exact quantum ordered search algorithm using 4 log_{605} N \approx 0.433 log_2 N queries, which improves upon the previously best known exact algorithm. }, doi = {10.1103/PhysRevA.75.032335}, url = {http://arxiv.org/abs/quant-ph/0608161v1}, author = {Andrew M. Childs and Andrew J. Landahl and Pablo A. Parrilo} } @article {1203, title = {Optimal control of light pulse storage and retrieval}, journal = {Physical Review Letters}, volume = {98}, year = {2007}, month = {2007/6/15}, abstract = { We demonstrate experimentally a procedure to obtain the maximum efficiency for the storage and retrieval of light pulses in atomic media. The procedure uses time reversal to obtain optimal input signal pulse-shapes. Experimental results in warm Rb vapor are in good agreement with theoretical predictions and demonstrate a substantial improvement of efficiency. This optimization procedure is applicable to a wide range of systems. }, doi = {10.1103/PhysRevLett.98.243602}, url = {http://arxiv.org/abs/quant-ph/0702266v1}, author = {Irina Novikova and Alexey V. Gorshkov and David F. Phillips and Anders S. Sorensen and Mikhail D. Lukin and Ronald L. Walsworth} } @article {1857, title = {Optimization of slow and stored light in atomic vapor}, journal = {Proc. SPIE}, volume = {6482}, year = {2007}, pages = {64820M}, url = {http://spiedigitallibrary.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PSISDG00648200000164820M000001\&idtype=cvips\&gifs=Yes\&bproc=volrange\&scode=6400\%20-\%206499}, author = {Novikova, I and Alexey V. Gorshkov and Phillips, D F and Xiao, Y and Klein, M and Walsworth, R L} } @article {1359, title = {Relaxation, dephasing, and quantum control of electron spins in double quantum dots}, journal = {Physical Review B}, volume = {76}, year = {2007}, month = {2007/7/13}, abstract = {Recent experiments have demonstrated quantum manipulation of two-electron spin states in double quantum dots using electrically controlled exchange interactions. Here, we present a detailed theory for electron spin dynamics in two-electron double dot systems that was used to guide these experiments and analyze experimental results. The theory treats both charge and spin degrees of freedom on an equal basis. Specifically, we analyze the relaxation and dephasing mechanisms that are relevant to experiments and discuss practical approaches for quantum control of two-electron system. We show that both charge and spin dephasing play important roles in the dynamics of the two-spin system, but neither represents a fundamental limit for electrical control of spin degrees of freedom in semiconductor quantum bits. }, doi = {10.1103/PhysRevB.76.035315}, url = {http://arxiv.org/abs/cond-mat/0602470v2}, author = {J. M. Taylor and J. R. Petta and A. C. Johnson and A. Yacoby and C. M. Marcus and M. D. Lukin} } @article {1323, title = {Two dogmas about quantum mechanics}, year = {2007}, month = {2007/12/27}, abstract = { We argue that the intractable part of the measurement problem -- the {\textquoteright}big{\textquoteright} measurement problem -- is a pseudo-problem that depends for its legitimacy on the acceptance of two dogmas. The first dogma is John Bell{\textquoteright}s assertion that measurement should never be introduced as a primitive process in a fundamental mechanical theory like classical or quantum mechanics, but should always be open to a complete analysis, in principle, of how the individual outcomes come about dynamically. The second dogma is the view that the quantum state has an ontological significance analogous to the significance of the classical state as the {\textquoteright}truthmaker{\textquoteright} for propositions about the occurrence and non-occurrence of events, i.e., that the quantum state is a representation of physical reality. We show how both dogmas can be rejected in a realist information-theoretic interpretation of quantum mechanics as an alternative to the Everett interpretation. The Everettian, too, regards the {\textquoteright}big{\textquoteright} measurement problem as a pseudo-problem, because the Everettian rejects the assumption that measurements have definite outcomes, in the sense that one particular outcome, as opposed to other possible outcomes, actually occurs in a quantum measurement process. By contrast with the Everettians, we accept that measurements have definite outcomes. By contrast with the Bohmians and the GRW {\textquoteright}collapse{\textquoteright} theorists who add structure to the theory and propose dynamical solutions to the {\textquoteright}big{\textquoteright} measurement problem, we take the problem to arise from the failure to see the significance of Hilbert space as a new kinematic framework for the physics of an indeterministic universe, in the sense that Hilbert space imposes kinematic (i.e., pre-dynamic) objective probabilistic constraints on correlations between events. }, url = {http://arxiv.org/abs/0712.4258v2}, author = {Jeffrey Bub and Itamar Pitowsky} } @article {1405, title = {Effects of finite temperature on the Mott insulator state}, journal = {Physical Review A}, volume = {73}, year = {2006}, month = {2006/1/20}, abstract = { We investigate the effects of finite temperature on ultracold Bose atoms confined in an optical lattice plus a parabolic potential in the Mott insulator state. In particular, we analyze the temperature dependence of the density distribution of atomic pairs in the lattice, by means of exact Monte-Carlo simulations. We introduce a simple model that quantitatively accounts for the computed pair density distributions at low enough temperatures. We suggest that the temperature dependence of the atomic pair statistics may be used to estimate the system{\textquoteright}s temperature at energies of the order of the atoms{\textquoteright} interaction energy. }, doi = {10.1103/PhysRevA.73.013408}, url = {http://arxiv.org/abs/cond-mat/0407075v3}, author = {Guido Pupillo and Carl J. Williams and Nikolay V. Prokof{\textquoteright}ev} } @article {1416, title = {Mean-field treatment of the damping of the oscillations of a 1D Bose gas in an optical lattice }, journal = {Physical Review A}, volume = {73}, year = {2006}, month = {2006/1/9}, abstract = { We present a theoretical treatment of the surprisingly large damping observed recently in one-dimensional Bose-Einstein atomic condensates in optical lattices. We show that time-dependent Hartree-Fock-Bogoliubov (HFB) calculations can describe qualitatively the main features of the damping observed over a range of lattice depths. We also derive a formula of the fluctuation-dissipation type for the damping, based on a picture in which the coherent motion of the condensate atoms is disrupted as they try to flow through the random local potential created by the irregular motion of noncondensate atoms. We expect this irregular motion to result from the well-known dynamical instability exhibited by the mean-field theory for these systems. When parameters for the characteristic strength and correlation times of the fluctuations, obtained from the HFB calculations, are substituted in the damping formula, we find very good agreement with the experimentally-observed damping, as long as the lattice is shallow enough for the fraction of atoms in the Mott insulator phase to be negligible. We also include, for completeness, the results of other calculations based on the Gutzwiller ansatz, which appear to work better for the deeper lattices. }, doi = {10.1103/PhysRevA.73.013605}, url = {http://arxiv.org/abs/cond-mat/0410677v4}, author = {Julio Gea-Banacloche and Ana Maria Rey and Guido Pupillo and Carl J. Williams and Charles W. Clark} } @article {1410, title = {Pseudo-fermionization of 1-D bosons in optical lattices}, journal = {New Journal of Physics}, volume = {8}, year = {2006}, month = {2006/08/30}, pages = {161 - 161}, abstract = { We present a model that generalizes the Bose-Fermi mapping for strongly correlated 1D bosons in an optical lattice, to cases in which the average number of atoms per site is larger than one. This model gives an accurate account of equilibrium properties of such systems, in parameter regimes relevant to current experiments. The application of this model to non-equilibrium phenomena is explored by a study of the dynamics of an atom cloud subject to a sudden displacement of the confining potential. Good agreement is found with results of recent experiments. The simplicity and intuitive appeal of this model make it attractive as a general tool for understanding bosonic systems in the strongly correlated regime. }, doi = {10.1088/1367-2630/8/8/161}, url = {http://arxiv.org/abs/cond-mat/0505325v2}, author = {Guido Pupillo and Ana Maria Rey and Carl J. Williams and Charles W. Clark} } @article {1415, title = {Bragg Spectroscopy of ultracold atoms loaded in an optical lattice}, journal = {Physical Review A}, volume = {72}, year = {2005}, month = {2005/8/12}, abstract = { We study Bragg spectroscopy of ultra-cold atoms in one-dimensional optical lattices as a method for probing the excitation spectrum in the Mott insulator phase, in particular the one particle-hole excitation band. Within the framework of perturbation theory we obtain an analytical expression for the dynamic structure factor $S(q,\omega)$ and use it to calculate the imparted energy which has shown to be a relevant observable in recent experiments. We test the accuracy of our approximations by comparing them with numerically exact solutions of the Bose-Hubbard model in restricted cases and establish the limits of validity of our linear response analysis. Finally we show that when the system is deep in the Mott insulator regime, its response to the Bragg perturbation is temperature dependent. We suggest that this dependence might be used as a tool to probe temperatures of order of the Mott gap. }, doi = {10.1103/PhysRevA.72.023407}, url = {http://arxiv.org/abs/cond-mat/0406552v2}, author = {Ana Maria Rey and P. Blair Blakie and Guido Pupillo and Carl J. Williams and Charles W. Clark} } @article {1418, title = {Scalable register initialization for quantum computing in an optical lattice }, journal = {Journal of Physics B: Atomic, Molecular and Optical Physics}, volume = {38}, year = {2005}, month = {2005/06/14}, pages = {1687 - 1694}, abstract = { The Mott insulator state created by loading an atomic Bose-Einstein condensate (BEC) into an optical lattice may be used as a means to prepare a register of atomic qubits in a quantum computer. Such architecture requires a lattice commensurately filled with atoms, which corresponds to the insulator state only in the limit of zero inter-well tunneling. We show that a lattice with spatial inhomogeneity created by a quadratic magnetic trapping potential can be used to isolate a subspace in the center which is impervious to hole-hoping. Components of the wavefunction with more than one atom in any well can be projected out by selective measurement on a molecular photo-associative transition. Maintaining the molecular coupling induces a quantum Zeno effect that can sustain a commensurately filled register for the duration of a quantum computation. }, doi = {10.1088/0953-4075/38/11/010}, url = {http://arxiv.org/abs/quant-ph/0312069v1}, author = {Gavin K. Brennen and Guido Pupillo and Ana Maria Rey and Charles W. Clark and Carl J. Williams} } @article {1409, title = {Ultracold atoms confined in an optical lattice plus parabolic potential: a closed-form approach }, journal = {Physical Review A}, volume = {72}, year = {2005}, month = {2005/9/22}, abstract = { We discuss interacting and non-interacting one dimensional atomic systems trapped in an optical lattice plus a parabolic potential. We show that, in the tight-binding approximation, the non-interacting problem is exactly solvable in terms of Mathieu functions. We use the analytic solutions to study the collective oscillations of ideal bosonic and fermionic ensembles induced by small displacements of the parabolic potential. We treat the interacting boson problem by numerical diagonalization of the Bose-Hubbard Hamiltonian. From analysis of the dependence upon lattice depth of the low-energy excitation spectrum of the interacting system, we consider the problems of "fermionization" of a Bose gas, and the superfluid-Mott insulator transition. The spectrum of the noninteracting system turns out to provide a useful guide to understanding the collective oscillations of the interacting system, throughout a large and experimentally relevant parameter regime. }, doi = {10.1103/PhysRevA.72.033616}, url = {http://arxiv.org/abs/cond-mat/0503477v2}, author = {Ana Maria Rey and Guido Pupillo and Charles W. Clark and Carl J. Williams} } @article {1365, title = {Quantum information processing using localized ensembles of nuclear spins}, year = {2004}, month = {2004/07/23}, abstract = {We describe a technique for quantum information processing based on localized en sembles of nuclear spins. A qubit is identified as the presence or absence of a collective excitation of a mesoscopic ensemble of nuclear spins surrounding a single quantum dot. All single and two-qubit operations can be effected using hyperfine interactions and single-electron spin rotations, hence the proposed scheme avoids gate errors arising from entanglement between spin and orbital degrees of freedom. Ultra-long coherence times of nuclear spins suggest that this scheme could be particularly well suited for applications where long lived memory is essential. }, url = {http://arxiv.org/abs/cond-mat/0407640v2}, author = {J. M. Taylor and G. Giedke and H. Christ and B. Paredes and J. I. Cirac and P. Zoller and M. D. Lukin and A. Imamoglu} } @article {1419, title = {Scalable quantum computation in systems with Bose-Hubbard dynamics}, journal = {Journal of Modern Optics}, volume = {51}, year = {2004}, month = {2004/02/15}, pages = {2395 - 2404}, abstract = { Several proposals for quantum computation utilize a lattice type architecture with qubits trapped by a periodic potential. For systems undergoing many body interactions described by the Bose-Hubbard Hamiltonian, the ground state of the system carries number fluctuations that scale with the number of qubits. This process degrades the initialization of the quantum computer register and can introduce errors during error correction. In an earlier manuscript we proposed a solution to this problem tailored to the loading of cold atoms into an optical lattice via the Mott Insulator phase transition. It was shown that by adding an inhomogeneity to the lattice and performing a continuous measurement, the unit filled state suitable for a quantum computer register can be maintained. Here, we give a more rigorous derivation of the register fidelity in homogeneous and inhomogeneous lattices and provide evidence that the protocol is effective in the finite temperature regime. }, doi = {10.1080/09500340408231798}, url = {http://arxiv.org/abs/quant-ph/0403052v2}, author = {Guido Pupillo and Ana Maria Rey and Gavin Brennen and Carl J. Williams and Charles W. Clark} } @article {1289, title = {Ultracold collision properties of metastable alkaline-earth atoms}, journal = {Physical Review Letters}, volume = {90}, year = {2003}, month = {2003/2/13}, abstract = { Ultra-cold collisions of spin-polarized 24Mg,40Ca, and 88Sr in the metastable 3P2 excited state are investigated. We calculate the long-range interaction potentials and estimate the scattering length and the collisional loss rate as a function of magnetic field. The estimates are based on molecular potentials between 3P2 alkaline-earth atoms obtained from ab initio atomic and molecular structure calculations. The scattering lengths show resonance behavior due to the appearance of a molecular bound state in a purely long-range interaction potential and are positive for magnetic fields below 50 mT. A loss-rate model shows that losses should be smallest near zero magnetic field and for fields slightly larger than the resonance field, where the scattering length is also positive. }, doi = {10.1103/PhysRevLett.90.063002}, url = {http://arxiv.org/abs/physics/0210076v1}, author = {Andrei Derevianko and Sergey G. Porsev and Svetlana Kotochigova and Eite Tiesinga and Paul S. Julienne} } @article {1248, title = {Exact sampling from non-attractive distributions using summary states}, journal = {Physical Review E}, volume = {63}, year = {2001}, month = {2001/2/22}, abstract = { Propp and Wilson{\textquoteright}s method of coupling from the past allows one to efficiently generate exact samples from attractive statistical distributions (e.g., the ferromagnetic Ising model). This method may be generalized to non-attractive distributions by the use of summary states, as first described by Huber. Using this method, we present exact samples from a frustrated antiferromagnetic triangular Ising model and the antiferromagnetic q=3 Potts model. We discuss the advantages and limitations of the method of summary states for practical sampling, paying particular attention to the slowing down of the algorithm at low temperature. In particular, we show that such a slowing down can occur in the absence of a physical phase transition. }, doi = {10.1103/PhysRevE.63.036113}, url = {http://arxiv.org/abs/cond-mat/0005132v1}, author = {Andrew M. Childs and Ryan B. Patterson and David J. C. MacKay} } @article {1209, title = {Robustness of adiabatic quantum computation}, journal = {Physical Review A}, volume = {65}, year = {2001}, month = {2001/12/14}, abstract = { We study the fault tolerance of quantum computation by adiabatic evolution, a quantum algorithm for solving various combinatorial search problems. We describe an inherent robustness of adiabatic computation against two kinds of errors, unitary control errors and decoherence, and we study this robustness using numerical simulations of the algorithm. }, doi = {10.1103/PhysRevA.65.012322}, url = {http://arxiv.org/abs/quant-ph/0108048v1}, author = {Andrew M. Childs and Edward Farhi and John Preskill} } @article {1236, title = {Quantum information and precision measurement}, journal = {Journal of Modern Optics}, volume = {47}, year = {2000}, month = {1999/04/07}, pages = {155 - 176}, abstract = { We describe some applications of quantum information theory to the analysis of quantum limits on measurement sensitivity. A measurement of a weak force acting on a quantum system is a determination of a classical parameter appearing in the master equation that governs the evolution of the system; limitations on measurement accuracy arise because it is not possible to distinguish perfectly among the different possible values of this parameter. Tools developed in the study of quantum information and computation can be exploited to improve the precision of physics experiments; examples include superdense coding, fast database search, and the quantum Fourier transform. }, doi = {10.1080/09500340008244034}, url = {http://arxiv.org/abs/quant-ph/9904021v2}, author = {Andrew M. Childs and John Preskill and Joseph Renes} }