@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 {3396, title = {Data Needs and Challenges of Quantum Dot Devices Automation: Workshop Report}, year = {2023}, month = {12/21/2023}, abstract = {

Gate-defined quantum dots are a promising candidate system to realize scalable, coupled qubit systems and serve as a fundamental building block for quantum computers. However, present-day quantum dot devices suffer from imperfections that must be accounted for, which hinders the characterization, tuning, and operation process. Moreover, with an increasing number of quantum dot qubits, the relevant parameter space grows sufficiently to make heuristic control infeasible. Thus, it is imperative that reliable and scalable autonomous tuning approaches are developed. In this report, we outline current challenges in automating quantum dot device tuning and operation with a particular focus on datasets, benchmarking, and standardization. We also present ideas put forward by the quantum dot community on how to overcome them.

}, doi = {https://doi.org/10.48550/arXiv.2312.14322}, url = {https://arxiv.org/abs/2312.14322}, author = {Justyna P. Zwolak and Jacob M. Taylor and Reed Andrews and Jared Benson and Garnett Bryant and Donovan Buterakos and Anasua Chatterjee and Sankar Das Sarma and Mark A. Eriksson and Eli{\v s}ka Greplov{\'a} and Michael J. Gullans and Fabian Hader and Tyler J. Kovach and Pranav S. Mundada and Mick Ramsey and Torbjoern Rasmussen and Brandon Severin and Anthony Sigillito and Brennan Undseth and Brian Weber} } @article {3421, title = {Digital quantum simulation of NMR experiments}, journal = {Science Advances}, volume = {9}, year = {2023}, month = {11/29/2023}, abstract = {

Simulations of nuclear magnetic resonance (NMR) experiments can be an important tool for extracting information about molecular structure and optimizing experimental protocols but are often intractable on classical computers for large molecules such as proteins and for protocols such as zero-field NMR. We demonstrate the first quantum simulation of an NMR spectrum, computing the zero-field spectrum of the methyl group of acetonitrile using four qubits of a trapped-ion quantum computer. We reduce the sampling cost of the quantum simulation by an order of magnitude using compressed sensing techniques. We show how the intrinsic decoherence of NMR systems may enable the zero-field simulation of classically hard molecules on relatively near-term quantum hardware and discuss how the experimentally demonstrated quantum algorithm can be used to efficiently simulate scientifically and technologically relevant solid-state NMR experiments on more mature devices. Our work opens a practical application for quantum computation.

}, issn = {2375-2548}, doi = {10.1126/sciadv.adh2594}, url = {https://arxiv.org/abs/2109.13298}, author = {Seetharam, Kushal and Biswas, Debopriyo and Noel, Crystal and Risinger, Andrew and Zhu, Daiwei and Katz, Or and Chattopadhyay, Sambuddha and Cetina, Marko and Monroe, Christopher and Demler, Eugene and Sels, Dries} } @article {3265, title = {Efficient learning of ground \& thermal states within phases of matter}, year = {2023}, month = {1/30/2023}, abstract = {

We consider two related tasks: (a) estimating a parameterisation of a given Gibbs state and expectation values of Lipschitz observables on this state; and (b) learning the expectation values of local observables within a thermal or quantum phase of matter. In both cases, we wish to minimise the number of samples we use to learn these properties to a given precision.
For the first task, we develop new techniques to learn parameterisations of classes of systems, including quantum Gibbs states of non-commuting Hamiltonians with exponential decay of correlations and the approximate Markov property. We show it is possible to infer the expectation values of all extensive properties of the state from a number of copies that not only scales polylogarithmically with the system size, but polynomially in the observable\&$\#$39;s locality -- an exponential improvement. This set of properties includes expected values of quasi-local observables and entropies.\ \ For the second task, we develop efficient algorithms for learning observables in a phase of matter of a quantum system. By exploiting the locality of the Hamiltonian, we show that M local observables can be learned with probability 1\−δ to precision ϵ with using only N=O(log(Mδ)epolylog(ϵ\−1)) samples -- an exponential improvement on the precision over previous bounds. Our results apply to both families of ground states of Hamiltonians displaying local topological quantum order, and thermal phases of matter with exponential decay of correlations. In addition, our sample complexity applies to the worse case setting whereas previous results only applied on average.
Furthermore, we develop tools of independent interest, such as robust shadow tomography algorithms, Gibbs approximations to ground states, and generalisations of transportation cost inequalities for Gibbs states.

}, url = {https://arxiv.org/abs/2301.12946}, author = {Emilio Onorati and Cambyse Rouz{\'e} and Daniel Stilck Fran{\c c}a and James D. Watson} } @article {3316, title = {Experimental Observation of Thermalization with Noncommuting Charges}, journal = {PRX Quantum}, volume = {4}, year = {2023}, month = {4/28/2023}, abstract = {

Quantum simulators have recently enabled experimental observations of quantum many-body systems\&$\#$39; internal thermalization. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number - in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6-21 spins in an approximate microcanonical subspace, a generalization of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivial values simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, whose predictions can now be tested.

}, doi = {10.1103/prxquantum.4.020318}, url = {https://arxiv.org/abs/2202.04652}, author = {Florian Kranzl and Aleksander Lasek and Manoj K. Joshi and Amir Kalev and Rainer Blatt and Christian F. Roos and Nicole Yunger Halpern} } @article {3408, title = {A general approach to backaction-evading receivers with magnetomechanical and electromechanical sensors}, year = {2023}, month = {11/16/2023}, abstract = {

Today\&$\#$39;s mechanical sensors are capable of detecting extremely weak perturbations while operating near the standard quantum limit. However, further improvements can be made in both sensitivity and bandwidth when we reduce the noise originating from the process of measurement itself -- the quantum-mechanical backaction of measurement -- and go below this \&$\#$39;standard\&$\#$39; limit, possibly approaching the Heisenberg limit. One of the ways to eliminate this noise is by measuring a quantum nondemolition variable such as the momentum in a free-particle system. Here, we propose and characterize theoretical models for direct velocity measurement that utilize traditional electric and magnetic transducer designs to generate a signal while enabling this backaction evasion. We consider the general readout of this signal via electric or magnetic field sensing by creating toy models analogous to the standard optomechanical position-sensing problem, thereby facilitating the assessment of measurement-added noise. Using simple models that characterize a wide range of transducers, we find that the choice of readout scheme -- voltage or current -- for each mechanical detector configuration implies access to either the position or velocity of the mechanical sub-system. This in turn suggests a path forward for key fundamental physics experiments such as the direct detection of dark matter particles.

}, url = {https://arxiv.org/abs/2311.09587}, author = {Brittany Richman and Sohitri Ghosh and Daniel Carney and Gerard Higgins and Peter Shawhan and C. J. Lobb 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 {3401, title = {Logical quantum processor based on reconfigurable atom arrays}, journal = {Nature}, year = {2023}, month = {12/7/2023}, issn = {1476-4687}, doi = {10.1038/s41586-023-06927-3}, url = {https://arxiv.org/abs/2312.03982}, author = {Bluvstein, Dolev and Evered, Simon J. and Geim, Alexandra A. and Li, Sophie H. and Zhou, Hengyun and Manovitz, Tom and Ebadi, Sepehr and Cain, Madelyn and Kalinowski, Marcin and Hangleiter, Dominik and Ataides, J. Pablo Bonilla and Maskara, Nishad and Cong, Iris and Gao, Xun and Rodriguez, Pedro Sales and Karolyshyn, Thomas and Semeghini, Giulia and Gullans, Michael J. and Greiner, Markus and Vuletic, Vladan and Lukin, Mikhail D.} } @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 {3427, title = {Provably Efficient Learning of Phases of Matter via Dissipative Evolutions}, year = {2023}, month = {11/13/2023}, abstract = {

The combination of quantum many-body and machine learning techniques has recently proved to be a fertile ground for new developments in quantum computing. Several works have shown that it is possible to classically efficiently predict the expectation values of local observables on all states within a phase of matter using a machine learning algorithm after learning from data obtained from other states in the same phase. However, existing results are restricted to phases of matter such as ground states of gapped Hamiltonians and Gibbs states that exhibit exponential decay of correlations. In this work, we drop this requirement and show how it is possible to learn local expectation values for all states in a phase, where we adopt the Lindbladian phase definition by Coser \& P{\'e}rez-Garc{\'\i}a [Coser \& P{\'e}rez-Garc{\'\i}a, Quantum 3, 174 (2019)], which defines states to be in the same phase if we can drive one to other rapidly with a local Lindbladian. This definition encompasses the better-known Hamiltonian definition of phase of matter for gapped ground state phases, and further applies to any family of states connected by short unitary circuits, as well as non-equilibrium phases of matter, and those stable under external dissipative interactions. Under this definition, we show that N=O(log(n/δ)2polylog(1/ϵ)) samples suffice to learn local expectation values within a phase for a system with n qubits, to error ϵ with failure probability δ. This sample complexity is comparable to previous results on learning gapped and thermal phases, and it encompasses previous results of this nature in a unified way. Furthermore, we also show that we can learn families of states which go beyond the Lindbladian definition of phase, and we derive bounds on the sample complexity which are dependent on the mixing time between states under a Lindbladian evolution.

}, url = {arXiv:2311.07506 Search...}, author = {Emilio Onorati and Cambyse Rouz{\'e} and Daniel Stilck Fran{\c c}a and James D. Watson} } @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 {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 {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 {3128, title = {CoVault: A Secure Analytics Platform}, year = {2022}, month = {8/7/2022}, abstract = {

In a secure analytics platform, data sources consent to the exclusive use of their data for a pre-defined set of analytics queries performed by a specific group of analysts, and for a limited period. If the platform is secure under a sufficiently strong threat model, it can provide the missing link to enabling powerful analytics of sensitive personal data, by alleviating data subjects\&$\#$39; concerns about leakage and misuse of data. For instance, many types of powerful analytics that benefit public health, mobility, infrastructure, finance, or sustainable energy can be made differentially private, thus alleviating concerns about privacy. However, no platform currently exists that is sufficiently secure to alleviate concerns about data leakage and misuse; as a result, many types of analytics that would be in the interest of data subjects and the public are not done. CoVault uses a new multi-party implementation of functional encryption (FE) for secure analytics, which relies on a unique combination of secret sharing, multi-party secure computation (MPC), and different trusted execution environments (TEEs). CoVault is secure under a very strong threat model that tolerates compromise and side-channel attacks on any one of a small set of parties and their TEEs. Despite the cost of MPC, we show that CoVault scales to very large data sizes using map-reduce based query parallelization. For example, we show that CoVault can perform queries relevant to epidemic analytics at scale.

}, keywords = {and Cluster Computing (cs.DC), Cryptography and Security (cs.CR), Distributed, FOS: Computer and information sciences, Parallel}, doi = {10.48550/ARXIV.2208.03784}, url = {https://arxiv.org/abs/2208.03784}, author = {De Viti, Roberta and Sheff, Isaac and Glaeser, Noemi and Dinis, Baltasar and Rodrigues, Rodrigo and Katz, Jonathan and Bhattacharjee, Bobby and Hithnawi, Anwar and Garg, Deepak and Druschel, Peter} } @article {3140, title = {Demonstration of three- and four-body interactions between trapped-ion spins}, year = {2022}, month = {9/12/2022}, abstract = {

Quantum processors use the native interactions between effective spins to simulate Hamiltonians or execute quantum gates. In most processors, the native interactions are pairwise, limiting the efficiency of controlling entanglement between many qubits. Here we experimentally demonstrate a new class of native interactions between trapped-ion qubits, extending conventional pairwise interactions to higher order. We realize three- and four-body spin interactions as examples, showing that high-order spin polynomials may serve as a new toolbox for quantum information applications.

}, keywords = {Atomic Physics (physics.atom-ph), FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2209.05691}, url = {https://arxiv.org/abs/2209.05691}, author = {Katz, Or and Feng, Lei and Risinger, Andrew and Monroe, Christopher and Cetina, Marko} } @article {3139, title = {Experimental Implementation of an Efficient Test of Quantumness}, year = {2022}, month = {9/28/2022}, abstract = {

A test of quantumness is a protocol where a classical user issues challenges to a quantum device to determine if it exhibits non-classical behavior, under certain cryptographic assumptions. Recent attempts to implement such tests on current quantum computers rely on either interactive challenges with efficient verification, or non-interactive challenges with inefficient (exponential time) verification. In this paper, we execute an efficient non-interactive test of quantumness on an ion-trap quantum computer. Our results significantly exceed the bound for a classical device\&$\#$39;s success.

}, keywords = {FOS: Physical sciences, Other Condensed Matter (cond-mat.other), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2209.14316}, url = {https://arxiv.org/abs/2209.14316}, author = {Lewis, Laura and Zhu, Daiwei and Gheorghiu, Alexandru and Noel, Crystal and Katz, Or and Harraz, Bahaa and Wang, Qingfeng and Risinger, Andrew and Feng, Lei and Biswas, Debopriyo and Egan, Laird and Vidick, Thomas and Cetina, Marko and Monroe, Christopher} } @article {3002, title = {Experimental observation of thermalisation with noncommuting charges}, year = {2022}, month = {2/9/2022}, abstract = {

Quantum simulators have recently enabled experimental observations of quantum many-body systems\&$\#$39; internal thermalisation. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number - in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6-15 spins in an approximate microcanonical subspace, a generalisation of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivial values simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, whose predictions can now be tested.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph), Statistical Mechanics (cond-mat.stat-mech)}, doi = {10.48550/ARXIV.2202.04652}, url = {https://arxiv.org/abs/2202.04652}, author = {Kranzl, Florian and Lasek, Aleksander and Joshi, Manoj K. and Kalev, Amir and Blatt, Rainer and Roos, Christian F. and Nicole Yunger Halpern} } @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 {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 Davoudi, Zohreh 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 {3186, title = {Qunity: A Unified Language for Quantum and Classical Computing (Extended Version)}, year = {2022}, month = {4/26/2022}, abstract = {

We introduce Qunity, a new quantum programming language designed to treat quantum computing as a natural generalization of classical computing. Qunity presents a unified syntax where familiar programming constructs can have both quantum and classical effects. For example, one can use sum types to implement the direct sum of linear operators, exception-handling syntax to implement projective measurements, and aliasing to induce entanglement. Further, Qunity takes advantage of the overlooked BQP subroutine theorem, allowing one to construct reversible subroutines from irreversible quantum algorithms through the uncomputation of \"garbage\" outputs. Unlike existing languages that enable quantum aspects with separate add-ons (like a classical language with quantum gates bolted on), Qunity provides a unified syntax and a novel denotational semantics that guarantees that programs are quantum mechanically valid. We present Qunity\&$\#$39;s syntax, type system, and denotational semantics, showing how it can cleanly express several quantum algorithms. We also detail how Qunity can be compiled into a low-level qubit circuit language like OpenQASM, proving the realizability of our design.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Logic in Computer Science (cs.LO), Programming Languages (cs.PL), Quantum Physics (quant-ph)}, doi = {https://doi.org/10.48550/arXiv.2204.12384}, url = {https://arxiv.org/abs/2204.12384}, author = {Voichick, Finn and Li, Liyi and Rand, Robert and Hicks, Michael} } @article {3136, title = {Scalably learning quantum many-body Hamiltonians from dynamical data}, year = {2022}, month = {9/28/2022}, abstract = {

The physics of a closed quantum mechanical system is governed by its Hamiltonian. However, in most practical situations, this Hamiltonian is not precisely known, and ultimately all there is are data obtained from measurements on the system. In this work, we introduce a highly scalable, data-driven approach to learning families of interacting many-body Hamiltonians from dynamical data, by bringing together techniques from gradient-based optimization from machine learning with efficient quantum state representations in terms of tensor networks. Our approach is highly practical, experimentally friendly, and intrinsically scalable to allow for system sizes of above 100 spins. In particular, we demonstrate on synthetic data that the algorithm works even if one is restricted to one simple initial state, a small number of single-qubit observables, and time evolution up to relatively short times. For the concrete example of the one-dimensional Heisenberg model our algorithm exhibits an error constant in the system size and scaling as the inverse square root of the size of the data set.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Machine Learning (cs.LG), Quantum Gases (cond-mat.quant-gas), Quantum Physics (quant-ph), Strongly Correlated Electrons (cond-mat.str-el)}, doi = {10.48550/ARXIV.2209.14328}, url = {https://arxiv.org/abs/2209.14328}, author = {Wilde, Frederik and Kshetrimayum, Augustine and Roth, Ingo and Hangleiter, Dominik and Sweke, Ryan and Eisert, Jens} } @article {3009, title = {Self-Testing of a Single Quantum System: Theory and Experiment}, year = {2022}, month = {3/17/2022}, abstract = {

Certifying individual quantum devices with minimal assumptions is crucial for the development of quantum technologies. Here, we investigate how to leverage single-system contextuality to realize self-testing. We develop a robust self-testing protocol based on the simplest contextuality witness for the simplest contextual quantum system, the Klyachko-Can-Binicio{\u g}lu-Shumovsky (KCBS) inequality for the qutrit. We establish a lower bound on the fidelity of the state and the measurements (to an ideal configuration) as a function of the value of the witness under a pragmatic assumption on the measurements we call the KCBS orthogonality condition. We apply the method in an experiment with randomly chosen measurements on a single trapped 40Ca+ and near-perfect detection efficiency. The observed statistics allow us to self-test the system and provide the first experimental demonstration of quantum self-testing of a single system. Further, we quantify and report that deviations from our assumptions are minimal, an aspect previously overlooked by contextuality experiments.

}, keywords = {Atomic Physics (physics.atom-ph), FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {https://doi.org/10.48550/arXiv.2203.09003}, url = {https://arxiv.org/abs/2203.09003}, author = {Hu, Xiao-Min and Xie, Yi and Arora, Atul Singh and Ai, Ming-Zhong and Bharti, Kishor and Zhang, Jie and Wu, Wei and Chen, Ping-Xing and Cui, Jin-Ming and Liu, Bi-Heng and Huang, Yun-Feng and Li, Chuan-Feng and Guo, Guang-Can and Roland, J{\'e}r{\'e}mie and Cabello, Ad{\'a}n and Kwek, Leong-Chuan} } @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 {3020, title = {Unlimited non-causal correlations and their relation to non-locality}, journal = {Quantum}, volume = {6}, year = {2022}, month = {3/22/2022}, pages = {673}, abstract = {

Non-causal correlations certify the lack of a definite causal order among localized space-time regions. In stark contrast to scenarios where a single region influences its own causal past, some processes that distribute non-causal correlations satisfy a series of natural desiderata: logical consistency, linear and reversible dynamics, and computational tameness. Here, we present such processes among arbitrary many regions where each region influences every other but itself, and show that the above desiderata are altogether insufficient to limit the amount of \"acausality\" of non-causal correlations. This leaves open the identification of a principle that forbids non-causal correlations. Our results exhibit qualitative and quantitative parallels with the non-local correlations due to Ardehali and Svetlichny.

}, doi = {https://doi.org/10.22331\%2Fq-2022-03-29-673}, url = {https://arxiv.org/abs/2104.06234}, author = {{\"A}min Baumeler and Amin Shiraz Gilani and Jibran Rashid} } @article {2684, title = {Circulation by microwave-induced vortex transport for signal isolation}, journal = {PRX Quantum}, volume = {2}, year = {2021}, month = {6/14/2021}, pages = {030309}, abstract = {

Magnetic fields break time-reversal symmetry, which is leveraged in many settings to enable the nonreciprocal behavior of light. This is the core physics of circulators and other elements used in a variety of microwave and optical settings. Commercial circulators in the microwave domain typically use ferromagnetic materials and wave interference, requiring large devices and large fields. However, quantum information devices for sensing and computation require small sizes, lower fields, and better on-chip integration. Equivalences to ferromagnetic order---such as the XY model---can be realized at much lower magnetic fields by using arrays of superconducting islands connected by Josephson junctions. Here we show that the quantum-coherent motion of a single vortex in such an array suffices to induce nonreciprocal behavior, enabling a small-scale, moderate-bandwidth, and low insertion loss circulator at very low magnetic fields and at microwave frequencies relevant for experiments with qubits.

}, doi = {https://doi.org/10.1103/PRXQuantum.2.030309}, url = {https://arxiv.org/abs/2010.04118}, author = {Brittany Richman and J. M. Taylor} } @article {2939, title = {Constructing quantum many-body scar Hamiltonians from Floquet automata}, year = {2021}, month = {12/22/2021}, abstract = {

We provide a systematic approach for constructing approximate quantum many-body scars(QMBS) starting from two-layer Floquet automaton circuits that exhibit trivial many-body revivals. We do so by applying successively more restrictions that force local gates of the automaton circuit to commute concomitantly more accurately when acting on select scar states. With these rules in place, an effective local, Floquet Hamiltonian is seen to capture dynamics of the automata over a long prethermal window, and neglected terms can be used to estimate the relaxation of revivals. We provide numerical evidence for such a picture and use our construction to derive several QMBS models, including the celebrated PXP model.

}, url = {https://arxiv.org/abs/2112.12153}, author = {Pierre-Gabriel Rozon and Michael Gullans and Kartiek Agarwal} } @article {2919, title = {Cross-Platform Comparison of Arbitrary Quantum Computations}, year = {2021}, month = {7/27/2021}, abstract = {

As we approach the era of quantum advantage, when quantum computers (QCs) can outperform any classical computer on particular tasks, there remains the difficult challenge of how to validate their performance. While algorithmic success can be easily verified in some instances such as number factoring or oracular algorithms, these approaches only provide pass/fail information for a single QC. On the other hand, a comparison between different QCs on the same arbitrary circuit provides a lower-bound for generic validation: a quantum computation is only as valid as the agreement between the results produced on different QCs. Such an approach is also at the heart of evaluating metrological standards such as disparate atomic clocks. In this paper, we report a cross-platform QC comparison using randomized and correlated measurements that results in a wealth of information on the QC systems. We execute several quantum circuits on widely different physical QC platforms and analyze the cross-platform fidelities.

}, url = {https://arxiv.org/abs/2107.11387}, author = {Daiwei Zhu and Ze-Pei Cian and Crystal Noel and Andrew Risinger and Debopriyo Biswas and Laird Egan and Yingyue Zhu and Alaina M. Green and Cinthia Huerta Alderete and Nhung H. Nguyen and Qingfeng Wang and Andrii Maksymov and Yunseong Nam and Marko Cetina and Norbert M. Linke and Mohammad Hafezi and Christopher Monroe} } @article {2815, title = {Crystallography of Hyperbolic Lattices}, year = {2021}, month = {5/3/2021}, abstract = {

Hyperbolic lattices are a revolutionary platform for tabletop simulations of holography and quantum physics in curved space and facilitate efficient quantum error correcting codes. Their underlying geometry is non-Euclidean, and the absence of Bloch\&$\#$39;s theorem precludes a simple understanding of their band structure. Motivated by recent insights into hyperbolic band theory, we initiate a crystallography of hyperbolic lattices. We show that many hyperbolic lattices feature a hidden crystal structure characterized by unit cells, hyperbolic Bravais lattices, and associated symmetry groups. Using the mathematical framework of higher-genus Riemann surfaces and Fuchsian groups, we derive, for the first time, a list of example hyperbolic {p,q} lattices and their hyperbolic Bravais lattices, including five infinite families and several graphs relevant for experiments in circuit quantum electrodynamics and topolectrical circuits. Our results find application for both finite and infinite hyperbolic lattices. We describe a method to efficiently generate finite hyperbolic lattices of arbitrary size and explain why the present crystallography is the first step towards a complete band theory of hyperbolic lattices and apply it to construct Bloch wave Hamiltonians. This work lays the foundation for generalizing some of the most powerful concepts of solid state physics, such as crystal momentum and Brillouin zone, to the emerging field of hyperbolic lattices and tabletop simulations of gravitational theories, and reveals the connections to concepts from topology and algebraic geometry.

}, url = {https://arxiv.org/abs/2105.01087}, author = {Igor Boettcher and Alexey V. Gorshkov and Alicia J. Koll{\'a}r and Joseph Maciejko and Steven Rayan and Ronny Thomale} } @article {2932, title = {Entangled quantum cellular automata, physical complexity, and Goldilocks rules}, journal = {Quantum Science and Technology}, volume = {6}, year = {2021}, month = {9/29/2021}, pages = {045017}, abstract = {

Cellular automata are interacting classical bits that display diverse emergent behaviors, from fractals to random-number generators to Turing-complete computation. We discover that quantum cellular automata (QCA) can exhibit complexity in the sense of the complexity science that describes biology, sociology, and economics. QCA exhibit complexity when evolving under \"Goldilocks rules\" that we define by balancing activity and stasis. Our Goldilocks rules generate robust dynamical features (entangled breathers), network structure and dynamics consistent with complexity, and persistent entropy fluctuations. Present-day experimental platforms -- Rydberg arrays, trapped ions, and superconducting qubits -- can implement our Goldilocks protocols, making testable the link between complexity science and quantum computation exposed by our QCA.

}, issn = {2058-9565}, doi = {10.1088/2058-9565/ac1c41}, url = {http://dx.doi.org/10.1088/2058-9565/ac1c41}, author = {Hillberry, Logan E and Jones, Matthew T and Vargas, David L and Rall, Patrick and Nicole Yunger Halpern and Bao, Ning and Notarnicola, Simone and Montangero, Simone and Carr, Lincoln D} } @article {2872, title = {Estimating distinguishability measures on quantum computers}, year = {2021}, month = {8/18/2021}, abstract = {

The performance of a quantum information processing protocol is ultimately judged by distinguishability measures that quantify how distinguishable the actual result of the protocol is from the ideal case. The most prominent distinguishability measures are those based on the fidelity and trace distance, due to their physical interpretations. In this paper, we propose and review several algorithms for estimating distinguishability measures based on trace distance and fidelity, and we evaluate their performance using simulators of quantum computers. The algorithms can be used for distinguishing quantum states, channels, and strategies (the last also known in the literature as \"quantum combs\"). The fidelity-based algorithms offer novel physical interpretations of these distinguishability measures in terms of the maximum probability with which a single prover (or competing provers) can convince a verifier to accept the outcome of an associated computation. We simulate these algorithms by using a variational approach with parameterized quantum circuits and find that they converge well for the examples that we consider.\ 

}, url = {https://arxiv.org/abs/2108.08406}, author = {Rochisha Agarwal and Soorya Rethinasamy and Kunal Sharma and Mark M. Wilde} } @conference {2941, title = {Expanding the VOQC Toolkit}, booktitle = {The Second International Workshop on Programming Languages for Quantum Computing (PLanQC 2021)}, year = {2021}, month = {06/2021}, abstract = {

voqc [Hietala et al. 2021b] (pronounced \“vox\”) is a compiler for quantum circuits, in the style of
tools like Qiskit [Aleksandrowicz et al. 2019], tket [Cambridge Quantum Computing Ltd 2019],
Quilc [Rigetti Computing 2019], and Cirq [Developers 2021]. What makes voqc different from these
tools is that it has been formally verified in the Coq proof assistant [Coq Development Team 2019].
voqc source programs are expressed in sqir, a simple quantum intermediate representation, which
has a precise mathematical semantics. We use Gallina, Coq\’s programming language, to implement
voqc transformations over sqir programs, and use Coq to prove the source program\’s semantics
are preserved. We then extract these Gallina definitions to OCaml, and compile the OCaml code to
a library that can operate on standard-formatted circuits.
voqc, and sqir, were built to be general-purpose. For example, while we originally designed sqir
for use in verified optimizations, we subsequently found sqir could also be suitable for writing, and
proving correct, source programs [Hietala et al. 2021a]. We have continued to develop the voqc
codebase to expand its reach and utility.
In this abstract, we present new extensions to voqc as an illustration of its flexibility. These
include support for calling voqc transformations from Python, added support for new gate sets
and optimizations, and the extension of our notion of correctness to include mapping-preservation,
which allows us to apply optimizations after mapping, reducing the cost introduced by making a
program conform to hardware constraints.

}, url = {http://rand.cs.uchicago.edu/files/planqc_2021c.pdf}, author = {Kesha Hietala and Liyi Li and Akshaj Gaur and Aaron Green and Robert Rand and Xiaodi Wu and Michael Hicks} } @article {3019, title = {Fully device-independent quantum key distribution using synchronous correlations}, year = {2021}, month = {10/27/2021}, abstract = {

We derive a device-independent quantum key distribution protocol based on synchronous correlations and their Bell inequalities. This protocol offers several advantages over other device-independent schemes including symmetry between the two users and no need for preshared randomness. We close a \"synchronicity\" loophole by showing that an almost synchronous correlation inherits the self-testing property of the associated synchronous correlation. We also pose a new security assumption that closes the \"locality\" (or \"causality\") loophole: an unbounded adversary with even a small uncertainty about the users\&$\#$39; choice of measurement bases cannot produce any almost synchronous correlation that approximately maximally violates a synchronous Bell inequality.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2110.14530}, url = {https://arxiv.org/abs/2110.14530}, author = {Rodrigues, Nishant and Lackey, Brad} } @article {2908, title = {Interactive Protocols for Classically-Verifiable Quantum Advantage}, year = {2021}, month = {12/9/2021}, abstract = {

Achieving quantum computational advantage requires solving a classically intractable problem on a quantum device. Natural proposals rely upon the intrinsic hardness of classically simulating quantum mechanics; however, verifying the output is itself classically intractable. On the other hand, certain quantum algorithms (e.g. prime factorization via Shor\&$\#$39;s algorithm) are efficiently verifiable, but require more resources than what is available on near-term devices. One way to bridge the gap between verifiability and implementation is to use \"interactions\" between a prover and a verifier. By leveraging cryptographic functions, such protocols enable the classical verifier to enforce consistency in a quantum prover\&$\#$39;s responses across multiple rounds of interaction. In this work, we demonstrate the first implementation of an interactive quantum advantage protocol, using an ion trap quantum computer. We execute two complementary protocols -- one based upon the learning with errors problem and another where the cryptographic construction implements a computational Bell test. To perform multiple rounds of interaction, we implement mid-circuit measurements on a subset of trapped ion qubits, with subsequent coherent evolution. For both protocols, the performance exceeds the asymptotic bound for classical behavior; maintaining this fidelity at scale would conclusively demonstrate verifiable quantum advantage.

}, url = {https://arxiv.org/abs/2112.05156}, author = {Daiwei Zhu and Gregory D. Kahanamoku-Meyer and Laura Lewis and Crystal Noel and Or Katz and Bahaa Harraz and Qingfeng Wang and Andrew Risinger and Lei Feng and Debopriyo Biswas and Laird Egan and Alexandru Gheorghiu and Yunseong Nam and Thomas Vidick and Umesh Vazirani and Norman Y. Yao and Marko Cetina and Christopher Monroe} } @article {2805, title = {Observation of measurement-induced quantum phases in a trapped-ion quantum computer}, year = {2021}, month = {6/10/2021}, abstract = {

Many-body open quantum systems balance internal dynamics against decoherence from interactions with an environment. Here, we explore this balance via random quantum circuits implemented on a trapped ion quantum computer, where the system evolution is represented by unitary gates with interspersed projective measurements. As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point akin to a fault-tolerent threshold. We probe the \"pure\" phase, where the system is rapidly projected to a deterministic state conditioned on the measurement outcomes, and the \"mixed\" or \"coding\" phase, where the initial state becomes partially encoded into a quantum error correcting codespace. We find convincing evidence of the two phases and show numerically that, with modest system scaling, critical properties of the transition clearly emerge.

}, url = {https://arxiv.org/abs/2106.05881}, author = {Crystal Noel and Pradeep Niroula and Daiwei Zhu and Andrew Risinger and Laird Egan and Debopriyo Biswas and Marko Cetina and Alexey V. Gorshkov and Michael Gullans and David A. Huse and Christopher Monroe} } @article {2827, title = {Precise Hamiltonian identification of a superconducting quantum processor}, year = {2021}, month = {8/18/2021}, abstract = {

The required precision to perform quantum simulations beyond the capabilities of classical computers imposes major experimental and theoretical challenges. Here, we develop a characterization technique to benchmark the implementation precision of a specific quantum simulation task. We infer all parameters of the bosonic Hamiltonian that governs the dynamics of excitations in a two-dimensional grid of nearest-neighbour coupled superconducting qubits. We devise a robust algorithm for identification of Hamiltonian parameters from measured times series of the expectation values of single-mode canonical coordinates. Using super-resolution and denoising methods, we first extract eigenfrequencies of the governing Hamiltonian from the complex time domain measurement; next, we recover the eigenvectors of the Hamiltonian via constrained manifold optimization over the orthogonal group. For five and six coupled qubits, we identify Hamiltonian parameters with sub-MHz precision and construct a spatial implementation error map for a grid of 27 qubits. Our approach enables us to distinguish and quantify the effects of state preparation and measurement errors and show that they are the dominant sources of errors in the implementation. Our results quantify the implementation accuracy of analog dynamics and introduce a diagnostic toolkit for understanding, calibrating, and improving analog quantum processors.

}, url = {https://arxiv.org/abs/2108.08319}, author = {Dominik Hangleiter and Ingo Roth and Jens Eisert and Pedram Roushan} } @article {2729, title = {Proving Quantum Programs Correct}, journal = {Schloss Dagstuhl}, year = {2021}, month = {7/13/2021}, abstract = {

As quantum computing steadily progresses from theory to practice, programmers are faced with a common problem: How can they be sure that their code does what they intend it to do? This paper presents encouraging results in the application of mechanized proof to the domain of quantum programming in the context of the SQIR development. It verifies the correctness of a range of a quantum algorithms including Simon\&$\#$39;s algorithm, Grover\&$\#$39;s algorithm, and quantum phase estimation, a key component of Shor\&$\#$39;s algorithm. In doing so, it aims to highlight both the successes and challenges of formal verification in the quantum context and motivate the theorem proving community to target quantum computing as an application domain.

}, doi = {https://doi.org/10.4230/LIPIcs.ITP.2021.21}, url = {https://arxiv.org/abs/2010.01240}, author = {Kesha Hietala and Robert Rand and Shih-Han Hung and Liyi Li and Michael Hicks} } @article {2942, title = {Proving Quantum Programs Correct}, journal = {12th International Conference on Interactive Theorem Proving (ITP 2021)}, volume = {193}, year = {2021}, month = {06/2021}, pages = {21:1{\textendash}21:19}, abstract = {

As quantum computing progresses steadily from theory into practice, programmers will face
a common problem: How can they be sure that their code does what they intend it to do? This
paper presents encouraging results in the application of mechanized proof to the domain of quantum
programming in the context of the sqir development. It verifies the correctness of a range of a
quantum algorithms including Grover\’s algorithm and quantum phase estimation, a key component
of Shor\’s algorithm. In doing so, it aims to highlight both the successes and challenges of formal
verification in the quantum context and motivate the theorem proving community to target quantum
computing as an application domain.

}, isbn = {978-3-95977-188-7}, issn = {1868-8969}, doi = {10.4230/LIPIcs.ITP.2021.21}, url = {https://drops.dagstuhl.de/opus/volltexte/2021/13916}, author = {Hietala, Kesha and Rand, Robert and Hung, Shih-Han and Li, Liyi and Hicks, Michael}, editor = {Cohen, Liron and Kaliszyk, Cezary} } @article {3022, title = {Quantum Algorithms for Reinforcement Learning with a Generative Model}, journal = {Proceedings of the 38th International Conference on Machine Learning, PMLR}, volume = {139}, year = {2021}, month = {12/15/2021}, chapter = {10916-10926}, abstract = {

Reinforcement learning studies how an agent should interact with an environment to maximize its cumulative reward. A standard way to study this question abstractly is to ask how many samples an agent needs from the environment to learn an optimal policy for a γ-discounted Markov decision process (MDP). For such an MDP, we design quantum algorithms that approximate an optimal policy (π\∗), the optimal value function (v\∗), and the optimal Q-function (q\∗), assuming the algorithms can access samples from the environment in quantum superposition. This assumption is justified whenever there exists a simulator for the environment; for example, if the environment is a video game or some other program. Our quantum algorithms, inspired by value iteration, achieve quadratic speedups over the best-possible classical sample complexities in the approximation accuracy (ϵ) and two main parameters of the MDP: the effective time horizon (11\−γ) and the size of the action space (A). Moreover, we show that our quantum algorithm for computing q\∗ is optimal by proving a matching quantum lower bound.

}, keywords = {FOS: Computer and information sciences, FOS: Physical sciences, Machine Learning (cs.LG), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2112.08451}, url = {https://arxiv.org/abs/2112.08451}, author = {Daochen Wang and Sundaram, Aarthi and Kothari, Robin and Kapoor, Ashish and Roetteler, Martin} } @article {2885, title = {Quantum Lattice Sieving}, year = {2021}, month = {10/26/2021}, abstract = {

Lattices are very important objects in the effort to construct cryptographic primitives that are secure against quantum attacks. A central problem in the study of lattices is that of finding the shortest non-zero vector in the lattice. Asymptotically, sieving is the best known technique for solving the shortest vector problem, however, sieving requires memory exponential in the dimension of the lattice. As a consequence, enumeration algorithms are often used in place of sieving due to their linear memory complexity, despite their super-exponential runtime. In this work, we present a heuristic quantum sieving algorithm that has memory complexity polynomial in the size of the length of the sampled vectors at the initial step of the sieve. In other words, unlike most sieving algorithms, the memory complexity of our algorithm does not depend on the number of sampled vectors at the initial step of the sieve.

}, url = {https://arxiv.org/abs/2110.13352}, author = {Nishant Rodrigues and Brad Lackey} } @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 {3025, title = {Relaxation of non-integrable systems and correlation functions}, year = {2021}, month = {12/17/2021}, abstract = {

We investigate early-time equilibration rates of observables in closed many-body quantum systems and compare them to those of two correlation functions, first introduced by Kubo and Srednicki. We explore whether these different rates coincide at a universal value that sets the timescales of processes at a finite energy density. We find evidence for this coincidence when the initial conditions are sufficiently generic, or typical. We quantify this with the effective dimension of the state and with a state-observable effective dimension, which estimate the number of energy levels that participate in the dynamics. Our findings are confirmed by proving that these different timescales coincide for dynamics generated by Haar-random Hamiltonians. This also allows to quantitatively understand the scope of previous theoretical results on equilibration timescales and on random matrix formalisms. We approach this problem with exact, full spectrum diagonalization. The numerics are carried out in a non-integrable Heisenberg-like Hamiltonian, and the dynamics are investigated for several pairs of observables and states.

}, keywords = {FOS: Physical sciences, Quantum Physics (quant-ph), Statistical Mechanics (cond-mat.stat-mech), Strongly Correlated Electrons (cond-mat.str-el)}, doi = {10.48550/ARXIV.2112.09475}, url = {https://arxiv.org/abs/2112.09475}, author = {Riddell, Jonathon and Garc{\'\i}a-Pintos, Luis Pedro and Alhambra, {\'A}lvaro M.} } @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 {2491, title = {A Verified Optimizer for Quantum Circuits}, journal = {Proceedings of the ACM on Programming Languages}, volume = {5}, year = {2021}, month = {11/12/2020}, abstract = {

We present VOQC, the first fully verified compiler for quantum circuits, written using the Coq proof assistant. Quantum circuits are expressed as programs in a simple, low-level language called SQIR, which is deeply embedded in Coq. Optimizations and other transformations are expressed as Coq functions, which are proved correct with respect to a semantics of SQIR programs. We evaluate VOQC\&$\#$39;s verified optimizations on a series of benchmarks, and it performs comparably to industrial-strength compilers. VOQC\&$\#$39;s optimizations reduce total gate counts on average by 17.7\% on a benchmark of 29 circuit programs compared to a 10.7\% reduction when using IBM\&$\#$39;s Qiskit compiler.

}, doi = {https://doi.org/10.1145/3434318}, url = {https://arxiv.org/abs/1912.02250}, author = {Kesha Hietala and Robert Rand and Shih-Han Hung and Xiaodi Wu and Michael Hicks} } @article {2627, title = {Efficient Simulation of Random States and Random Unitaries}, journal = {In: Canteaut A., Ishai Y. (eds) Advances in Cryptology {\textendash} EUROCRYPT 2020. Lecture Notes in Computer Science, Springer, Cham}, volume = {12107}, year = {2020}, month = {5/1/2020}, pages = {759-787}, type = {inproceedings}, abstract = {

We consider the problem of efficiently simulating random quantum states and random unitary operators, in a manner which is convincing to unbounded adversaries with black-box oracle access.

This problem has previously only been considered for restricted adversaries. Against adversaries with an a priori bound on the number of queries, it is well-known that t-designs suffice. Against polynomial-time adversaries, one can use pseudorandom states (PRS) and pseudorandom unitaries (PRU), as defined in a recent work of Ji, Liu, and Song; unfortunately, no provably secure construction is known for PRUs.

In our setting, we are concerned with unbounded adversaries. Nonetheless, we are able to give stateful quantum algorithms which simulate the ideal object in both settings of interest. In the case of Haar-random states, our simulator is polynomial-time, has negligible error, and can also simulate verification and reflection through the simulated state. This yields an immediate application to quantum money: a money scheme which is information-theoretically unforgeable and untraceable. In the case of Haar-random unitaries, our simulator takes polynomial space, but simulates both forward and inverse access with zero error.

These results can be seen as the first significant steps in developing a theory of lazy sampling for random quantum objects.

}, doi = {https://doi.org/10.1007/978-3-030-45727-3_26}, author = {Gorjan Alagic and Christian Majenz and Alexander Russell} } @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 {2686, title = {Fault-Tolerant Operation of a Quantum Error-Correction Code}, year = {2020}, month = {9/24/2020}, abstract = {

Quantum error correction protects fragile quantum information by encoding it in a larger quantum system whose extra degrees of freedom enable the detection and correction of errors. An encoded logical qubit thus carries increased complexity compared to a bare physical qubit. Fault-tolerant protocols contain the spread of errors and are essential for realizing error suppression with an error-corrected logical qubit. Here we experimentally demonstrate fault-tolerant preparation, rotation, error syndrome extraction, and measurement on a logical qubit encoded in the 9-qubit Bacon-Shor code. For the logical qubit, we measure an average fault-tolerant preparation and measurement error of 0.6\% and a transversal Clifford gate with an error of 0.3\% after error correction. The result is an encoded logical qubit whose logical fidelity exceeds the fidelity of the entangling operations used to create it. We compare these operations with non-fault-tolerant protocols capable of generating arbitrary logical states, and observe the expected increase in error. We directly measure the four Bacon-Shor stabilizer generators and are able to detect single qubit Pauli errors. These results show that fault-tolerant quantum systems are currently capable of logical primitives with error rates lower than their constituent parts. With the future addition of intermediate measurements, the full power of scalable quantum error-correction can be achieved.\ 

}, url = {https://arxiv.org/abs/2009.11482}, author = {Laird Egan and Dripto M. Debroy and Crystal Noel and Andrew Risinger and Daiwei Zhu and Debopriyo Biswas and Michael Newman and Muyuan Li and Kenneth R. Brown and Marko Cetina 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 {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 {2635, title = {Optical quantum memory with optically inaccessible noble-gas spins}, year = {2020}, month = {7/17/2020}, abstract = {

Optical quantum memories, which store and preserve the quantum state of photons, rely on a coherent mapping of the photonic state onto matter states that are optically accessible. Here we outline a new physical mechanism to map the state of photons onto the long-lived but optically inaccessible collective state of noble-gas spins. The mapping employs the coherent spin-exchange interaction arising from random collisions with alkali vapor. We analyze optimal strategies for high-efficiency storage and retrieval of non-classical light at various parameter regimes. Based on these strategies, we identify feasible experimental conditions for realizing efficient quantum memories with noble-gas spins having hours-long coherence times at room temperature and above

}, url = {https://arxiv.org/abs/2007.08770}, author = {Or Katz and Eran Reches and Roy Shaham and Alexey V. Gorshkov and Ofer Firstenberg} } @article {2515, title = {Optimal Two-Qubit Circuits for Universal Fault-Tolerant Quantum Computation}, year = {2020}, month = {1/16/2020}, abstract = {

We study two-qubit circuits over the Clifford+CS gate set which consists of Clifford gates together with the controlled-phase gate CS=diag(1,1,1,i). The Clifford+CS gate set is universal for quantum computation and its elements can be implemented fault-tolerantly in most error-correcting schemes with magic state distillation. However, since non-Clifford gates are typically more expensive to perform in a fault-tolerant manner, it is desirable to construct circuits that use few CS gates. In the present paper, we introduce an algorithm to construct optimal circuits for two-qubit Clifford+CS operators. Our algorithm inputs a Clifford+CS operator U and efficiently produces a Clifford+CS circuit for U using the least possible number of CS gates. Because our algorithm is deterministic, the circuit it associates to a Clifford+CS operator can be viewed as a normal form for the operator. We give a formal description of these normal forms as walks over certain graphs and use this description to derive an asymptotic lower bound of 5log(1/epsilon)+O(1) on the number CS gates required to epsilon-approximate any 4x4 unitary matrix.\ 

}, url = {https://arxiv.org/abs/2001.05997}, author = {Andrew N. Glaudell and Neil J. Ross and J. M. Taylor} } @article {2557, title = {Quantum Coupon Collector}, journal = {Proceedings of the 15th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2020), Leibniz International Proceedings in Informatics}, volume = {158}, year = {2020}, month = {2/18/2020}, pages = {10:1-10:17}, abstract = {

We study how efficiently a k-element set S\⊆[n] can be learned from a uniform superposition |S\⟩ of its elements. One can think of |S\⟩=\∑i\∈S|i\⟩/|S|\−\−\−\√ as the quantum version of a uniformly random sample over S, as in the classical analysis of the {\textquoteleft}{\textquoteleft}coupon collector problem.\&$\#$39;\&$\#$39; We show that if k is close to n, then we can learn S using asymptotically fewer quantum samples than random samples. In particular, if there are n\−k=O(1) missing elements then O(k) copies of |S\⟩ suffice, in contrast to the Θ(klogk) random samples needed by a classical coupon collector. On the other hand, if n\−k=Ω(k), then Ω(klogk) quantum samples are~necessary. More generally, we give tight bounds on the number of quantum samples needed for every k and n, and we give efficient quantum learning algorithms. We also give tight bounds in the model where we can additionally reflect through |S\⟩. Finally, we relate coupon collection to a known example separating proper and improper PAC learning that turns out to show no separation in the quantum case.

}, doi = {10.4230/LIPIcs.TQC.2020.10}, url = {https://arxiv.org/abs/2002.07688}, author = {Srinivasan Arunachalam and Aleksandrs Belovs and Andrew M. Childs and Robin Kothari and Ansis Rosmanis and Ronald de Wolf} } @article {2628, title = {Quantum-Access-Secure Message Authentication via Blind-Unforgeability}, journal = {In: Canteaut A., Ishai Y. (eds) Advances in Cryptology {\textendash} EUROCRYPT 2020. Lecture Notes in Computer Science, Springer, Cham}, volume = {12-17}, year = {2020}, month = {5/1/2020}, pages = {788-817 }, type = {inproceedings}, abstract = {

Formulating and designing authentication of classical messages in the presence of adversaries with quantum query access has been a longstanding challenge, as the familiar classical notions of unforgeability do not directly translate into meaningful notions in the quantum setting. A particular difficulty is how to fairly capture the notion of \“predicting an unqueried value\” when the adversary can query in quantum superposition.

We propose a natural definition of unforgeability against quantum adversaries called blind unforgeability. This notion defines a function to be predictable if there exists an adversary who can use \“partially blinded\” oracle access to predict values in the blinded region. We support the proposal with a number of technical results. We begin by establishing that the notion coincides with EUF-CMA in the classical setting and go on to demonstrate that the notion is satisfied by a number of simple guiding examples, such as random functions and quantum-query-secure pseudorandom functions. We then show the suitability of blind unforgeability for supporting canonical constructions and reductions. We prove that the \“hash-and-MAC\” paradigm and the Lamport one-time digital signature scheme are indeed unforgeable according to the definition. To support our analysis, we additionally define and study a new variety of quantum-secure hash functions called Bernoulli-preserving.

Finally, we demonstrate that blind unforgeability is strictly stronger than a previous definition of Boneh and Zhandry [EUROCRYPT \’13, CRYPTO \’13] and resolve an open problem concerning this previous definition by constructing an explicit function family which is forgeable yet satisfies the definition.

}, doi = {https://doi.org/10.1007/978-3-030-45727-3_27}, author = {Gorjan Alagic and Christian Majenz and Alexander Russell and Fang Song} } @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 {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} } @article {2519, title = {Time evolution of correlation functions in quantum many-body systems}, journal = {Phys. Rev. Lett}, volume = {124}, year = {2020}, month = {3/19/2020}, abstract = {

We give rigorous analytical results on the temporal behavior of two-point correlation functions --also known as dynamical response functions or Green\&$\#$39;s functions-- in closed many-body quantum systems. We show that in a large class of translation-invariant models the correlation functions factorize at late times \⟨A(t)B\⟩β\→\⟨A\⟩β\⟨B\⟩β, thus proving that dissipation emerges out of the unitary dynamics of the system. We also show that for systems with a generic spectrum the fluctuations around this late-time value are bounded by the purity of the thermal ensemble, which generally decays exponentially with system size. For auto-correlation functions we provide an upper bound on the timescale at which they reach the factorized late time value. Remarkably, this bound is only a function of local expectation values, and does not increase with system size. We give numerical examples that show that this bound is a good estimate in non-integrable models, and argue that the timescale that appears can be understood in terms of an emergent fluctuation-dissipation theorem. Our study extends to further classes of two point functions such as the symmetrized ones and the Kubo function that appears in linear response theory, for which we give analogous results.

}, doi = {https://doi.org/10.1103/PhysRevLett.124.110605}, url = {https://arxiv.org/abs/1906.11280}, author = {{\'A}lvaro M. Alhambra and Jonathon Riddell and Luis Pedro Garc{\'\i}a-Pintos} } @article {2271, title = {Canonical forms for single-qutrit Clifford+T operators}, journal = {Annals of Physics}, volume = {406}, year = {2019}, month = {8/19/2019}, pages = {54-70}, abstract = {

We introduce canonical forms for single qutrit Clifford+T circuits and prove that every single-qutrit Clifford+T operator admits a unique such canonical form. We show that our canonical forms are T-optimal in the sense that among all the single-qutrit Clifford+T circuits implementing a given operator our canonical form uses the least number of T gates. Finally, we provide an algorithm which inputs the description of an operator (as a matrix or a circuit) and constructs the canonical form for this operator. The algorithm runs in time linear in the number of T gates. Our results provide a higher-dimensional generalization of prior work by Matsumoto and Amano who introduced similar canonical forms for single-qubit Clifford+T circuits.\ 

}, doi = {https://doi.org/10.1016/j.aop.2019.04.001}, url = {https://arxiv.org/abs/1803.05047}, author = {Andrew N. Glaudell and Neil J. Ross and J. M. Taylor} } @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 {1995, title = {Graphical Methods in Device-Independent Quantum Cryptography}, journal = {Quantum}, volume = {3}, year = {2019}, month = {05/20/2019}, abstract = {

We introduce a framework for graphical security proofs in device-independent quantum cryptography using the methods of categorical quantum mechanics. We are optimistic that this approach will make some of the highly complex proofs in quantum cryptography more accessible, facilitate the discovery of new proofs, and enable automated proof verification. As an example of our framework, we reprove a recent result from device-independent quantum cryptography: any linear randomness expansion protocol can be converted into an unbounded randomness expansion protocol. We give a graphical exposition of a proof of this result and implement parts of it in the Globular proof assistant.

}, doi = {https://doi.org/10.22331/q-2019-05-27-146}, url = {https://arxiv.org/abs/1705.09213}, author = {Spencer Breiner and Carl Miller and Neil J. Ross} } @article {2464, title = {Number-Theoretic Characterizations of Some Restricted Clifford+T Circuits}, year = {2019}, month = {8/16/2019}, abstract = {

Kliuchnikov, Maslov, and Mosca proved in 2012 that a 2\×2 unitary matrix V can be exactly represented by a single-qubit Clifford+T circuit if and only if the entries of V belong to the ring Z[1/2\–\√,i]. Later that year, Giles and Selinger showed that the same restriction applies to matrices that can be exactly represented by a multi-qubit Clifford+T circuit. These number-theoretic characterizations shed new light upon the structure of Clifford+T circuits and led to remarkable developments in the field of quantum compiling. In the present paper, we provide number-theoretic characterizations for certain restricted Clifford+T circuits by considering unitary matrices over subrings of Z[1/2\–\√,i]. We focus on the subrings Z[1/2], Z[1/2\–\√], Z[1/-2\−\−\√], and Z[1/2,i], and we prove that unitary matrices with entries in these rings correspond to circuits over well-known universal gate sets. In each case, the desired gate set is obtained by extending the set of classical reversible gates {X,CX,CCX} with an analogue of the Hadamard gate and an optional phase gate.

}, url = {https://arxiv.org/abs/1908.06076}, author = {Matthew Amy and Andrew N. Glaudell and Neil J. Ross} } @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 {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 {2384, title = {Verification Logics for Quantum Programs}, year = {2019}, type = {Originally submitted in March 2016 as a qualifying examination (WPE-II) for the PhD program at the University of Pennsylvania}, abstract = {

We survey the landscape of Hoare logics for quantum programs. We review three papers: \"Reasoning about imperative quantum programs\" by Chadha, Mateus and Sernadas; \"A logic for formal verification of quantum programs\" by Yoshihiko Kakutani; and \"Floyd-hoare logic for quantum programs\" by Mingsheng Ying. We compare the mathematical foundations of the logics, their underlying languages, and the expressivity of their assertions. We also use the languages to verify the Deutsch-Jozsa Algorithm, and discuss their relative usability in practice.

}, url = {https://arxiv.org/abs/1904.04304}, author = {Robert Rand} } @article {2404, title = {Verified Optimization in a Quantum Intermediate Representation}, year = {2019}, month = {04/12/2019}, abstract = {

We present sqire, a low-level language for quantum computing and verification. sqire uses a global register of quantum bits, allowing easy compilation to and from existing {\textquoteleft}quantum assembly\&$\#$39; languages and simplifying the verification process. We demonstrate the power of sqire as an intermediate representation of quantum programs by verifying a number of useful optimizations, and we demonstrate sqire\&$\#$39;s use as a tool for general verification by proving several quantum programs correct.

}, url = {https://arxiv.org/abs/1904.06319}, author = {Kesha Hietala and Robert Rand and Shih-Han Hung and Xiaodi Wu and Michael Hicks} } @article {2108, title = {Absence of Thermalization in Finite Isolated Interacting Floquet Systems}, journal = {Physical Review B}, volume = {97}, year = {2018}, month = {2018/01/29}, pages = {014311}, abstract = {

Conventional wisdom suggests that the long time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy non-thermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit non-thermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even with no static component of the interaction, the quasienergy spectrum contains gaps and a significant fraction of the Floquet eigenstates, at all quasienergies, have non-thermal average doublon densities. We show that this non-thermal behavior arises due to emergent integrability at large interaction strength and discuss how the integrability breaks down with power-law behavior in system size.

}, doi = {10.1103/PhysRevB.97.014311}, url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.014311}, author = {Karthik Seetharam and Paraj Titum and Michael Kolodrubetz and Gil Refael} } @article {2067, title = {Automated optimization of large quantum circuits with continuous parameters}, journal = {npj:Quantum Information}, volume = {4}, year = {2018}, month = {2017/10/19}, abstract = {

We develop and implement automated methods for optimizing quantum circuits of the size and type expected in quantum computations that outperform classical computers. We show how to handle continuous gate parameters and report a collection of fast algorithms capable of optimizing large-scale quantum circuits. For the suite of benchmarks considered, we obtain substantial reductions in gate counts. In particular, we provide better optimization in significantly less time than previous approaches, while making minimal structural changes so as to preserve the basic layout of the underlying quantum algorithms. Our results help bridge the gap between the computations that can be run on existing hardware and those that are expected to outperform classical computers.\ 

}, doi = {https://doi.org/10.1038/s41534-018-0072-4}, url = {https://arxiv.org/abs/1710.07345}, author = {Yunseong Nam and Neil J. Ross and Yuan Su and Andrew M. Childs and Dmitri Maslov} } @article {2145, title = {An autonomous single-piston engine with a quantum rotor}, year = {2018}, month = {2018/02/15}, abstract = {

Pistons are elementary components of a wide variety of thermal engines, converting input fuel into rotational motion. Here, we propose a single-piston engine where the rotational degree of freedom is effectively realized by the flux of a superconducting island -- a quantum rotor -- while the working volume corresponds to the effective length of a superconducting resonator. Our autonomous design implements a Carnot cycle, relies solely on standard thermal baths and can be implemented with circuit quantum electrodynamics. We demonstrate how the piston is able to extract a net positive work via its built-in synchronicity using a filter cavity as an effective valve, eliminating the need for external control.

}, doi = {https://doi.org/10.1088/2058-9565/aac40d}, url = {https://arxiv.org/abs/1802.05486}, author = {Alexandre Roulet and Stefan Nimmrichter and J. M. Taylor} } @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 {2298, title = {Black Hole Microstate Cosmology}, year = {2018}, abstract = {

In this note, we explore the possibility that certain high-energy holographic CFT states correspond to black hole microstates with a geometrical behind-the-horizon region, modelled by a portion of a second asymptotic region terminating at an end-of-the-world (ETW) brane. We study the time-dependent physics of this behind-the-horizon region, whose ETW boundary geometry takes the form of a closed FRW spacetime. We show that in many cases, this behind-the-horizon physics can be probed directly by looking at the time dependence of entanglement entropy for sufficiently large spatial CFT subsystems. We study in particular states defined via Euclidean evolution from conformal boundary states and give specific predictions for the behavior of the entanglement entropy in this case. We perform analogous calculations for the SYK model and find qualitative agreement with our expectations. A fascinating possibility is that for certain states, we might have gravity localized to the ETW brane as in the Randall-Sundrum II scenario for cosmology. In this case, the effective description of physics beyond the horizon could be a big bang/big crunch cosmology of the same dimensionality as the CFT. In this case, the d-dimensional CFT describing the black hole microstate would give a precise, microscopic description of the d-dimensional cosmological physics.\ 

}, url = {https://arxiv.org/abs/1810.10601}, author = {Sean Cooper and Moshe Rozali and Brian Swingle and Mark Van Raamsdonk and Christopher Waddell and David Wakeham} } @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 {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 {2282, title = {Experimentally Generated Randomness Certified by the Impossibility of Superluminal Signals}, journal = {Nature}, volume = {556}, year = {2018}, month = {2018/04/11}, pages = {223-226}, abstract = {

From dice to modern complex circuits, there have been many attempts to build increasingly better devices to generate random numbers. Today, randomness is fundamental to security and cryptographic systems, as well as safeguarding privacy. A key challenge with random number generators is that it is hard to ensure that their outputs are unpredictable. For a random number generator based on a physical process, such as a noisy classical system or an elementary quantum measurement, a detailed model describing the underlying physics is required to assert unpredictability. Such a model must make a number of assumptions that may not be valid, thereby compromising the integrity of the device. However, it is possible to exploit the phenomenon of quantum nonlocality with a loophole-free Bell test to build a random number generator that can produce output that is unpredictable to any adversary limited only by general physical principles. With recent technological developments, it is now possible to carry out such a loophole-free Bell test. Here we present certified randomness obtained from a photonic Bell experiment and extract 1024 random bits uniform to within 10\−12. These random bits could not have been predicted within any physical theory that prohibits superluminal signaling and allows one to make independent measurement choices. To certify and quantify the randomness, we describe a new protocol that is optimized for apparatuses characterized by a low per-trial violation of Bell inequalities. We thus enlisted an experimental result that fundamentally challenges the notion of determinism to build a system that can increase trust in random sources. In the future, random number generators based on loophole-free Bell tests may play a role in increasing the security and trust of our cryptographic systems and infrastructure.

}, doi = {https://doi.org/10.1038/s41586-018-0019-0}, url = {https://arxiv.org/abs/1803.06219}, author = {Peter Bierhorst and Emanuel Knill and Scott Glancy and Yanbao Zhang and Alan Mink and Stephen Jordan and Andrea Rommal and Yi-Kai Liu and Bradley Christensen and Sae Woo Nam and Martin J. Stevens and Lynden K. Shalm} } @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 {2279, title = {Morphisms in categories of nonlocal games}, year = {2018}, abstract = {

Synchronous correlations provide a class of nonlocal games that behave like functions between finite sets. In this work we examine categories whose morphisms are games with synchronous classical, quantum, or general nonsignaling correlations. In particular, we characterize when morphisms in these categories are monic, epic, sections, or retractions.

}, url = {https://arxiv.org/abs/1810.10074}, author = {Brad Lackey and Nishant Rodrigues} } @article {1836, title = {Optimal and Secure Measurement Protocols for Quantum Sensor Networks}, year = {2018}, month = {2018/03/23}, abstract = {

Studies of quantum metrology have shown that the use of many-body entangled states can lead to an enhancement in sensitivity when compared to product states. In this paper, we quantify the metrological advantage of entanglement in a setting where the quantity to be measured is a linear function of parameters coupled to each qubit individually. We first generalize the Heisenberg limit to the measurement of non-local observables in a quantum network, deriving a bound based on the multi-parameter quantum Fisher information. We then propose a protocol that can make use of GHZ states or spin-squeezed states, and show that in the case of GHZ states the procedure is optimal, i.e., it saturates our bound.

}, doi = {https://doi.org/10.1103/PhysRevA.97.042337}, url = {http://arxiv.org/abs/1607.04646}, author = {Zachary Eldredge and Michael Foss-Feig and Steven L. Rolston and Alexey V. Gorshkov} } @article {2259, title = {Optimal Pure-State Qubit Tomography via Sequential Weak Measurements}, journal = {Phys. Rev. Lett. }, volume = {121}, year = {2018}, abstract = {

The spin-coherent-state positive-operator-valued-measure (POVM) is a fundamental measurement in quantum science, with applications including tomography, metrology, teleportation, benchmarking, and measurement of Husimi phase space probabilities. We prove that this POVM is achieved by collectively measuring the spin projection of an ensemble of qubits weakly and isotropically. We apply this in the context of optimal tomography of pure qubits. We show numerically that through a sequence of weak measurements of random directions of the collective spin component, sampled discretely or in a continuous measurement with random controls, one can approach the optimal bound.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.130404}, url = {https://arxiv.org/abs/1805.01012}, author = {Ezad Shojaee and Christopher S. Jackson and Carlos A. Riofrio and Amir Kalev and Ivan H. Deutsch} } @article {2268, title = {QFlow lite dataset: A machine-learning approach to the charge states in quantum dot experiments}, journal = {PLOS ONE}, volume = {13}, year = {2018}, month = {2018}, pages = {e0205844}, type = {2018/10/17}, abstract = {

Over the past decade, machine learning techniques have revolutionized how research is done, from designing new materials and predicting their properties to assisting drug discovery to advancing cybersecurity. Recently, we added to this list by showing how a machine learning algorithm (a so-called learner) combined with an optimization routine can assist experimental efforts in the realm of tuning semiconductor quantum dot (QD) devices. Among other applications, semiconductor QDs are a candidate system for building quantum computers. The present-day tuning techniques for bringing the QD devices into a desirable configuration suitable for quantum computing that rely on heuristics do not scale with the increasing size of the quantum dot arrays required for even near-term quantum computing demonstrations. Establishing a reliable protocol for tuning that does not rely on the gross-scale heuristics developed by experimentalists is thus of great importance. To implement the machine learning-based approach, we constructed a dataset of simulated QD device characteristics, such as the conductance and the charge sensor response versus the applied electrostatic gate voltages. Here, we describe the methodology for generating the dataset, as well as its validation in training convolutional neural networks. We show that the learner\&$\#$39;s accuracy in recognizing the state of a device is ~96.5 \% in both current- and charge-sensor-based training. We also introduce a tool that enables other researchers to use this approach for further research: QFlow lite - a Python-based mini-software suite that uses the dataset to train neural networks to recognize the state of a device and differentiate between states in experimental data. This work gives the definitive reference for the new dataset that will help enable researchers to use it in their experiments or to develop new machine learning approaches and concepts

}, doi = {https://doi.org/10.1371/journal.pone.0205844}, url = {https://arxiv.org/abs/1809.10018}, author = {Justyna P. Zwolak and Sandesh S. Kalantre and Xingyao Wu and Stephen Ragole and J. M. Taylor} } @article {2308, title = {Quantum-secure message authentication via blind-unforgeability}, year = {2018}, abstract = {

Formulating and designing unforgeable authentication of classical messages in the presence of quantum adversaries has been a challenge, as the familiar classical notions of unforgeability do not directly translate into meaningful notions in the quantum setting. A particular difficulty is how to fairly capture the notion of \"predicting an unqueried value\" when the adversary can query in quantum superposition. In this work, we uncover serious shortcomings in existing approaches, and propose a new definition. We then support its viability by a number of constructions and characterizations. Specifically, we demonstrate a function which is secure according to the existing definition by Boneh and Zhandry, but is clearly vulnerable to a quantum forgery attack, whereby a query supported only on inputs that start with 0 divulges the value of the function on an input that starts with 1. We then propose a new definition, which we call \"blind-unforgeability\" (or BU.) This notion matches \"intuitive unpredictability\" in all examples studied thus far. It defines a function to be predictable if there exists an adversary which can use \"partially blinded\" oracle access to predict values in the blinded region. Our definition (BU) coincides with standard unpredictability (EUF-CMA) in the classical-query setting. We show that quantum-secure pseudorandom functions are BU-secure MACs. In addition, we show that BU satisfies a composition property (Hash-and-MAC) using \"Bernoulli-preserving\" hash functions, a new notion which may be of independent interest. Finally, we show that BU is amenable to security reductions by giving a precise bound on the extent to which quantum algorithms can deviate from their usual behavior due to the blinding in the BU security experiment.\ 

}, url = {https://arxiv.org/abs/1803.03761}, author = {Gorjan Alagic and Christian Majenz and Alexander Russell and Fang Song} } @article {2138, title = {Recovering quantum gates from few average gate fidelities}, journal = {Phys. Rev. Lett. }, volume = {121}, year = {2018}, month = {2018/03/01}, pages = {170502}, abstract = {

Characterising quantum processes is a key task in and constitutes a challenge for the development of quantum technologies, especially at the noisy intermediate scale of today\&$\#$39;s devices. One method for characterising processes is randomised benchmarking, which is robust against state preparation and measurement (SPAM) errors, and can be used to benchmark Clifford gates. A complementing approach asks for full tomographic knowledge. Compressed sensing techniques achieve full tomography of quantum channels essentially at optimal resource efficiency. So far, guarantees for compressed sensing protocols rely on unstructured random measurements and can not be applied to the data acquired from randomised benchmarking experiments. It has been an open question whether or not the favourable features of both worlds can be combined. In this work, we give a positive answer to this question. For the important case of characterising multi-qubit unitary gates, we provide a rigorously guaranteed and practical reconstruction method that works with an essentially optimal number of average gate fidelities measured respect to random Clifford unitaries. Moreover, for general unital quantum channels we provide an explicit expansion into a unitary 2-design, allowing for a practical and guaranteed reconstruction also in that case. As a side result, we obtain a new statistical interpretation of the unitarity -- a figure of merit that characterises the coherence of a process. In our proofs we exploit recent representation theoretic insights on the Clifford group, develop a version of Collins\&$\#$39; calculus with Weingarten functions for integration over the Clifford group, and combine this with proof techniques from compressed sensing.

}, doi = {https://doi.org/10.1103/PhysRevLett.121.170502}, url = {https://arxiv.org/abs/1803.00572}, author = {Ingo Roth and Richard Kueng and Shelby Kimmel and Yi-Kai Liu and David Gross and Jens Eisert and Martin Kliesch} } @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 {1837, title = {Spectrum estimation of density operators with alkaline-earth atoms}, volume = {120}, year = {2018}, month = {2018/01/09}, abstract = {

We show that Ramsey spectroscopy of fermionic alkaline-earth atoms in a square-well trap provides an efficient and accurate estimate for the eigenspectrum of a density matrix whose n copies are stored in the nuclear spins of n such atoms. This spectrum estimation is enabled by the high symmetry of the interaction Hamiltonian, dictated, in turn, by the decoupling of the nuclear spin from the electrons and by the shape of the square-well trap. Practical performance of this procedure and its potential applications to quantum computing, quantum simulation, and time-keeping with alkalineearth atoms are discussed.

}, doi = {https://doi.org/10.1103/PhysRevLett.120.025301}, url = {http://arxiv.org/abs/1608.02045}, author = {Michael E. Beverland and Jeongwan Haah and Gorjan Alagic and Gretchen K. Campbell and Ana Maria Rey and Alexey V. Gorshkov} } @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 {2220, title = {Toward the first quantum simulation with quantum speedup}, journal = {Proceedings of the National Academy of Sciences}, volume = {115 }, year = {2018}, pages = {9456-9461}, abstract = {

With quantum computers of significant size now on the horizon, we should understand how to best exploit their initially limited abilities. To this end, we aim to identify a practical problem that is beyond the reach of current classical computers, but that requires the fewest resources for a quantum computer. We consider quantum simulation of spin systems, which could be applied to understand condensed matter phenomena. We synthesize explicit circuits for three leading quantum simulation algorithms, using diverse techniques to tighten error bounds and optimize circuit implementations. Quantum signal processing appears to be preferred among algorithms with rigorous performance guarantees, whereas higher-order product formulas prevail if empirical error estimates suffice. Our circuits are orders of magnitude smaller than those for the simplest classically infeasible instances of factoring and quantum chemistry, bringing practical quantum computation closer to reality.

}, doi = {https://doi.org/10.1073/pnas.1801723115}, url = {https://arxiv.org/abs/1711.10980}, author = {Andrew M. Childs and Dmitri Maslov and Yunseong Nam and Neil J. Ross and Yuan Su} } @article {2062, title = {Disorder induced transitions in resonantly driven Floquet Topological Insulators}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/08/16}, pages = {054207}, abstract = {

We investigate the effects of disorder in Floquet topological insulators (FTIs) occurring in semiconductor quantum wells. Such FTIs are induced by resonantly driving a transition between the valence and conduction band. We show that when disorder is added, the topological nature of such FTIs persists as long as there is a mobility gap at the resonant quasi-energy. For strong enough disorder, this gap closes and all the states become localized as the system undergoes a transition to a trivial insulator. Interestingly, the effects of disorder are not necessarily adverse: we show that in the same quantum well, disorder can also induce a transition from a trivial to a topological system, thereby establishing a Floquet Topological Anderson Insulator (FTAI). We identify the conditions on the driving field necessary for observing such a transition.

}, doi = {10.1103/PhysRevB.96.054207}, url = {https://arxiv.org/abs/1702.02956}, author = {Paraj Titum and Netanel H. Lindner and Gil Refael} } @article {2059, title = {Efimov States of Strongly Interacting Photons}, journal = {Physical Review Letters}, volume = {119}, year = {2017}, month = {2017/12/04}, pages = {233601}, abstract = {

We demonstrate the emergence of universal Efimov physics for interacting photons in cold gases of Rydberg atoms. We consider the behavior of three photons injected into the gas in their propagating frame, where a paraxial approximation allows us to consider them as massive particles. In contrast to atoms and nuclei, the photons have a large anisotropy between their longitudinal mass, arising from dispersion, and their transverse mass, arising from diffraction. Nevertheless, we show that in suitably rescaled coordinates the effective interactions become dominated by s-wave scattering near threshold and, as a result, give rise to an Efimov effect near unitarity, but with spatially anisotropic wavefunctions in the original coordinates. We show that the three-body loss of these Efimov trimers can be strongly suppressed and determine conditions under which these states are observable in current experiments. These effects can be naturally extended to probe few-body universality beyond three bodies, as well as the role of Efimov physics in the non-equilbrium, many-body regime.

}, doi = {10.1103/PhysRevLett.119.233601}, url = {https://arxiv.org/abs/1709.01955}, author = {Michael Gullans and S. Diehl and S. T. Rittenhouse and B. P. Ruzic and J. P. D{\textquoteright}Incao and P. Julienne and Alexey V. Gorshkov and J. M. Taylor} } @proceedings {1935, title = {Experimental Comparison of Two Quantum Computing Architectures}, volume = {114}, year = {2017}, month = {2017/03/21}, pages = {3305-3310}, edition = {13}, abstract = {

We run a selection of algorithms on two state-of-the-art 5-qubit quantum computers that are based on different technology platforms. One is a publicly accessible superconducting transmon device [1] with limited connectivity, and the other is a fully connected trapped-ion system [2]. Even though the two systems have different native quantum interactions, both can be programmed in a way that is blind to the underlying hardware, thus allowing the first comparison of identical quantum algorithms between different physical systems. We show that quantum algorithms and circuits that employ more connectivity clearly benefit from a better connected system of qubits. While the quantum systems here are not yet large enough to eclipse classical computers, this experiment exposes critical factors of scaling quantum computers, such as qubit connectivity and gate expressivity. In addition, the results suggest that co-designing particular quantum applications with the hardware itself will be paramount in successfully using quantum computers in the future.

}, doi = {10.1073/pnas.1618020114}, url = {http://www.pnas.org/content/114/13/3305}, author = {N.M. Linke and Dmitri Maslov and Martin Roetteler and S. Debnath and C. Figgatt and K. A. Landsman and K. Wright and Christopher Monroe} } @article {1944, title = {Experimental demonstration of cheap and accurate phase estimation}, journal = {Physical Review Letters}, volume = {118}, year = {2017}, month = {2017/05/12}, pages = {190502}, abstract = {

We demonstrate experimental implementation of robust phase estimation (RPE) to learn the phases of X and Y rotations on a trapped Yb+ ion qubit. We estimate these phases with uncertainties less than 4 \· 10\−4 radians using as few as 176 total experimental samples per phase, and our estimates exhibit Heisenberg scaling. Unlike standard phase estimation protocols, RPE neither assumes perfect state preparation and measurement, nor requires access to ancillae. We cross-validate the results of RPE with the more resource-intensive protocol of gate set tomography.

}, doi = {doi.org/10.1103/PhysRevLett.118.190502}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.190502}, author = {Kenneth Rudinger and Shelby Kimmel and Daniel Lobser and Peter Maunz} } @article {1945, title = {Experimentally Generated Random Numbers Certified by the Impossibility of Superluminal Signaling}, year = {2017}, month = {2017/02/16}, abstract = {

Random numbers are an important resource for applications such as numerical simulation and secure communication. However, it is difficult to certify whether a physical random number generator is truly unpredictable. Here, we exploit the phenomenon of quantum nonlocality in a loophole-free photonic Bell test experiment for the generation of randomness that cannot be predicted within any physical theory that allows one to make independent measurement choices and prohibits superluminal signaling. To certify and quantify the randomness, we describe a new protocol that performs well in an experimental regime characterized by low violation of Bell inequalities. Applying an extractor function to our data, we obtained 256 new random bits, uniform to within 0.001.

}, url = {https://arxiv.org/abs/1702.05178$\#$}, author = {Peter Bierhorst and Emanuel Knill and Scott Glancy and Alan Mink and Stephen P. Jordan and Andrea Rommal and Yi-Kai Liu and Bradley Christensen and Sae Woo Nam and Lynden K. Shalm} } @article {2103, title = {Machine Learning techniques for state recognition and auto-tuning in quantum dots}, year = {2017}, month = {2017/12/13}, abstract = {

Recent progress in building large-scale quantum devices for exploring quantum computing and simulation paradigms has relied upon effective tools for achieving and maintaining good experimental parameters, i.e. tuning up devices. In many cases, including in quantum-dot based architectures, the parameter space grows substantially with the number of qubits, and may become a limit to scalability. Fortunately, machine learning techniques for pattern recognition and image classification using so-called deep neural networks have shown surprising successes for computer-aided understanding of complex systems. In this work, we use deep and convolutional neural networks to characterize states and charge configurations of semiconductor quantum dot arrays when one can only measure a current-voltage characteristic of transport (here conductance) through such a device. For simplicity, we model a semiconductor nanowire connected to leads and capacitively coupled to depletion gates using the Thomas-Fermi approximation and Coulomb blockade physics. We then generate labeled training data for the neural networks, and find at least 90 \% accuracy for charge and state identification for single and double dots purely from the dependence of the nanowire\’s conductance upon gate voltages. Using these characterization networks, we can then optimize the parameter space to achieve a desired configuration of the array, a technique we call \‘auto-tuning\’. Finally, we show how such techniques can be implemented in an experimental setting by applying our approach to an experimental data set, and outline further problems in this domain, from using charge sensing data to extensions to full one and two-dimensional arrays, that can be tackled with machine learning.

}, url = {https://arxiv.org/abs/1712.04914}, author = {Sandesh S. Kalantre and Justyna P. Zwolak and Stephen Ragole and Xingyao Wu and Neil M. Zimmerman and M. D. Stewart and J. M. Taylor} } @article {2049, title = {Nonlocal games, synchronous correlations, and Bell inequalities}, year = {2017}, month = {2017/09/21}, abstract = {

A nonlocal game with a synchronous correlation is a natural generalization of a function between two finite sets, and has recently appeared in the context of quantum graph homomorphisms. In this work we examine analogues of Bell\&$\#$39;s inequalities for synchronous correlations. We show that, unlike general correlations and the CHSH inequality, there can be no quantum Bell violation among synchronous correlations with two measurement settings. However we exhibit explicit analogues of Bell\&$\#$39;s inequalities for synchronous correlations with three measurement settings and two outputs, provide an analogue of Tsirl\&$\#$39;son\&$\#$39;s bound in this setting, and give explicit quantum correlations that saturate this bound.

}, url = {https://arxiv.org/abs/1707.06200}, author = {Brad Lackey and Nishant Rodrigues} } @article {2621, title = {Quantum-Secure Symmetric-Key Cryptography Based on Hidden Shifts}, journal = {In: Coron JS., Nielsen J. (eds) Advances in Cryptology {\textendash} EUROCRYPT 2017. Lecture Notes in Computer Science, Springer, Cham}, volume = {10212}, year = {2017}, abstract = {

Recent results of Kaplan et al., building on work by Kuwakado and Morii, have shown that a wide variety of classically-secure symmetric-key cryptosystems can be completely broken by quantum chosen-plaintext attacks (qCPA). In such an attack, the quantum adversary has the ability to query the cryptographic functionality in superposition. The vulnerable cryptosystems include the Even-Mansour block cipher, the three-round Feistel network, the Encrypted-CBC-MAC, and many others.

In this article, we study simple algebraic adaptations of such schemes that replace\ \  (Z/2)n\  addition with operations over alternate finite groups\—such as\ \  Z/2n \—and provide evidence that these adaptations are qCPA-secure. These adaptations furthermore retain the classical security properties and basic structural features enjoyed by the original schemes.

We establish security by treating the (quantum) hardness of the well-studied Hidden Shift problem as a cryptographic assumption. We observe that this problem has a number of attractive features in this cryptographic context, including random self-reducibility, hardness amplification, and\—in many cases of interest\—a reduction from the \“search version\” to the \“decisional version.\” We then establish, under this assumption, the qCPA-security of several such Hidden Shift adaptations of symmetric-key constructions. We show that a Hidden Shift version of the Even-Mansour block cipher yields a quantum-secure pseudorandom function, and that a Hidden Shift version of the Encrypted CBC-MAC yields a collision-resistant hash function. Finally, we observe that such adaptations frustrate the direct Simon\’s algorithm-based attacks in more general circumstances, e.g., Feistel networks and slide attacks.

}, doi = {https://doi.org/10.1007/978-3-319-56617-7_3}, author = {Gorjan Alagic and Alexander Russell} } @article {1970, title = {Shorter stabilizer circuits via Bruhat decomposition and quantum circuit transformations}, year = {2017}, month = {2017/05/25}, abstract = {

In this paper we improve the layered implementation of arbitrary stabilizer circuits introduced by Aaronson and Gottesman in Phys. Rev. A 70(052328), 2004: to implement a general stabilizer circuit, we reduce their 11-stage computation -HC-P-C-P-C-H-P-C-P-C- over the gate set consisting of Hadamard, Controlled-NOT, and Phase gates, into a 7-stage computation of the form -C-CZ-P-H-P-CZ-C-. We show arguments in support of using -CZ- stages over the -C- stages: not only the use of -CZ- stages allows a shorter layered expression, but -CZ- stages are simpler and appear to be easier to implement compared to the -C- stages. Based on this decomposition, we develop a twoqubit gate depth-(14n\−4) implementation of stabilizer circuits over the gate library {H, P, CNOT}, executable in the LNN architecture, improving best previously known depth-25n circuit, also executable in the LNN architecture. Our constructions rely on Bruhat decomposition of the symplectic group and on folding arbitrarily long sequences of the form (-P-C-) m into a 3-stage computation -P-CZ-C-. Our results include the reduction of the 11-stage decomposition -H-C-P-C-P-C-H-P-C-P-C- into a 9-stage decomposition of the form -C-P-C-P-H-C-P-C-P-. This reduction is based on the Bruhat decomposition of the symplectic group. This result also implies a new normal form for stabilizer circuits. We show that a circuit in this normal form is optimal in the number of Hadamard gates used. We also show that the normal form has an asymptotically optimal number of parameters.

}, url = {https://arxiv.org/abs/1705.09176}, author = {Dmitri Maslov and Martin Roetteler} } @article {2055, title = {Thermodynamic limits for optomechanical systems with conservative potentials}, journal = {Physical Review B}, volume = {96}, year = {2017}, month = {2017/11/13}, pages = {184106}, abstract = {

The mechanical force from light \– radiation pressure \– provides an intrinsic nonlinear interaction. Consequently, optomechanical systems near their steady state, such as the canonical optical spring, can display non-analytic behavior as a function of external parameters. This non-analyticity, a key feature of thermodynamic phase transitions, suggests that there could be an effective thermodynamic description of optomechanical systems. Here we explicitly define the thermodynamic limit for optomechanical systems and derive a set of sufficient constraints on the system parameters as the mechanical system grows large. As an example, we show how these constraints can be satisfied in a system with Z2 symmetry and derive a free energy, allowing us to characterize this as an equilibrium phase transition.

}, doi = {10.1103/PhysRevB.96.184106}, url = {https://arxiv.org/abs/1707.05771}, author = {Stephen Ragole and Haitan Xu and John Lawall and J. M. Taylor} } @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 {1925, title = {A finite presentation of CNOT-dihedral operators}, year = {2016}, month = {2016/12/31}, abstract = {

We give a finite presentation by generators and relations of unitary operators expressible over the {CNOT, T, X} gate set, also known as CNOT-dihedral operators. To this end, we introduce a notion of normal form for CNOT-dihedral circuits and prove that every CNOT-dihedral operator admits a unique normal form. Moreover, we show that in the presence of certain structural rules only finitely many circuit identities are required to reduce an arbitrary CNOT-dihedral circuit to its normal form. By appropriately restricting our relations, we obtain a finite presentation of unitary operators expressible over the {CNOT, T } gate set as a corollary.

}, url = {https://arxiv.org/abs/1701.00140}, author = {Matthew Amy and Jianxin Chen and Neil J. Ross} } @article {1736, title = {Interacting atomic interferometry for rotation sensing approaching the Heisenberg Limit}, journal = {Physical Review Letters}, volume = {117}, year = {2016}, month = {2016/11/11}, pages = {203002}, abstract = {

Atom interferometers provide exquisite measurements of the properties of non-inertial frames. While atomic interactions are typically detrimental to good sensing, efforts to harness entanglement to improve sensitivity remain tantalizing. Here we explore the role of interactions in an analogy between atomic gyroscopes and SQUIDs, motivated by recent experiments realizing ring shaped traps for ultracold atoms. We explore the one-dimensional limit of these ring systems with a moving weak barrier, such as that provided by a blue-detuned laser beam. In this limit, we employ Luttinger liquid theory and find an analogy with the superconducting phase-slip qubit, in which the topological charge associated with persistent currents can be put into superposition. In particular, we find that strongly-interacting atoms in such a system could be used for precision rotation sensing. We compare the performance of this new sensor to an equivalent non-interacting atom interferometer, and find improvements in sensitivity and bandwidth beyond the atomic shot-noise limit.

}, doi = {10.1103/PhysRevLett.117.203002}, url = {https://doi.org/10.1103/PhysRevLett.117.203002}, author = {Stephen Ragole and J. M. Taylor} } @article {1695, title = {Kaleidoscope of quantum phases in a long-range interacting spin-1 chain}, journal = {Physical Review B}, volume = {93}, year = {2016}, month = {2016/05/11}, pages = {205115}, abstract = {Motivated by recent trapped-ion quantum simulation experiments, we carry out a comprehensive study of the phase diagram of a spin-1 chain with XXZ-type interactions that decay as 1/rα, using a combination of finite and infinite-size DMRG calculations, spin-wave analysis, and field theory. In the absence of long-range interactions, varying the spin-coupling anisotropy leads to four distinct phases: a ferromagnetic Ising phase, a disordered XY phase, a topological Haldane phase, and an antiferromagnetic Ising phase. If long-range interactions are antiferromagnetic and thus frustrated, we find primarily a quantitative change of the phase boundaries. On the other hand, ferromagnetic (non-frustrated) long-range interactions qualitatively impact the entire phase diagram. Importantly, for α≲3, long-range interactions destroy the Haldane phase, break the conformal symmetry of the XY phase, give rise to a new phase that spontaneously breaks a U(1) continuous symmetry, and introduce an exotic tricritical point with no direct parallel in short-range interacting spin chains. We show that the main signatures of all five phases found could be observed experimentally in the near future. }, doi = {http://dx.doi.org/10.1103/PhysRevB.93.205115}, url = {http://arxiv.org/abs/1510.02108}, author = {Zhe-Xuan Gong and Mohammad F. Maghrebi and Anzi Hu and Michael Foss-Feig and Philip Richerme and Christopher Monroe and Alexey V. Gorshkov} } @article {1271, title = {Many-body localization in a quantum simulator with programmable random disorder}, journal = {Nature Physics}, year = {2016}, month = {2016/06/06}, abstract = {

When a system thermalizes it loses all local memory of its initial conditions. This is a general feature of open systems and is well described by equilibrium statistical mechanics. Even within a closed (or reversible) quantum system, where unitary time evolution retains all information about its initial state, subsystems can still thermalize using the rest of the system as an effective heat bath. Exceptions to quantum thermalization have been predicted and observed, but typically require inherent symmetries or noninteracting particles in the presence of static disorder. The prediction of many-body localization (MBL), in which disordered quantum systems can fail to thermalize in spite of strong interactions and high excitation energy, was therefore surprising and has attracted considerable theoretical attention. Here we experimentally generate MBL states by applying an Ising Hamiltonian with long-range interactions and programmably random disorder to ten spins initialized far from equilibrium. We observe the essential signatures of MBL: memory retention of the initial state, a Poissonian distribution of energy level spacings, and entanglement growth in the system at long times. Our platform can be scaled to higher numbers of spins, where detailed modeling of MBL becomes impossible due to the complexity of representing such entangled quantum states. Moreover, the high degree of control in our experiment may guide the use of MBL states as potential quantum memories in naturally disordered quantum systems.

}, doi = {10.1038/nphys3783}, url = {http://arxiv.org/abs/1508.07026v1}, author = {Jacob Smith and Aaron Lee and Philip Richerme and Brian Neyenhuis and Paul W. Hess and Philipp Hauke and Markus Heyl and David A. Huse and Christopher Monroe} } @article {1774, title = {Mapping constrained optimization problems to quantum annealing with application to fault diagnosis}, year = {2016}, abstract = {Current quantum annealing (QA) hardware suffers from practical limitations such as finite temperature, sparse connectivity, small qubit numbers, and control error. We propose new algorithms for mapping boolean constraint satisfaction problems (CSPs) onto QA hardware mitigating these limitations. In particular we develop a new embedding algorithm for mapping a CSP onto a hardware Ising model with a fixed sparse set of interactions, and propose two new decomposition algorithms for solving problems too large to map directly into hardware. The mapping technique is locally-structured, as hardware compatible Ising models are generated for each problem constraint, and variables appearing in different constraints are chained together using ferromagnetic couplings. In contrast, global embedding techniques generate a hardware independent Ising model for all the constraints, and then use a minor-embedding algorithm to generate a hardware compatible Ising model. We give an example of a class of CSPs for which the scaling performance of D-Wave{\textquoteright}s QA hardware using the local mapping technique is significantly better than global embedding. We validate the approach by applying D-Wave{\textquoteright}s hardware to circuit-based fault-diagnosis. For circuits that embed directly, we find that the hardware is typically able to find all solutions from a min-fault diagnosis set of size N using 1000N samples, using an annealing rate that is 25 times faster than a leading SAT-based sampling method. Further, we apply decomposition algorithms to find min-cardinality faults for circuits that are up to 5 times larger than can be solved directly on current hardware.}, url = {http://arxiv.org/abs/1603.03111}, author = {Bian, Zhengbing and Chudak, Fabian and Israel, Robert and Lackey, Brad and Macready, William G and Roy, Aidan} } @article {1859, title = {Mapping contrained optimization problems to quantum annealing with application to fault diagnosis}, journal = {Frontiers in ICT}, volume = {3}, year = {2016}, month = {2016/07/28}, pages = {14}, abstract = {

Current quantum annealing (QA) hardware suffers from practical limitations such as finite\ temperature, sparse connectivity, small qubit numbers, and control error. We propose new algorithms for\ mapping Boolean constraint satisfaction problems (CSPs) onto QA hardware mitigating these limitations.\ In particular, we develop a new embedding algorithm for mapping a CSP onto a hardware Ising model with\ a fixed sparse set of interactions and propose two new decomposition algorithms for solving problems too\ large to map directly into hardware. The mapping technique is locally structured, as hardware compatible\ Ising models are generated for each problem constraint, and variables appearing in different constraints are\ chained together using ferromagnetic couplings. By contrast, global embedding techniques generate a\ hardware-independent Ising model for all the constraints, and then use a minor-embedding algorithm to\ generate\ a hardware compatible Ising model. We give an example of a class of CSPs for which the scaling\ performance\ of the D-Wave hardware using the local mapping technique is significantly better than global\ embedding. We validate\ the approach by applying D- Wave\’s QA hardware to circuit-based fault diagnosis.\ For circuits that embed directly, we\ find that the hardware is typically able to find all solutions from a\ min-fault diagnosis set of size N using 1000 N samples,\ using an annealing rate that is 25 times faster than\ a leading SAT-based sampling method. Furthermore, we apply\ decomposition algorithms to find min-cardinality\ faults for circuits that are up to 5 times larger than can be solved directly on current hardware.

}, url = {http://journal.frontiersin.org/article/10.3389/fict.2016.00014/full}, author = {Bian, Zhengbing and Chudak, Fabian and Robert Brian Israel and Brad Lackey and Macready, William G and Aiden Roy} } @article {1914, title = {Multiple scattering dynamics of fermions at an isolated p-wave resonance}, journal = {Nature Communications}, volume = {7}, year = {2016}, month = {2016/07/11}, pages = {12069}, abstract = {

The wavefunction for indistinguishable fermions is anti-symmetric under particle exchange, which directly leads to the Pauli exclusion principle, and hence underlies the structure of atoms and the properties of almost all materials. In the dynamics of collisions between two indistinguishable fermions this requirement strictly prohibits scattering into 90 degree angles. Here we experimentally investigate the collisions of ultracold clouds fermionic\ 40K\ atoms by directly measuring scattering distributions. With increasing collision energy we identify the Wigner threshold for p-wave scattering with its tell-tale dumb-bell shape and no\ 90o\ yield. Above this threshold effects of multiple scattering become manifest as deviations from the underlying binary p-wave shape, adding particles either isotropically or axially. A shape resonance for\ 40K\ facilitates the separate observation of these two processes. The isotropically enhanced multiple scattering mode is a generic p-wave threshold phenomenon, while the axially enhanced mode should occur in any colliding particle system with an elastic scattering resonance.

}, doi = {10.1038/ncomms12069}, url = {http://www.nature.com/articles/ncomms12069}, author = {Ryan Thomas and Kris O. Roberts and Eite Tiesinga and Andrew C.J. Wade and P. Blair Blakie and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {2005, title = {{O}bservation of {P}rethermalization in {L}ong-{R}ange {I}nteracting {S}pin {C}hains}, year = {2016}, month = {2016/08/02}, abstract = {

Statistical mechanics can predict thermal equilibrium states for most classical systems, but for an isolated quantum system there is no general understanding on how equilibrium states dynamically emerge from the microscopic Hamiltonian. For instance, quantum systems that are near-integrable usually fail to thermalize in an experimentally realistic time scale and, instead, relax to quasi-stationary prethermal states that can be described by statistical mechanics when approximately conserved quantities are appropriately included in a generalized Gibbs ensemble (GGE). Here we experimentally study the relaxation dynamics of a chain of up to 22 spins evolving under a long-range transverse field Ising Hamiltonian following a sudden quench. For sufficiently long-ranged interactions the system relaxes to a new type of prethermal state that retains a strong memory of the initial conditions. In this case, the prethermal state cannot be described by a GGE, but rather arises from an emergent double-well potential felt by the spin excitations. This result shows that prethermalization occurs in a significantly broader context than previously thought, and reveals new challenges for a generic understanding of the thermalization of quantum systems, particularly in the presence of long-range interactions.

}, url = {https://arxiv.org/abs/1608.00681}, author = {B. Neyenhuis and J. Smith and A. C. Lee and J. Zhang and P. Richerme and P. W. Hess and Z. -X. Gong and Alexey V. Gorshkov and C. Monroe} } @article {1533, title = {Optimal ancilla-free Clifford+T approximation of z-rotations}, journal = {Quantum Information and Computation}, volume = {16}, year = {2016}, pages = {901-953}, abstract = {

We consider the problem of decomposing arbitrary single-qubit z-rotations into ancilla-free Clifford+T circuits, up to given epsilon. We present a new efficient algorithm for solving this problem optimally, i.e., for finding the shortest possible circuit whatsoever for the given problem instance. The algorithm requires a factoring oracle (such as a quantum computer). Even in the absence of a factoring oracle, the algorithm is still near-optimal: In this case, it finds a solution of T-count m + O(log(log(1/epsilon))), where m is the T-count of the second-to-optimal solution. In the typical case, this yields circuit decompositions of T-count 3log_2(1/epsilon) + O(log(log(1/epsilon))).

}, url = {http://arxiv.org/abs/1403.2975v2}, author = {Neil J. Ross and Peter Selinger} } @article {1909, title = {Optimized tomography of continuous variable systems using excitation counting}, journal = {Physical Review A}, volume = {94}, year = {2016}, month = {2016/11/21}, pages = {052327}, abstract = {

We propose a systematic procedure to optimize quantum state tomography protocols for continuous variable systems based on excitation counting preceded by a displacement operation. Compared with conventional tomography based on Husimi or Wigner function measurement, the excitation counting approach can significantly reduce the number of measurement settings. We investigate both informational completeness and robustness, and provide a bound of reconstruction error involving the condition number of the sensing map. We also identify the measurement settings that optimize this error bound, and demonstrate that the improved reconstruction robustness can lead to an order-of-magnitude reduction of estimation error with given resources. This optimization procedure is general and can incorporate prior information of the unknown state to further simplify the protocol.

}, doi = {10.1103/PhysRevA.94.052327}, url = {http://link.aps.org/doi/10.1103/PhysRevA.94.052327}, author = {Shen, Chao and Heeres, Reinier W. and Reinhold, Philip and Jiang, Luyao and Yi-Kai Liu and Schoelkopf, Robert J. and Jiang, Liang} } @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 {1187, title = {Realizing Exactly Solvable SU(N) Magnets with Thermal Atoms}, journal = {Physical Review A}, volume = {93}, year = {2016}, month = {2016/05/06}, abstract = {

We show that n thermal fermionic alkaline-earth-metal atoms in a flat-bottom trap allow one to robustly implement a spin model displaying two symmetries: the Sn symmetry that permutes atoms occupying different vibrational levels of the trap and the SU(N) symmetry associated with N nuclear spin states. The symmetries make the model exactly solvable, which, in turn, enables the analytic study of dynamical processes such as spin diffusion in this SU(N) system. We also show how to use this system to generate entangled states that allow for Heisenberg-limited metrology. This highly symmetric spin model should be experimentally realizable even when the vibrational levels are occupied according to a high-temperature thermal or an arbitrary nonthermal distribution.

}, doi = {10.1103/PhysRevA.93.051601}, url = {http://journals.aps.org/pra/abstract/10.1103/PhysRevA.93.051601}, author = {Michael E. Beverland and Gorjan Alagic and Michael J. Martin and Andrew P. Koller and Ana M. Rey and Alexey V. Gorshkov} } @article {1907, title = {Steady-state superradiance with Rydberg polaritons}, journal = {arXiv:1611.00797}, year = {2016}, month = {2016/11/02}, abstract = {

A steady-state superradiant laser can be used to generate ultranarrow-linewidth light, and thus has important applications in the fields of quantum information and precision metrology. However, the light produced by such a laser is still essentially classical. Here, we show that the introduction of a Rydberg medium into a cavity containing atoms with a narrow optical transition can lead to the steady-state superradiant emission of ultranarrow-linewidth\ nonclassical\ light. The cavity nonlinearity induced by the Rydberg medium strongly modifies the superradiance threshold, and leads to a Mollow triplet in the cavity output spectrum\−this behavior can be understood as an unusual analogue of resonance fluorescence. The cavity output spectrum has an extremely sharp central peak, with a linewidth that can be far narrower than that of a classical superradiant laser. This unprecedented spectral sharpness, together with the nonclassical nature of the light, could lead to new applications in which spectrally pure\ quantum\ light is desired.

}, url = {https://arxiv.org/abs/1611.00797}, author = {Zhe-Xuan Gong and Minghui Xu and Michael Foss-Feig and James K. Thompson and Ana Maria Rey and Murray Holland and Alexey V. Gorshkov} } @article {1515, title = {Demonstration of Robust Quantum Gate Tomography via Randomized Benchmarking}, journal = {New Journal of Physics}, volume = {17}, year = {2015}, month = {2015/11/05}, pages = {113019}, abstract = { Typical quantum gate tomography protocols struggle with a self-consistency problem: the gate operation cannot be reconstructed without knowledge of the initial state and final measurement, but such knowledge cannot be obtained without well-characterized gates. A recently proposed technique, known as randomized benchmarking tomography (RBT), sidesteps this self-consistency problem by designing experiments to be insensitive to preparation and measurement imperfections. We implement this proposal in a superconducting qubit system, using a number of experimental improvements including implementing each of the elements of the Clifford group in single {\textquoteleft}atomic{\textquoteright} pulses and custom control hardware to enable large overhead protocols. We show a robust reconstruction of several single-qubit quantum gates, including a unitary outside the Clifford group. We demonstrate that RBT yields physical gate reconstructions that are consistent with fidelities obtained by randomized benchmarking. }, doi = {10.1088/1367-2630/17/11/113019}, url = {http://arxiv.org/abs/1505.06686}, author = {Blake R. Johnson and Marcus P. da Silva and Colm A. Ryan and Shelby Kimmel and Jerry M. Chow and Thomas A. Ohki} } @article {1526, title = {Entanglement entropy of dispersive media from thermodynamic entropy in one higher dimension}, journal = {Physical Review Letters}, volume = {114}, year = {2015}, month = {2015/04/16}, pages = {151602}, abstract = { A dispersive medium becomes entangled with zero-point fluctuations in the vacuum. We consider an arbitrary array of material bodies weakly interacting with a quantum field and compute the quantum mutual information between them. It is shown that the mutual information in D dimensions can be mapped to classical thermodynamic entropy in D+1 dimensions. As a specific example, we compute the mutual information both analytically and numerically for a range of separation distances between two bodies in D=2 dimensions and find a logarithmic correction to the area law at short separations. A key advantage of our method is that it allows the strong subadditivity property---notoriously difficult to prove for quantum systems---to be easily verified. }, doi = {10.1103/PhysRevLett.114.151602}, url = {http://arxiv.org/abs/1412.5613v2}, author = {Mohammad F. Maghrebi and Homer Reid} } @article {1473, title = {Entangling two transportable neutral atoms via local spin exchange}, journal = {Nature}, volume = {527}, year = {2015}, month = {2015/11/02}, pages = {208-211}, abstract = { To advance quantum information science a constant pursuit is the search for physical systems that meet the stringent requirements for creating and preserving quantum entanglement. In atomic physics, robust two-qubit entanglement is typically achieved by strong, long-range interactions in the form of Coulomb interactions between ions or dipolar interactions between Rydberg atoms. While these interactions allow fast gates, atoms subject to these interactions must overcome the associated coupling to the environment and cross-talk among qubits. Local interactions, such as those requiring significant wavefunction overlap, can alleviate these detrimental effects yet present a new challenge: To distribute entanglement, qubits must be transported, merged for interaction, and then isolated for storage and subsequent operations. Here we show how, via a mobile optical tweezer, it is possible to prepare and locally entangle two ultracold neutral atoms, and then separate them while preserving their entanglement. While ultracold neutral atom experiments have measured dynamics consistent with spin entanglement, we are now able to demonstrate two-particle coherence via application of a local gradient and parity measurements; this new entanglement-verification protocol could be applied to arbitrary spin-entangled states of spatially-separated atoms. The local entangling operation is achieved via ultracold spin-exchange interactions, and quantum tunneling is used to combine and separate atoms. Our toolset provides a framework for dynamically entangling remote qubits via local operations within a large-scale quantum register. }, doi = {10.1038/nature16073}, url = {http://arxiv.org/abs/1507.05586}, author = {A. M. Kaufman and B. J. Lester and Michael Foss-Feig and M. L. Wall and A. M. Rey and C. A. Regal} } @article {1259, title = {Momentum switches}, journal = {Quantum Information and Computation}, volume = {15}, year = {2015}, month = {2015/05/01}, pages = {601-621}, abstract = { Certain continuous-time quantum walks can be viewed as scattering processes. These processes can perform quantum computations, but it is challenging to design graphs with desired scattering behavior. In this paper, we study and construct momentum switches, graphs that route particles depending on their momenta. We also give an example where there is no exact momentum switch, although we construct an arbitrarily good approximation. }, url = {http://arxiv.org/abs/1406.4510v1}, author = {Andrew M. Childs and David Gosset and Daniel Nagaj and Mouktik Raha and Zak Webb} } @article {1604, title = {Observation of optomechanical buckling phase transitions}, year = {2015}, month = {2015/10/16}, abstract = {

Correlated phases of matter provide long-term stability for systems as diverse as solids, magnets, and potential exotic quantum materials. Mechanical systems, such as relays and buckling transition spring switches can yield similar stability by exploiting non-equilibrium phase transitions. Curiously, in the optical domain, observations of such phase transitions remain elusive. However, efforts to integrate optical and mechanical systems -- optomechanics -- suggest that a hybrid approach combining the quantum control of optical systems with the engineerability of mechanical systems may provide a new avenue for such explorations. Here we report the first observation of the buckling of an optomechanical system, in which transitions between stable mechanical states corresponding to both first- and second-order phase transitions are driven by varying laser power and detuning. Our results enable new applications in photonics and, given rapid progress in pushing optomechanical systems into the quantum regime, the potential for explorations of quantum phase transitions.

}, url = {http://arxiv.org/abs/1510.04971v1}, author = {Haitan Xu and Utku Kemiktarak and Jingyun Fan and Stephen Ragole and John Lawall and J. M. Taylor} } @article {1532, title = {Optimal ancilla-free Clifford+V approximation of z-rotations}, journal = {Quantum Information and Computation}, volume = {15}, year = {2015}, month = {2015/03/06}, pages = {932-950}, abstract = { We describe a new efficient algorithm to approximate z-rotations by ancilla-free Clifford+V circuits, up to a given precision epsilon. Our algorithm is optimal in the presence of an oracle for integer factoring: it outputs the shortest Clifford+V circuit solving the given problem instance. In the absence of such an oracle, our algorithm is still near-optimal, producing circuits of V-count m + O(log(log(1/epsilon))), where m is the V-count of the third-to-optimal solution. A restricted version of the algorithm approximates z-rotations in the Pauli+V gate set. Our method is based on previous work by the author and Selinger on the optimal ancilla-free approximation of z-rotations using Clifford+T gates and on previous work by Bocharov, Gurevich, and Svore on the asymptotically optimal ancilla-free approximation of z-rotations using Clifford+V gates. }, url = {http://arxiv.org/abs/1409.4355v2}, author = {Neil J. Ross} } @article {1565, title = {Programming the Quantum Future}, journal = {Communications of the ACM}, volume = {58}, year = {2015}, month = {2015/08/01}, pages = {52-61}, abstract = {The earliest computers, like the ENIAC, were rare and heroically difficult to program. That difficulty stemmed from the requirement that algorithms be expressed in a "vocabulary" suited to the particular hardware available, ranging from function tables for the ENIAC to more conventional arithmetic and movement operations on later machines. Introduction of symbolic programming languages, exemplified by FORTRAN, solved a major difficulty for the next generation of computing devices by enabling specification of an algorithm in a form more suitable for human understanding, then translating this specification to a form executable by the machine. The "programming language" used for such specification bridged a semantic gap between the human and the computing device. It provided two important features: high-level abstractions, taking care of automated bookkeeping, and modularity, making it easier to reason about sub-parts of programs.}, doi = {10.1145/2699415}, url = {http://cacm.acm.org/magazines/2015/8/189851-programming-the-quantum-future/fulltext$\#$comments}, author = {D. Scott Alexander and Neil J. Ross and Peter Selinger and Jonathan M. Smith and Beno{\^\i}t Valiron} } @article {1839, title = {Beyond the spin model approximation for Ramsey spectroscopy}, journal = {Phys. Rev. Lett.}, volume = {112}, year = {2014}, pages = {123001}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.112.123001}, author = {A P Koller and M Beverland and Alexey V. Gorshkov and A M Rey} } @article {bian2014discrete, title = {Discrete optimization using quantum annealing on sparse Ising models}, journal = {Frontiers in Physics}, volume = {2}, year = {2014}, month = {2014/09/01}, pages = {56}, publisher = {Frontiers}, abstract = {This paper discusses techniques for solving discrete optimization problems using quantum annealing. Practical issues likely to affect the computation include precision limitations, finite temperature, bounded energy range, sparse connectivity, and small numbers of qubits. To address these concerns we propose a way of finding energy representations with large classical gaps between ground and first excited states, efficient algorithms for mapping non-compatible Ising models into the hardware, and the use of decomposition methods for problems that are too large to fit in hardware. We validate the approach by describing experiments with D-Wave quantum hardware for low density parity check decoding with up to 1000 variables.}, author = {Bian, Zhengbing and Chudak, Fabian and Israel, Robert and Brad Lackey and Macready, William G and Roy, Aidan} } @article {1791, title = {Extended order parameter and conjugate field for the dynamic phase transition in a Ginzburg-Landau mean-field model in an oscillating field}, journal = {Physical Review E}, volume = {89}, year = {2014}, month = {2014/02/12}, pages = {022114}, abstract = {We present numerical evidence for an extended order parameter and conjugate field for the dynamic phase transition in a Ginzburg-Landau mean-field model driven by an oscillating field. The order parameter, previously taken to be the time-averaged magnetization, comprises the deviations of the Fourier components of the magnetization from their values at the critical period. The conjugate field, previously taken to be the time-averaged magnetic field, comprises the even Fourier components of the field. The scaling exponents β and δ associated with the extended order parameter and conjugate field are shown numerically to be consistent with their values in the equilibrium mean-field model.}, doi = {10.1103/PhysRevE.89.022114}, url = {http://link.aps.org/doi/10.1103/PhysRevE.89.022114}, author = {Daniel T. Robb and Aaron Ostrander} } @article {1477, title = {Hong-Ou-Mandel atom interferometry in tunnel-coupled optical tweezers}, journal = {Science}, volume = {345}, year = {2014}, month = {2014/06/26}, pages = {306 - 309}, abstract = { The quantum statistics of atoms is typically observed in the behavior of an ensemble via macroscopic observables. However, quantum statistics modifies the behavior of even two particles, inducing remarkable consequences that are at the heart of quantum science. Here we demonstrate near-complete control over all the internal and external degrees of freedom of two laser-cooled 87Rb atoms trapped in two optical tweezers. This full controllability allows us to implement a massive-particle analog of a Hong-Ou-Mandel interferometer where atom tunneling plays the role of a photon beamsplitter. We use the interferometer to probe the effect of quantum statistics on the two-atom dynamics under tunable initial conditions, chosen to adjust the degree of atomic indistinguishability. Our work thereby establishes laser-cooled atoms in optical tweezers as a new route to bottom-up engineering of scalable, low-entropy quantum systems. }, doi = {10.1126/science.1250057}, url = {http://arxiv.org/abs/1312.7182v2}, author = {A. M. Kaufman and B. J. Lester and C. M. Reynolds and M. L. Wall and Michael Foss-Feig and K. R. A. Hazzard and A. M. Rey and C. A. Regal} } @article {1480, title = {Many-body dynamics of dipolar molecules in an optical lattice}, journal = {Physical Review Letters}, volume = {113}, year = {2014}, month = {2014/11/7}, abstract = { Understanding the many-body dynamics of isolated quantum systems is one of the central challenges in modern physics. To this end, the direct experimental realization of strongly correlated quantum systems allows one to gain insights into the emergence of complex phenomena. Such insights enable the development of theoretical tools that broaden our understanding. Here, we theoretically model and experimentally probe with Ramsey spectroscopy the quantum dynamics of disordered, dipolar-interacting, ultracold molecules in a partially filled optical lattice. We report the capability to control the dipolar interaction strength, and we demonstrate that the many-body dynamics extends well beyond a nearest-neighbor or mean-field picture, and cannot be quantitatively described using previously available theoretical tools. We develop a novel cluster expansion technique and demonstrate that our theoretical method accurately captures the measured dependence of the spin dynamics on molecule number and on the dipolar interaction strength. In the spirit of quantum simulation, this agreement simultaneously benchmarks the new theoretical method and verifies our microscopic understanding of the experiment. Our findings pave the way for numerous applications in quantum information science, metrology, and condensed matter physics. }, doi = {10.1103/PhysRevLett.113.195302}, url = {http://arxiv.org/abs/1402.2354v1}, author = {Kaden R. A. Hazzard and Bryce Gadway and Michael Foss-Feig and Bo Yan and Steven A. Moses and Jacob P. Covey and Norman Y. Yao and Mikhail D. Lukin and Jun Ye and Deborah S. Jin and Ana Maria Rey} } @article {1202, title = {Non-local propagation of correlations in long-range interacting quantum systems }, journal = {Nature}, volume = {511}, year = {2014}, month = {2014/7/9}, pages = {198 - 201}, abstract = { The maximum speed with which information can propagate in a quantum many-body system directly affects how quickly disparate parts of the system can become correlated and how difficult the system will be to describe numerically. For systems with only short-range interactions, Lieb and Robinson derived a constant-velocity bound that limits correlations to within a linear effective light cone. However, little is known about the propagation speed in systems with long-range interactions, since the best long-range bound is too loose to give the correct light-cone shape for any known spin model and since analytic solutions rarely exist. In this work, we experimentally determine the spatial and time-dependent correlations of a far-from-equilibrium quantum many-body system evolving under a long-range Ising- or XY-model Hamiltonian. For several different interaction ranges, we extract the shape of the light cone and measure the velocity with which correlations propagate through the system. In many cases we find increasing propagation velocities, which violate the Lieb-Robinson prediction, and in one instance cannot be explained by any existing theory. Our results demonstrate that even modestly-sized quantum simulators are well-poised for studying complicated many-body systems that are intractable to classical computation. }, doi = {10.1038/nature13450}, url = {http://arxiv.org/abs/1401.5088v1}, author = {Philip Richerme and Zhe-Xuan Gong and Aaron Lee and Crystal Senko and Jacob Smith and Michael Foss-Feig and Spyridon Michalakis and Alexey V. Gorshkov and Christopher Monroe} } @article {1840, title = {Probing many-body interactions in an optical lattice clock}, journal = {Ann. Phys.}, volume = {340}, year = {2014}, pages = {311}, url = {http://www.sciencedirect.com/science/article/pii/S0003491613002546}, author = {Rey, A M and Alexey V. Gorshkov and Kraus, C V and Martin, M J and Bishof, M and Swallows, M D and Zhang, X and Benko, C and Ye, J and Lemke, N D and Ludlow, A D} } @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 {1534, title = {Quipper: Concrete Resource Estimation in Quantum Algorithms}, year = {2014}, month = {2014/12/01}, abstract = {

Despite the rich literature on quantum algorithms, there is a surprisingly small amount of coverage of their concrete logical design and implementation. Most resource estimation is done at the level of complexity analysis, but actual concrete numbers (of quantum gates, qubits, etc.) can differ by orders of magnitude. The line of work we present here is a formal framework to write, and reason about, quantum algorithms. Specifically, we designed a language, Quipper, with scalability in mind, and we are able to report actual resource counts for seven non-trivial algorithms found in the quantum computer science literature.

}, url = {http://arxiv.org/abs/1412.0625v1}, author = {Jonathan M. Smith and Neil J. Ross and Peter Selinger and Beno{\^\i}t Valiron} } @article {1788, title = {Remote tomography and entanglement swapping via von Neumann{\textendash}Arthurs{\textendash}Kelly interaction }, journal = {Physical Review A}, volume = {89}, year = {2014}, month = {2014/05/09}, pages = {052107}, abstract = {We propose an interaction-based method for remote tomography and entanglement swapping. Alice arranges a von Neumann-Arthurs-Kelly interaction between a system particle P and two apparatus particles A1,A2, and then transports the latter to Bob. Bob can reconstruct the unknown initial state of particle P not received by him by quadrature measurements on A1,A2. Further, if another particle P' in Alice{\textquoteright}s hands is EPR entangled with P, it will be EPR entangled with the distant pair A1,A2. This method will be contrasted with the usual teleportation protocols.}, doi = {http://dx.doi.org/10.1103/PhysRevA.89.052107}, url = {http://journals.aps.org/pra/abstract/10.1103/PhysRevA.89.052107}, author = {S. M. Roy and Abhinav Deshpande and Nitica Sakharwade} } @article {1514, title = {Robust Extraction of Tomographic Information via Randomized Benchmarking}, journal = {Physical Review X}, volume = {4}, year = {2014}, month = {2014/3/25}, abstract = { We describe how randomized benchmarking can be used to reconstruct the unital part of any trace-preserving quantum map, which in turn is sufficient for the full characterization of any unitary evolution, or more generally, any unital trace-preserving evolution. This approach inherits randomized benchmarking{\textquoteright}s robustness to preparation and measurement imperfections, therefore avoiding systematic errors caused by these imperfections. We also extend these techniques to efficiently estimate the average fidelity of a quantum map to unitary maps outside of the Clifford group. The unitaries we consider include operations commonly used to achieve universal quantum computation in a fault-tolerant setting. In addition, we rigorously bound the time and sampling complexities of randomized benchmarking procedures. }, doi = {10.1103/PhysRevX.4.011050}, url = {http://arxiv.org/abs/1306.2348v1}, author = {Shelby Kimmel and Marcus P. da Silva and Colm A. Ryan and Blake R. Johnson and Thomas Ohki} } @article {1479, title = {Suppressing the loss of ultracold molecules via the continuous quantum Zeno effect }, journal = {Physical Review Letters}, volume = {112}, year = {2014}, month = {2014/2/20}, abstract = { We investigate theoretically the suppression of two-body losses when the on-site loss rate is larger than all other energy scales in a lattice. This work quantitatively explains the recently observed suppression of chemical reactions between two rotational states of fermionic KRb molecules confined in one-dimensional tubes with a weak lattice along the tubes [Yan et al., Nature 501, 521-525 (2013)]. New loss rate measurements performed for different lattice parameters but under controlled initial conditions allow us to show that the loss suppression is a consequence of the combined effects of lattice confinement and the continuous quantum Zeno effect. A key finding, relevant for generic strongly reactive systems, is that while a single-band theory can qualitatively describe the data, a quantitative analysis must include multiband effects. Accounting for these effects reduces the inferred molecule filling fraction by a factor of five. A rate equation can describe much of the data, but to properly reproduce the loss dynamics with a fixed filling fraction for all lattice parameters we develop a mean-field model and benchmark it with numerically exact time-dependent density matrix renormalization group calculations. }, doi = {10.1103/PhysRevLett.112.070404}, url = {http://arxiv.org/abs/1310.2221v2}, author = {Bihui Zhu and Bryce Gadway and Michael Foss-Feig and Johannes Schachenmayer and Michael Wall and Kaden R. A. Hazzard and Bo Yan and Steven A. Moses and Jacob P. Covey and Deborah S. Jin and Jun Ye and Murray Holland and Ana Maria Rey} } @article {1475, title = {Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence }, journal = {New Journal of Physics}, volume = {15}, year = {2013}, month = {2013/11/07}, pages = {113008}, abstract = { Ising models, and the physical systems described by them, play a central role in generating entangled states for use in quantum metrology and quantum information. In particular, ultracold atomic gases, trapped ion systems, and Rydberg atoms realize long-ranged Ising models, which even in the absence of a transverse field can give rise to highly non-classical dynamics and long-range quantum correlations. In the first part of this paper, we present a detailed theoretical framework for studying the dynamics of such systems driven (at time t=0) into arbitrary unentangled non-equilibrium states, thus greatly extending and unifying the work of Ref. [1]. Specifically, we derive exact expressions for closed-time-path ordered correlation functions, and use these to study experimentally relevant observables, e.g. Bloch vector and spin-squeezing dynamics. In the second part, these correlation functions are then used to derive closed-form expressions for the dynamics of arbitrary spin-spin correlation functions in the presence of both T_1 (spontaneous spin relaxation/excitation) and T_2 (dephasing) type decoherence processes. Even though the decoherence is local, our solution reveals that the competition between Ising dynamics and T_1 decoherence gives rise to an emergent non-local dephasing effect, thereby drastically amplifying the degradation of quantum correlations. In addition to identifying the mechanism of this deleterious effect, our solution points toward a scheme to eliminate it via measurement-based coherent feedback. }, doi = {10.1088/1367-2630/15/11/113008}, url = {http://arxiv.org/abs/1306.0172v1}, author = {Michael Foss-Feig and Kaden R A Hazzard and John J Bollinger and Ana Maria Rey and Charles W Clark} } @article {1245, title = {Easy and hard functions for the Boolean hidden shift problem}, journal = {Proceedings of TQC 2013}, volume = {22}, year = {2013}, month = {2013/04/16}, pages = {50-79}, abstract = { We study the quantum query complexity of the Boolean hidden shift problem. Given oracle access to f(x+s) for a known Boolean function f, the task is to determine the n-bit string s. The quantum query complexity of this problem depends strongly on f. We demonstrate that the easiest instances of this problem correspond to bent functions, in the sense that an exact one-query algorithm exists if and only if the function is bent. We partially characterize the hardest instances, which include delta functions. Moreover, we show that the problem is easy for random functions, since two queries suffice. Our algorithm for random functions is based on performing the pretty good measurement on several copies of a certain state; its analysis relies on the Fourier transform. We also use this approach to improve the quantum rejection sampling approach to the Boolean hidden shift problem. }, doi = {10.4230/LIPIcs.TQC.2013.50}, url = {http://arxiv.org/abs/1304.4642v1}, author = {Andrew M. Childs and Robin Kothari and Maris Ozols and Martin Roetteler} } @article {1268, title = {Experimental Performance of a Quantum Simulator: Optimizing Adiabatic Evolution and Identifying Many-Body Ground States }, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/7/31}, abstract = { We use local adiabatic evolution to experimentally create and determine the ground state spin ordering of a fully-connected Ising model with up to 14 spins. Local adiabatic evolution -- in which the system evolution rate is a function of the instantaneous energy gap -- is found to maximize the ground state probability compared with other adiabatic methods while only requiring knowledge of the lowest $\sim N$ of the $2^N$ Hamiltonian eigenvalues. We also demonstrate that the ground state ordering can be experimentally identified as the most probable of all possible spin configurations, even when the evolution is highly non-adiabatic. }, doi = {10.1103/PhysRevA.88.012334}, url = {http://arxiv.org/abs/1305.2253v1}, author = {Philip Richerme and Crystal Senko and Jacob Smith and Aaron Lee and Simcha Korenblit and Christopher Monroe} } @article {1474, title = {Far from equilibrium quantum magnetism with ultracold polar molecules}, journal = {Physical Review Letters}, volume = {110}, year = {2013}, month = {2013/2/11}, abstract = { Recent theory has indicated how to emulate tunable models of quantum magnetism with ultracold polar molecules. Here we show that present molecule optical lattice experiments can accomplish three crucial goals for quantum emulation, despite currently being well below unit filling and not quantum degenerate. The first is to verify and benchmark the models proposed to describe these systems. The second is to prepare correlated and possibly useful states in well-understood regimes. The third is to explore many-body physics inaccessible to existing theoretical techniques. Our proposal relies on a non-equilibrium protocol that can be viewed either as Ramsey spectroscopy or an interaction quench. It uses only routine experimental tools available in any ultracold molecule experiment. }, doi = {10.1103/PhysRevLett.110.075301}, url = {http://arxiv.org/abs/1209.4076v1}, author = {Kaden R. A. Hazzard and Salvatore R. Manmana and Michael Foss-Feig and Ana Maria Rey} } @article {1536, title = {An Introduction to Quantum Programming in Quipper}, journal = {Lecture Notes in Computer Science}, volume = {7948}, year = {2013}, month = {2013/07/05}, pages = {110-124}, abstract = { Quipper is a recently developed programming language for expressing quantum computations. This paper gives a brief tutorial introduction to the language, through a demonstration of how to make use of some of its key features. We illustrate many of Quipper{\textquoteright}s language features by developing a few well known examples of Quantum computation, including quantum teleportation, the quantum Fourier transform, and a quantum circuit for addition. }, isbn = {978-3-642-38986-3}, doi = {10.1007/978-3-642-38986-3_10}, url = {http://arxiv.org/abs/1304.5485v1}, author = {Alexander S. Green and Peter LeFanu Lumsdaine and Neil J. Ross and Peter Selinger and Beno{\^\i}t Valiron} } @article {1175, title = {Kitaev honeycomb and other exotic spin models with polar molecules}, journal = {Molecular Physics}, volume = {111}, year = {2013}, month = {2013/01/01}, pages = {1908 - 1916}, abstract = { We show that ultracold polar molecules pinned in an optical lattice can be used to access a variety of exotic spin models, including the Kitaev honeycomb model. Treating each molecule as a rigid rotor, we use DC electric and microwave fields to define superpositions of rotational levels as effective spin degrees of freedom, while dipole-dipole interactions give rise to interactions between the spins. In particular, we show that, with sufficient microwave control, the interaction between two spins can be written as a sum of five independently controllable Hamiltonian terms proportional to the five rank-2 spherical harmonics Y_{2,q}(theta,phi), where (theta,phi) are the spherical coordinates of the vector connecting the two molecules. To demonstrate the potential of this approach beyond the simplest examples studied in [S. R. Manmana et al., arXiv:1210.5518v2], we focus on the realization of the Kitaev honeycomb model, which can support exotic non-Abelian anyonic excitations. We also discuss the possibility of generating spin Hamiltonians with arbitrary spin S, including those exhibiting SU(N=2S+1) symmetry. }, doi = {10.1080/00268976.2013.800604}, url = {http://arxiv.org/abs/1301.5636v1}, author = {Alexey V. Gorshkov and Kaden R. A. Hazzard and Ana Maria Rey} } @article {1471, title = {Non-equilibrium dynamics of Ising models with decoherence: an exact solution }, journal = {Physical Review A}, volume = {87}, year = {2013}, month = {2013/4/3}, abstract = { The interplay between interactions and decoherence in many-body systems is of fundamental importance in quantum physics: Decoherence can degrade correlations, but can also give rise to a variety of rich dynamical and steady-state behaviors. We obtain an exact analytic solution for the non-equilibrium dynamics of Ising models with arbitrary interactions and subject to the most general form of local Markovian decoherence. Our solution shows that decoherence affects the relaxation of observables more than predicted by single-particle considerations. It also reveals a dynamical phase transition, specifically a Hopf bifurcation, which is absent at the single-particle level. These calculations are applicable to ongoing quantum information and emulation efforts using a variety of atomic, molecular, optical, and solid-state systems. }, doi = {10.1103/PhysRevA.87.042101}, url = {http://arxiv.org/abs/1209.5795v2}, author = {Michael Foss-Feig and Kaden R. A. Hazzard and John J. Bollinger and Ana Maria Rey} } @article {1270, title = {Quantum Catalysis of Magnetic Phase Transitions in a Quantum Simulator}, journal = {Physical Review Letters}, volume = {111}, year = {2013}, month = {2013/9/5}, abstract = { We control quantum fluctuations to create the ground state magnetic phases of a classical Ising model with a tunable longitudinal magnetic field using a system of 6 to 10 atomic ion spins. Due to the long-range Ising interactions, the various ground state spin configurations are separated by multiple first-order phase transitions, which in our zero temperature system cannot be driven by thermal fluctuations. We instead use a transverse magnetic field as a quantum catalyst to observe the first steps of the complete fractal devil{\textquoteright}s staircase, which emerges in the thermodynamic limit and can be mapped to a large number of many-body and energy-optimization problems. }, doi = {10.1103/PhysRevLett.111.100506}, url = {http://arxiv.org/abs/1303.6983v2}, author = {Philip Richerme and Crystal Senko and Simcha Korenblit and Jacob Smith and Aaron Lee and Rajibul Islam and Wesley C. Campbell and Christopher Monroe} } @article {1842, title = {A quantum many-body spin system in an optical lattice clock}, journal = {Science}, volume = {341}, year = {2013}, pages = {632}, url = {http://www.sciencemag.org/content/341/6146/632.abstract}, author = {M J Martin and Bishof, M and Swallows, M D and X Zhang and C Benko and J von-Stecher and Alexey V. Gorshkov and Rey, A M and Jun Ye} } @article {1535, title = {Quipper: A Scalable Quantum Programming Language}, journal = {ACM SIGPLAN Notices}, volume = {48}, year = {2013}, month = {2013/06/23}, pages = {333-342}, abstract = {

The field of quantum algorithms is vibrant. Still, there is currently a lack of programming languages for describing quantum computation on a practical scale, i.e., not just at the level of toy problems. We address this issue by introducing Quipper, a scalable, expressive, functional, higher-order quantum programming language. Quipper has been used to program a diverse set of non-trivial quantum algorithms, and can generate quantum gate representations using trillions of gates. It is geared towards a model of computation that uses a classical computer to control a quantum device, but is not dependent on any particular model of quantum hardware. Quipper has proven effective and easy to use, and opens the door towards using formal methods to analyze quantum algorithms.

}, doi = {10.1145/2499370.2462177}, url = {http://arxiv.org/abs/1304.3390v1}, author = {Alexander S. Green and Peter LeFanu Lumsdaine and Neil J. Ross and Peter Selinger and Beno{\^\i}t Valiron} } @article {1344, title = {The Resonant Exchange Qubit}, journal = {Physical Review Letters}, volume = {111}, year = {2013}, month = {2013/7/31}, abstract = {We introduce a solid-state qubit in which exchange interactions among confined electrons provide both the static longitudinal field and the oscillatory transverse field, allowing rapid and full qubit control via rf gate-voltage pulses. We demonstrate two-axis control at a detuning sweet-spot, where leakage due to hyperfine coupling is suppressed by the large exchange gap. A {\pi}/2-gate time of 2.5 ns and a coherence time of 19 {\mu}s, using multi-pulse echo, are also demonstrated. Model calculations that include effects of hyperfine noise are in excellent quantitative agreement with experiment. }, doi = {10.1103/PhysRevLett.111.050501}, url = {http://arxiv.org/abs/1304.3413v2}, author = {J. Medford and J. Beil and J. M. Taylor and E. I. Rashba and H. Lu and A. C. Gossard and C. M. Marcus} } @article {1345, title = {Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit}, journal = {Nature Nanotechnology}, volume = {8}, year = {2013}, month = {2013/9/1}, pages = {654 - 659}, abstract = {We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes. }, doi = {10.1038/nnano.2013.168}, url = {http://arxiv.org/abs/1302.1933v1}, author = {J. Medford and J. Beil and J. M. Taylor and S. D. Bartlett and A. C. Doherty and E. I. Rashba and D. P. DiVincenzo and H. Lu and A. C. Gossard and C. M. Marcus} } @article {1184, title = {Topological phases in ultracold polar-molecule quantum magnets}, journal = {Physical Review B}, volume = {87}, year = {2013}, month = {2013/2/26}, abstract = { We show how to use polar molecules in an optical lattice to engineer quantum spin models with arbitrary spin S >= 1/2 and with interactions featuring a direction-dependent spin anisotropy. This is achieved by encoding the effective spin degrees of freedom in microwave-dressed rotational states of the molecules and by coupling the spins through dipolar interactions. We demonstrate how one of the experimentally most accessible anisotropies stabilizes symmetry protected topological phases in spin ladders. Using the numerically exact density matrix renormalization group method, we find that these interacting phases -- previously studied only in the nearest-neighbor case -- survive in the presence of long-range dipolar interactions. We also show how to use our approach to realize the bilinear-biquadratic spin-1 and the Kitaev honeycomb models. Experimental detection schemes and imperfections are discussed. }, doi = {10.1103/PhysRevB.87.081106}, url = {http://arxiv.org/abs/1210.5518v2}, author = {Salvatore R. Manmana and E. M. Stoudenmire and Kaden R. A. Hazzard and Ana Maria Rey and Alexey V. Gorshkov} } @article {1458, title = {Comment on some results of Erdahl and the convex structure of reduced density matrices}, journal = {Journal of Mathematical Physics}, volume = {53}, year = {2012}, month = {2012/05/16}, pages = {072203}, abstract = { In J. Math. Phys. 13, 1608-1621 (1972), Erdahl considered the convex structure of the set of $N$-representable 2-body reduced density matrices in the case of fermions. Some of these results have a straightforward extension to the $m$-body setting and to the more general quantum marginal problem. We describe these extensions, but can not resolve a problem in the proof of Erdahl{\textquoteright}s claim that every extreme point is exposed in finite dimensions. Nevertheless, we can show that when $2m \geq N$ every extreme point of the set of $N$-representable $m$-body reduced density matrices has a unique pre-image in both the symmetric and anti-symmetric setting. Moreover, this extends to the quantum marginal setting for a pair of complementary $m$-body and $(N-m)$-body reduced density matrices. }, doi = {10.1063/1.4736842}, url = {http://arxiv.org/abs/1205.3682v1}, author = {Jianxin Chen and Zhengfeng Ji and Mary Beth Ruskai and Bei Zeng and Duan-Lu Zhou} } @article {1563, title = {Full Abstraction for Set-Based Models of the Symmetric Interaction Combinators}, journal = {Proceedings of the 15th International Conference on Foundations of Software Science and Computation Structures}, volume = {7213}, year = {2012}, month = {2012/01/01}, pages = {316-330}, abstract = {The symmetric interaction combinators are a model of distributed and deterministic computation based on Lafont{\textquoteright}s interaction nets, a special form of graph rewriting. The interest of the symmetric interaction combinators lies in their universality, that is, the fact that they may encode all other interaction net systems; for instance, several implementations of the lambda-calculus in the symmetric interaction combinators exist, related to Lamping{\textquoteright}s sharing graphs for optimal reduction. A certain number of observational equivalences were introduced for this system, by Lafont, Fernandez and Mackie, and the first author. In this paper, we study the problem of full abstraction with respect to one of these equivalences, using a class of very simple denotational models based on pointed sets.}, url = {https://lipn.univ-paris13.fr/~mazza/papers/CombSetSem-FOSSACS2012.pdf}, author = {Damiano Mazza and Neil J. Ross} } @article {1476, title = {Long-lived dipolar molecules and Feshbach molecules in a 3D optical lattice }, journal = {Physical Review Letters}, volume = {108}, year = {2012}, month = {2012/2/23}, abstract = { We have realized long-lived ground-state polar molecules in a 3D optical lattice, with a lifetime of up to 25 s, which is limited only by off-resonant scattering of the trapping light. Starting from a 2D optical lattice, we observe that the lifetime increases dramatically as a small lattice potential is added along the tube-shaped lattice traps. The 3D optical lattice also dramatically increases the lifetime for weakly bound Feshbach molecules. For a pure gas of Feshbach molecules, we observe a lifetime of >20 s in a 3D optical lattice; this represents a 100-fold improvement over previous results. This lifetime is also limited by off-resonant scattering, the rate of which is related to the size of the Feshbach molecule. Individually trapped Feshbach molecules in the 3D lattice can be converted to pairs of K and Rb atoms and back with nearly 100\% efficiency. }, doi = {10.1103/PhysRevLett.108.080405}, url = {http://arxiv.org/abs/1110.4420v1}, author = {Amodsen Chotia and Brian Neyenhuis and Steven A. Moses and Bo Yan and Jacob P. Covey and Michael Foss-Feig and Ana Maria Rey and Deborah S. Jin and Jun Ye} } @article {1470, title = {Steady-state many-body entanglement of hot reactive fermions}, journal = {Physical Review Letters}, volume = {109}, year = {2012}, month = {2012/12/4}, abstract = { Entanglement is typically created via systematic intervention in the time evolution of an initially unentangled state, which can be achieved by coherent control, carefully tailored non-demolition measurements, or dissipation in the presence of properly engineered reservoirs. In this paper we show that two-component Fermi gases at ~\mu K temperatures naturally evolve, in the presence of reactive two-body collisions, into states with highly entangled (Dicke-type) spin wavefunctions. The entanglement is a steady-state property that emerges---without any intervention---from uncorrelated initial states, and could be used to improve the accuracy of spectroscopy in experiments with fermionic alkaline earth atoms or fermionic groundstate molecules. }, doi = {10.1103/PhysRevLett.109.230501}, url = {http://arxiv.org/abs/1207.4741v1}, author = {Michael Foss-Feig and Andrew J. Daley and James K. Thompson and Ana Maria Rey} } @article {1531, title = {Casimir force between sharp-shaped conductors}, journal = {Proceedings of the National Academy of Sciences}, volume = {108}, year = {2011}, month = {2011/04/11}, pages = {6867 - 6871}, abstract = { Casimir forces between conductors at the sub-micron scale cannot be ignored in the design and operation of micro-electromechanical (MEM) devices. However, these forces depend non-trivially on geometry, and existing formulae and approximations cannot deal with realistic micro-machinery components with sharp edges and tips. Here, we employ a novel approach to electromagnetic scattering, appropriate to perfect conductors with sharp edges and tips, specifically to wedges and cones. The interaction of these objects with a metal plate (and among themselves) is then computed systematically by a multiple-scattering series. For the wedge, we obtain analytical expressions for the interaction with a plate, as functions of opening angle and tilt, which should provide a particularly useful tool for the design of MEMs. Our result for the Casimir interactions between conducting cones and plates applies directly to the force on the tip of a scanning tunneling probe; the unexpectedly large temperature dependence of the force in these configurations should attract immediate experimental interest. }, doi = {10.1073/pnas.1018079108}, url = {http://arxiv.org/abs/1010.3223v1}, author = {Mohammad F. Maghrebi and Sahand Jamal Rahi and Thorsten Emig and Noah Graham and Robert L. Jaffe and Mehran Kardar} } @article {1171, title = {d-Wave Superfluidity in Optical Lattices of Ultracold Polar Molecules}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/12/29}, abstract = { Recent work on ultracold polar molecules, governed by a generalization of the t-J Hamiltonian, suggests that molecules may be better suited than atoms for studying d-wave superfluidity due to stronger interactions and larger tunability of the system. We compute the phase diagram for polar molecules in a checkerboard lattice consisting of weakly coupled square plaquettes. In the simplest experimentally realizable case where there is only tunneling and an XX-type spin-spin interaction, we identify the parameter regime where d-wave superfluidity occurs. We also find that the inclusion of a density-density interaction destroys the superfluid phase and that the inclusion of a spin-density or an Ising-type spin-spin interaction can enhance the superfluid phase. We also propose schemes for experimentally realizing the perturbative calculations exhibiting enhanced d-wave superfluidity. }, doi = {10.1103/PhysRevA.84.063639}, url = {http://arxiv.org/abs/1110.5330v2}, author = {Kevin A. Kuns and Ana Maria Rey and Alexey V. Gorshkov} } @article {1467, title = {Phase diagram of the Bose Kondo-Hubbard model}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/11/16}, abstract = { We study a bosonic version of the Kondo lattice model with an on-site repulsion in the conduction band, implemented with alkali atoms in two bands of an optical lattice. Using both weak and strong-coupling perturbation theory, we find that at unit filling of the conduction bosons the superfluid to Mott insulator transition should be accompanied by a magnetic transition from a ferromagnet (in the superfluid) to a paramagnet (in the Mott insulator). Furthermore, an analytic treatment of Gutzwiller mean-field theory reveals that quantum spin fluctuations induced by the Kondo exchange cause the otherwise continuous superfluid to Mott-insulator phase transition to be first order. We show that lattice separability imposes a serious constraint on proposals to exploit excited bands for quantum simulations, and discuss a way to overcome this constraint in the context of our model by using an experimentally realized non-separable lattice. A method to probe the first-order nature of the transition based on collapses and revivals of the matter-wave field is also discussed. }, doi = {10.1103/PhysRevA.84.053619}, url = {http://arxiv.org/abs/1103.0245v2}, author = {Michael Foss-Feig and Ana Maria Rey} } @article {1181, title = {Quantum Magnetism with Polar Alkali Dimers}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/9/15}, abstract = { We show that dipolar interactions between ultracold polar alkali dimers in optical lattices can be used to realize a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. The model features long-range spin-spin interactions J_z and J_perp of XXZ type, long-range density-density interaction V, and long-range density-spin interaction W, all of which can be controlled in both magnitude and sign independently of each other and of the tunneling t. The "spin" is encoded in the rotational degree of freedom of the molecules, while the interactions are controlled by applied static electric and continuous-wave microwave fields. Furthermore, we show that nuclear spins of the molecules can be used to implement an additional (orbital) degree of freedom that is coupled to the original rotational degree of freedom in a tunable way. The presented system is expected to exhibit exotic physics and to provide insights into strongly correlated phenomena in condensed matter systems. Realistic experimental imperfections are discussed. }, doi = {10.1103/PhysRevA.84.033619}, url = {http://arxiv.org/abs/1106.1655v1}, author = {Alexey V. Gorshkov and Salvatore R. Manmana and Gang Chen and Eugene Demler and Mikhail D. Lukin and Ana Maria Rey} } @article {1846, title = {Quantum magnetism with polar alkali-metal dimers}, journal = {Phys. Rev. A}, volume = {84}, year = {2011}, pages = {033619}, url = {http://link.aps.org/abstract/PRA/v84/e033619/}, author = {Alexey V. Gorshkov and Manmana, S R and Chen, G and Demler, E and Lukin, M D and Rey, A M} } @article {1183, title = {Resolved atomic interaction sidebands in an optical clock transition}, journal = {Physical Review Letters}, volume = {106}, year = {2011}, month = {2011/6/22}, abstract = { We report the observation of resolved atomic interaction sidebands (ISB) in the ${}^{87}$Sr optical clock transition when atoms at microkelvin temperatures are confined in a two-dimensional (2D) optical lattice. The ISB are a manifestation of the strong interactions that occur between atoms confined in a quasi-one-dimensional geometry and disappear when the confinement is relaxed along one dimension. The emergence of ISB is linked to the recently observed suppression of collisional frequency shifts in [1]. At the current temperatures, the ISB can be resolved but are broad. At lower temperatures, ISB are predicted to be substantially narrower and usable as powerful spectroscopic tools in strongly interacting alkaline-earth gases. }, doi = {10.1103/PhysRevLett.106.250801}, url = {http://arxiv.org/abs/1102.1016v2}, author = {Michael Bishof and Yige Lin and Matthew D. Swallows and Alexey V. Gorshkov and Jun Ye and Ana Maria Rey} } @article {2617, title = {Spectral Concentration of Positive Functions on Compact Groups}, journal = {Journal of Fourier Analysis and Applications }, volume = {17}, year = {2011}, pages = {355-373}, abstract = {

The problem of understanding the Fourier-analytic structure of the cone of
positive functions on a group has a long history. In this article, we develop the first
quantitative spectral concentration results for such functions over arbitrary compact
groups. Specifically, we describe a family of finite, positive quadrature rules for the
Fourier coefficients of band-limited functions on compact groups. We apply these
quadrature rules to establish a spectral concentration result for positive functions:
given appropriately nested band limits A \⊂ B \⊂ G, we prove a lower bound on the
fraction of L2-mass that any B-band-limited positive function has in A. Our bounds
are explicit and depend only on elementary properties of A and B; they are the first
such bounds that apply to arbitrary compact groups. They apply to finite groups as
a special case, where the quadrature rule is given by the Fourier transform on the
smallest quotient whose dual contains the Fourier support of the function.

}, doi = {https://doi.org/10.1007/s00041-011-9174-5}, author = {Gorjan Alagic and Alexander Russell} } @article {1172, title = {Spectroscopy of dipolar fermions in 2D pancakes and 3D lattices}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/9/6}, abstract = { Motivated by ongoing measurements at JILA, we calculate the recoil-free spectra of dipolar interacting fermions, for example ultracold heteronuclear molecules, in a one-dimensional lattice of two-dimensional pancakes, spectroscopically probing transitions between different internal (e.g., rotational) states. We additionally incorporate p-wave interactions and losses, which are important for reactive molecules such as KRb. Moreover, we consider other sources of spectral broadening: interaction-induced quasiparticle lifetimes and the different polarizabilities of the different rotational states used for the spectroscopy. Although our main focus is molecules, some of the calculations are also useful for optical lattice atomic clocks. For example, understanding the p-wave shifts between identical fermions and small dipolar interactions coming from the excited clock state are necessary to reach future precision goals. Finally, we consider the spectra in a deep 3D lattice and show how they give a great deal of information about static correlation functions, including \textit{all} the moments of the density correlations between nearby sites. The range of correlations measurable depends on spectroscopic resolution and the dipole moment. }, doi = {10.1103/PhysRevA.84.033608}, url = {http://arxiv.org/abs/1106.1718v1}, author = {Kaden R. A. Hazzard and Alexey V. Gorshkov and Ana Maria Rey} } @article {1847, title = {Spectroscopy of dipolar fermions in layered two-dimensional and three-dimensional lattices}, journal = {Phys. Rev. A}, volume = {84}, year = {2011}, pages = {033608}, url = {http://link.aps.org/abstract/PRA/v84/e033608/}, author = {Hazzard, K R A and Alexey V. Gorshkov and Rey, A M} } @article {1198, title = {Tunable Superfluidity and Quantum Magnetism with Ultracold Polar Molecules }, journal = {Physical Review Letters}, volume = {107}, year = {2011}, month = {2011/9/8}, abstract = { By selecting two dressed rotational states of ultracold polar molecules in an optical lattice, we obtain a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. In addition to XXZ spin exchange, the model features density-density interactions and novel density-spin interactions; all interactions are dipolar. We show that full control of all interaction parameters in both magnitude and sign can be achieved independently of each other and of the tunneling. As a first step towards demonstrating the potential of the system, we apply the density matrix renormalization group method (DMRG) to obtain the 1D phase diagram of the simplest experimentally realizable case. Specifically, we show that the tunability and the long-range nature of the interactions in the t-J-V-W model enable enhanced superfluidity. Finally, we show that Bloch oscillations in a tilted lattice can be used to probe the phase diagram experimentally. }, doi = {10.1103/PhysRevLett.107.115301}, url = {http://arxiv.org/abs/1106.1644v1}, author = {Alexey V. Gorshkov and Salvatore R. Manmana and Gang Chen and Jun Ye and Eugene Demler and Mikhail D. Lukin and Ana Maria Rey} } @article {1401, title = {Approximating Turaev-Viro 3-manifold invariants is universal for quantum computation }, journal = {Physical Review A}, volume = {82}, year = {2010}, month = {2010/10/8}, abstract = { The Turaev-Viro invariants are scalar topological invariants of compact, orientable 3-manifolds. We give a quantum algorithm for additively approximating Turaev-Viro invariants of a manifold presented by a Heegaard splitting. The algorithm is motivated by the relationship between topological quantum computers and (2+1)-D topological quantum field theories. Its accuracy is shown to be nontrivial, as the same algorithm, after efficient classical preprocessing, can solve any problem efficiently decidable by a quantum computer. Thus approximating certain Turaev-Viro invariants of manifolds presented by Heegaard splittings is a universal problem for quantum computation. This establishes a novel relation between the task of distinguishing non-homeomorphic 3-manifolds and the power of a general quantum computer. }, doi = {10.1103/PhysRevA.82.040302}, url = {http://arxiv.org/abs/1003.0923v1}, author = {Gorjan Alagic and Stephen P. Jordan and Robert Koenig and Ben W. Reichardt} } @article {1469, title = {Heavy fermions in an optical lattice}, journal = {Physical Review A}, volume = {82}, year = {2010}, month = {2010/11/22}, abstract = { We employ a mean-field theory to study ground-state properties and transport of a two-dimensional gas of ultracold alklaline-earth metal atoms governed by the Kondo Lattice Hamiltonian plus a parabolic confining potential. In a homogenous system this mean-field theory is believed to give a qualitatively correct description of heavy fermion metals and Kondo insulators: it reproduces the Kondo-like scaling of the quasiparticle mass in the former, and the same scaling of the excitation gap in the latter. In order to understand ground-state properties in a trap we extend this mean-field theory via local-density approximation. We find that the Kondo insulator gap manifests as a shell structure in the trapped density profile. In addition, a strong signature of the large Fermi surface expected for heavy fermion systems survives the confinement, and could be probed in time-of-flight experiments. From a full self-consistent diagonalization of the mean-field theory we are able to study dynamics in the trap. We find that the mass enhancement of quasiparticle excitations in the heavy Fermi liquid phase manifests as slowing of the dipole oscillations that result from a sudden displacement of the trap center. }, doi = {10.1103/PhysRevA.82.053624}, url = {http://arxiv.org/abs/1007.5083v1}, author = {Michael Foss-Feig and Michael Hermele and Victor Gurarie and Ana Maria Rey} } @article {1455, title = {Principle of Maximum Entropy and Ground Spaces of Local Hamiltonians}, year = {2010}, month = {2010/10/13}, abstract = { The structure of the ground spaces of quantum systems consisting of local interactions is of fundamental importance to different areas of physics. In this Letter, we present a necessary and sufficient condition for a subspace to be the ground space of a k-local Hamiltonian. Our analysis are motivated by the concept of irreducible correlations studied by [Linden et al., PRL 89, 277906] and [Zhou, PRL 101, 180505], which is in turn based on the principle of maximum entropy. It establishes a better understanding of the ground spaces of local Hamiltonians and builds an intimate link of ground spaces to the correlations of quantum states. }, url = {http://arxiv.org/abs/1010.2739v4}, author = {Jianxin Chen and Zhengfeng Ji and Mary Beth Ruskai and Bei Zeng and Duanlu Zhou} } @article {1468, title = {Probing the Kondo Lattice Model with Alkaline Earth Atoms}, journal = {Physical Review A}, volume = {81}, year = {2010}, month = {2010/5/7}, abstract = { We study transport properties of alkaline-earth atoms governed by the Kondo Lattice Hamiltonian plus a harmonic confining potential, and suggest simple dynamical probes of several different regimes of the phase diagram that can be implemented with current experimental techniques. In particular, we show how Kondo physics at strong coupling, low density, and in the heavy fermion phase is manifest in the dipole oscillations of the conduction band upon displacement of the trap center. }, doi = {10.1103/PhysRevA.81.051603}, url = {http://arxiv.org/abs/0912.4762v1}, author = {Michael Foss-Feig and Michael Hermele and Ana Maria Rey} } @article {2616, title = {Quantum Algorithms for Simon{\textquoteright}s Problem over Nonabelian Groups}, journal = {ACM Trans. Algorithms}, volume = {6}, year = {2010}, abstract = {

Daniel Simon\&$\#$39;s 1994 discovery of an efficient quantum algorithm for finding \“hidden shifts\” of Z2n provided the first algebraic problem for which quantum computers are exponentially faster than their classical counterparts. In this article, we study the generalization of Simon\&$\#$39;s problem to arbitrary groups. Fixing a finite group G, this is the problem of recovering an involution m = (m1,\…,mn) \∈ Gn from an oracle f with the property that f(x \⋅ y) = f(x) \⇔ y \∈ {1, m}. In the current parlance, this is the hidden subgroup problem (HSP) over groups of the form Gn, where G is a nonabelian group of constant size, and where the hidden subgroup is either trivial or has order two.

Although groups of the form Gn have a simple product structure, they share important representation--theoretic properties with the symmetric groups Sn, where a solution to the HSP would yield a quantum algorithm for Graph Isomorphism. In particular, solving their HSP with the so-called \“standard method\” requires highly entangled measurements on the tensor product of many coset states.

In this article, we provide quantum algorithms with time complexity 2O(\√n) that recover hidden involutions m = (m1,\…mn) \∈ Gn where, as in Simon\&$\#$39;s problem, each mi is either the identity or the conjugate of a known element m which satisfies κ(m) = \−κ(1) for some κ \∈ Gh. Our approach combines the general idea behind Kuperberg\&$\#$39;s sieve for dihedral groups with the \“missing harmonic\” approach of Moore and Russell. These are the first nontrivial HSP algorithms for group families that require highly entangled multiregister Fourier sampling.

}, doi = {https://doi.org/10.1145/1644015.1644034}, author = {Gorjan Alagic and Cristopher Moore and Alexander Russell} } @article {1850, title = {Two-orbital SU(N) magnetism with ultracold alkaline-earth atoms}, journal = {Nature Phys.}, volume = {6}, year = {2010}, pages = {289}, url = {http://www.nature.com/nphys/journal/v6/n4/abs/nphys1535.html}, author = {Alexey V. Gorshkov and Hermele, M and Gurarie, V and Xu, C and Julienne, P S and Ye, J and Zoller, P and Demler, E and Lukin, M D and Rey, A M} } @article {1193, title = {Alkaline-Earth-Metal Atoms as Few-Qubit Quantum Registers}, journal = {Physical Review Letters}, volume = {102}, year = {2009}, month = {2009/3/18}, abstract = { We propose and analyze a novel approach to quantum information processing, in which multiple qubits can be encoded and manipulated using electronic and nuclear degrees of freedom associated with individual alkaline-earth atoms trapped in an optical lattice. Specifically, we describe how the qubits within each register can be individually manipulated and measured with sub-wavelength optical resolution. We also show how such few-qubit registers can be coupled to each other in optical superlattices via conditional tunneling to form a scalable quantum network. Finally, potential applications to quantum computation and precision measurements are discussed. }, doi = {10.1103/PhysRevLett.102.110503}, url = {http://arxiv.org/abs/0812.3660v2}, author = {Alexey V. Gorshkov and Ana Maria Rey and Andrew J. Daley and Martin M. Boyd and Jun Ye and Peter Zoller and Mikhail D. Lukin} } @article {1851, title = {Many-Body Treatment of the Collisional Frequency Shift in Fermionic Atoms}, journal = {Phys. Rev. Lett.}, volume = {103}, year = {2009}, pages = {260402}, url = {http://link.aps.org/abstract/PRL/v103/e260402/}, author = {Rey, A M and Alexey V. Gorshkov and Rubbo, C} } @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 {2615, title = {Uncertainty principles for compact groups}, journal = {Illinois J. Math. }, volume = {52}, year = {2008}, pages = {1315-1324}, abstract = {

We establish an uncertainty principle over arbitrary compact groups, generalizing several previous results. Specifically, we show that if P and R are operators on L2(G) such that P commutes with projection onto every measurable subset of G and R commutes with left-multiplication by elements of G, then ||PR||\≤||P\⋅χG||2||R||2, where χG:g↦1 is the characteristic function of G. As a consequence, we show that every nonzero function f in L2(G) satisfies μ(suppf)\⋅\∑ρ\∈G^dρrankf^(ρ)\≥1.

}, doi = {doi:10.1215/ijm/1258554365}, url = {http://projecteuclid.org/euclid.ijm/1258554365}, author = {Gorjan Alagic and Alexander Russell} } @article {1255, title = {Every NAND formula of size N can be evaluated in time N^{1/2+o(1)} on a quantum computer }, year = {2007}, month = {2007/03/02}, abstract = { For every NAND formula of size N, there is a bounded-error N^{1/2+o(1)}-time quantum algorithm, based on a coined quantum walk, that evaluates this formula on a black-box input. Balanced, or {\textquoteleft}{\textquoteleft}approximately balanced,{\textquoteright}{\textquoteright} NAND formulas can be evaluated in O(sqrt{N}) queries, which is optimal. It follows that the (2-o(1))-th power of the quantum query complexity is a lower bound on the formula size, almost solving in the positive an open problem posed by Laplante, Lee and Szegedy. }, url = {http://arxiv.org/abs/quant-ph/0703015v3}, author = {Andrew M. Childs and Ben W. Reichardt and Robert Spalek and Shengyu Zhang} } @article {2613, title = {Quantum Algorithms for Simon{\textquoteright}s Problem over General Groups}, journal = {SODA {\textquoteright}07: Proceedings of the eighteenth annual ACM-SIAM symposium on Discrete algorithms}, year = {2007}, month = {1/25/2007}, pages = {1217{\textendash}1224}, abstract = {

Daniel Simon\&$\#$39;s 1994 discovery of an efficient quantum algorithm for solving the hidden subgroup problem (HSP) over Zn2 provided one of the first algebraic problems for which quantum computers are exponentially faster than their classical counterparts. In this paper, we study the generalization of Simon\&$\#$39;s problem to arbitrary groups. Fixing a finite group G, this is the problem of recovering an involution m = (m1,...,mn) ε Gn from an oracle f with the property that f(x) = f(x \· y) \⇔ y ε {1, m}. In the current parlance, this is the hidden subgroup problem (HSP) over groups of the form Gn, where G is a nonabelian group of constant size, and where the hidden subgroup is either trivial or has order two.

Although groups of the form Gn have a simple product structure, they share important representation-theoretic properties with the symmetric groups Sn, where a solution to the HSP would yield a quantum algorithm for Graph Isomorphism. In particular, solving their HSP with the so-called \"standard method\" requires highly entangled measurements on the tensor product of many coset states.

Here we give quantum algorithms with time complexity 2O(\√n log n) that recover hidden involutions m = (m1,..., mn) ε Gn where, as in Simon\&$\#$39;s problem, each mi is either the identity or the conjugate of a known element m and there is a character X of G for which X(m) = - X(1). Our approach combines the general idea behind Kuperberg\&$\#$39;s sieve for dihedral groups with the \"missing harmonic\" approach of Moore and Russell. These are the first nontrivial hidden subgroup algorithms for group families that require highly entangled multiregister Fourier sampling.

}, doi = {https://dl.acm.org/doi/10.5555/1283383.1283514}, url = {https://arxiv.org/abs/quant-ph/0603251}, author = {Gorjan Alagic and Cristopher Moore and Alexander Russell} } @article {2614, title = {Quantum Computing and the Hunt for Hidden Symmetry}, journal = {Bulletin of the EATCS}, volume = {93}, year = {2007}, pages = {53-75}, abstract = {

In 1994, Peter Shor gave e cient quantum algorithms for factoring integers and extracting discrete logarithms [20]. If we believe that nature will permit us to faithfully implement our current model of quantum computation, then these algorithms dramatically contradict the Strong Church-Turing thesis. The e ect is heightened by the fact that these algorithms solve computational problems with long histories of attention by the computational and mathematical communities alike. In this article we discuss the branch of quantum algorithms research arising from attempts to generalize the core quantum algorithmic aspects of Shor\&$\#$39;s algorithms. Roughly, this can be viewed as the problem of generalizing algorithms of Simon [21] and Shor [20], which work over abelian groups, to general nonabelian groups. The article is meant to be self-contained, assuming no knowledge of quantum computing or the representation theory of nite groups. We begin in earnest in Section 2, describing the problem of symmetry nding : given a function f : G \→ S on a group G, this is the problem of determining {g \∈ G | \∀x, f(x) = f(gx)}, the set of symmetries of f . We switch gears in Section 3, giving a short introduction to the circuit model of quantum computation. The connection between these two sections is eventually established in Section 4, where we discuss the representation theory of nite groups and the quantum Fourier transform a unitary transformation speci cally tuned to the symmetries of the underlying group. Section 4.2 is devoted to Fourier

}, url = {https://pdfs.semanticscholar.org/08f7/abc04ca0bd38c1351ee1179139d8b0fc172b.pdf?_ga=2.210619804.800377824.1595266095-1152452310.1595266095}, author = {Gorjan Alagic and Alexander Russell} } @article {1194, title = {Signatures of incoherence in a quantum information processor}, year = {2007}, month = {2007/05/24}, abstract = { Incoherent noise is manifest in measurements of expectation values when the underlying ensemble evolves under a classical distribution of unitary processes. While many incoherent processes appear decoherent, there are important differences. The distribution functions underlying incoherent processes are either static or slowly varying with respect to control operations and so the errors introduced by these distributions are refocusable. The observation and control of incoherence in small Hilbert spaces is well known. Here we explore incoherence during an entangling operation, such as is relevant in quantum information processing. As expected, it is more difficult to separate incoherence and decoherence over such processes. However, by studying the fidelity decay under a cyclic entangling map we are able to identify distinctive experimental signatures of incoherence. This result is demonstrated both through numerical simulations and experimentally in a three qubit nuclear magnetic resonance implementation. }, url = {http://arxiv.org/abs/0705.3666v2}, author = {Michael K. Henry and Alexey V. Gorshkov and Yaakov S. Weinstein and Paola Cappellaro and Joseph Emerson and Nicolas Boulant and Jonathan S. Hodges and Chandrasekhar Ramanathan and Timothy F. Havel and Rudy Martinez and David G. Cory} } @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 {2612, title = {Decoherence in Quantum Walks on the Hypercube }, journal = {Phys. Rev. A }, volume = {76}, year = {2005}, month = {12/5/2005}, pages = {062304}, abstract = {

We study a natural notion of decoherence on quantum random walks over the hypercube. We prove that in this model there is a decoherence threshold beneath which the essential properties of the hypercubic quantum walk, such as linear mixing times, are preserved. Beyond the threshold, we prove that the walks behave like their classical counterparts.

}, doi = {https://doi.org/10.1103/PhysRevA.72.062304}, url = {https://arxiv.org/abs/quant-ph/0501169}, author = {Gorjan Alagic and Alexander Russell} } @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 {2611, title = {Strong Fourier Sampling Fails over Gn}, year = {2005}, month = {11/7/2005}, abstract = {

We present a negative result regarding the hidden subgroup problem on the powers Gn of a fixed group G. Under a condition on the base group G, we prove that strong Fourier sampling cannot distinguish some subgroups of Gn. Since strong sampling is in fact the optimal measurement on a coset state, this shows that we have no hope of efficiently solving the hidden subgroup problem over these groups with separable measurements on coset states (that is, using any polynomial number of single-register coset state experiments). Base groups satisfying our condition include all nonabelian simple groups. We apply our results to show that there exist uniform families of nilpotent groups whose normal series factors have constant size and yet are immune to strong Fourier sampling.

}, url = {https://arxiv.org/abs/quant-ph/0511054}, author = {Gorjan Alagic and Cristopher Moore and Alexander Russell} } @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 {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 {1414, title = {Bogoliubov approach to superfluidity of atoms in an optical lattice}, journal = {Journal of Physics B: Atomic, Molecular and Optical Physics}, volume = {36}, year = {2003}, month = {2003/03/14}, pages = {825 - 841}, abstract = { We use the Bogoliubov theory of atoms in an optical lattice to study the approach to the Mott-insulator transition. We derive an explicit expression for the superfluid density based on the rigidity of the system under phase variations. This enables us to explore the connection between the quantum depletion of the condensate and the quasi-momentum distribution on the one hand and the superfluid fraction on the other. The approach to the insulator phase may be characterized through the filling of the band by quantum depletion, which should be directly observable via the matter wave interference patterns. We complement these findings by self-consistent Hartree-Fock-Bogoliubov-Popov calculations for one-dimensional lattices including the effects of a parabolic trapping potential. }, doi = {10.1088/0953-4075/36/5/304}, url = {http://arxiv.org/abs/cond-mat/0210550v2}, author = {Ana Maria Rey and Keith Burnett and Robert Roth and Mark Edwards and Carl J. Williams and Charles W. Clark} } @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} }