TY - RPRT T1 - Data Needs and Challenges of Quantum Dot Devices Automation: Workshop Report Y1 - 2023 A1 - Justyna P. Zwolak A1 - Jacob M. Taylor A1 - Reed Andrews A1 - Jared Benson A1 - Garnett Bryant A1 - Donovan Buterakos A1 - Anasua Chatterjee A1 - Sankar Das Sarma A1 - Mark A. Eriksson A1 - Eliška Greplová A1 - Michael J. Gullans A1 - Fabian Hader A1 - Tyler J. Kovach A1 - Pranav S. Mundada A1 - Mick Ramsey A1 - Torbjoern Rasmussen A1 - Brandon Severin A1 - Anthony Sigillito A1 - Brennan Undseth A1 - Brian Weber AB -

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.

UR - https://arxiv.org/abs/2312.14322 U5 - https://doi.org/10.48550/arXiv.2312.14322 ER - TY - JOUR T1 - Decoherence from Long-Range Forces in Atom Interferometry JF - Phys. Rev. A Y1 - 2023 A1 - Jonathan Kunjummen A1 - Daniel Carney A1 - Jacob M. Taylor VL - 107 UR - https://arxiv.org/abs/2205.03006 CP - 033319 U5 - https://doi.org/10.1103/PhysRevA.107.033319 ER - TY - JOUR T1 - Digital quantum simulation of NMR experiments JF - Science Advances Y1 - 2023 A1 - Seetharam, Kushal A1 - Biswas, Debopriyo A1 - Noel, Crystal A1 - Risinger, Andrew A1 - Zhu, Daiwei A1 - Katz, Or A1 - Chattopadhyay, Sambuddha A1 - Cetina, Marko A1 - Monroe, Christopher A1 - Demler, Eugene A1 - Sels, Dries AB -

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.

VL - 9 UR - https://arxiv.org/abs/2109.13298 U5 - 10.1126/sciadv.adh2594 ER - TY - JOUR T1 - The discrete adiabatic quantum linear system solver has lower constant factors than the randomized adiabatic solver Y1 - 2023 A1 - Pedro C. S. Costa A1 - Dong An A1 - Ryan Babbush A1 - Dominic Berry AB -

The solution of linear systems of equations is the basis of many other quantum algorithms, and recent results provided an algorithm with optimal scaling in both the condition number κ and the allowable error ϵ [PRX Quantum \textbf{3}, 0403003 (2022)]. That work was based on the discrete adiabatic theorem, and worked out an explicit constant factor for an upper bound on the complexity. Here we show via numerical testing on random matrices that the constant factor is in practice about 1,500 times smaller than the upper bound found numerically in the previous results. That means that this approach is far more efficient than might naively be expected from the upper bound. In particular, it is over an order of magnitude more efficient than using a randomised approach from [arXiv:2305.11352] that claimed to be more efficient.

UR - https://arxiv.org/abs/2312.07690 ER - TY - JOUR T1 - DiVincenzo-like criteria for autonomous quantum machines Y1 - 2023 A1 - José Antonio Marín Guzmán A1 - Paul Erker A1 - Simone Gasparinetti A1 - Marcus Huber A1 - Nicole Yunger Halpern AB -

Controlled quantum machines have matured significantly. A natural next step is to grant them autonomy, freeing them from timed external control. For example, autonomy could unfetter quantum computers from classical control wires that heat and decohere them; and an autonomous quantum refrigerator recently reset superconducting qubits to near their ground states, as is necessary before a computation. What conditions are necessary for realizing useful autonomous quantum machines? Inspired by recent quantum thermodynamics and chemistry, we posit conditions analogous to DiVincenzo's criteria for quantum computing. Our criteria are intended to foment and guide the development of useful autonomous quantum machines.

UR - https://arxiv.org/abs/2307.08739 ER - TY - JOUR T1 - Dark Solitons in Bose-Einstein Condensates: A Dataset for Many-body Physics Research Y1 - 2022 A1 - Amilson R. Fritsch A1 - Shangjie Guo A1 - Sophia M. Koh A1 - I. B. Spielman A1 - Justyna P. Zwolak AB -

We establish a dataset of over 1.6×104 experimental images of Bose-Einstein condensates containing solitonic excitations to enable machine learning (ML) for many-body physics research. About 33 % of this dataset has manually assigned and carefully curated labels. The remainder is automatically labeled using SolDet -- an implementation of a physics-informed ML data analysis framework -- consisting of a convolutional-neural-network-based classifier and object detector as well as a statistically motivated physics-informed classifier and a quality metric. This technical note constitutes the definitive reference of the dataset, providing an opportunity for the data science community to develop more sophisticated analysis tools, to further understand nonlinear many-body physics, and even advance cold atom experiments.

UR - https://arxiv.org/abs/2205.09114 ER - TY - JOUR T1 - Deconfinement and Error Thresholds in Holography Y1 - 2022 A1 - Bao, Ning A1 - Cao, Charles A1 - Zhu, Guanyu KW - FOS: Physical sciences KW - High Energy Physics - Theory (hep-th) KW - Nuclear Theory (nucl-th) KW - Quantum Physics (quant-ph) KW - Strongly Correlated Electrons (cond-mat.str-el) AB -

We study the error threshold properties of holographic quantum error-correcting codes. We demonstrate that holographic CFTs admit an algebraic threshold, which is related to the confinement-deconfinement phase transition. We then apply geometric intuition from holography and the Hawking-Page phase transition to motivate the CFT result, and comment on potential extensions to other confining theories.

UR - https://arxiv.org/abs/2202.04710 U5 - 10.48550/ARXIV.2202.04710 ER - TY - JOUR T1 - Demonstration of three- and four-body interactions between trapped-ion spins Y1 - 2022 A1 - Katz, Or A1 - Feng, Lei A1 - Risinger, Andrew A1 - Monroe, Christopher A1 - Cetina, Marko KW - Atomic Physics (physics.atom-ph) KW - FOS: Physical sciences KW - Quantum Physics (quant-ph) AB -

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.

UR - https://arxiv.org/abs/2209.05691 U5 - 10.48550/ARXIV.2209.05691 ER - TY - JOUR T1 - Differentiable Quantum Programming with Unbounded Loops Y1 - 2022 A1 - Fang, Wang A1 - Ying, Mingsheng A1 - Wu, Xiaodi KW - FOS: Computer and information sciences KW - FOS: Physical sciences KW - Machine Learning (cs.LG) KW - Programming Languages (cs.PL) KW - Quantum Physics (quant-ph) AB -

The emergence of variational quantum applications has led to the development of automatic differentiation techniques in quantum computing. Recently, Zhu et al. (PLDI 2020) have formulated differentiable quantum programming with bounded loops, providing a framework for scalable gradient calculation by quantum means for training quantum variational applications. However, promising parameterized quantum applications, e.g., quantum walk and unitary implementation, cannot be trained in the existing framework due to the natural involvement of unbounded loops. To fill in the gap, we provide the first differentiable quantum programming framework with unbounded loops, including a newly designed differentiation rule, code transformation, and their correctness proof. Technically, we introduce a randomized estimator for derivatives to deal with the infinite sum in the differentiation of unbounded loops, whose applicability in classical and probabilistic programming is also discussed. We implement our framework with Python and Q#, and demonstrate a reasonable sample efficiency. Through extensive case studies, we showcase an exciting application of our framework in automatically identifying close-to-optimal parameters for several parameterized quantum applications.

UR - https://arxiv.org/abs/2211.04507 U5 - 10.48550/ARXIV.2211.04507 ER - TY - JOUR T1 - Disordered Lieb-Robinson bounds in one dimension Y1 - 2022 A1 - Baldwin, Christopher L. A1 - Ehrenberg, Adam A1 - Guo, Andrew Y. A1 - Alexey V. Gorshkov KW - Disordered Systems and Neural Networks (cond-mat.dis-nn) KW - FOS: Physical sciences KW - Mathematical Physics (math-ph) KW - Quantum Physics (quant-ph) AB -

By tightening the conventional Lieb-Robinson bounds to better handle systems which lack translation invariance, we determine the extent to which "weak links" suppress operator growth in disordered one-dimensional spin chains. In particular, we prove that ballistic growth is impossible when the distribution of coupling strengths μ(J) has a sufficiently heavy tail at small J, and identify the correct dynamical exponent to use instead. Furthermore, through a detailed analysis of the special case in which the couplings are genuinely random and independent, we find that the standard formulation of Lieb-Robinson bounds is insufficient to capture the complexity of the dynamics -- we must distinguish between bounds which hold for all sites of the chain and bounds which hold for a subsequence of sites, and we show by explicit example that these two can have dramatically different behaviors. All the same, our result for the dynamical exponent is tight, in that we prove by counterexample that there cannot exist any Lieb-Robinson bound with a smaller exponent. We close by discussing the implications of our results, both major and minor, for numerous applications ranging from quench dynamics to the structure of ground states.

UR - https://arxiv.org/abs/2208.05509 U5 - 10.48550/ARXIV.2208.05509 ER - TY - JOUR T1 - Decoding conformal field theories: from supervised to unsupervised learning Y1 - 2021 A1 - En-Jui Kuo A1 - Alireza Seif A1 - Rex Lundgren A1 - Seth Whitsitt A1 - Mohammad Hafezi AB -

We use machine learning to classify rational two-dimensional conformal field theories. We first use the energy spectra of these minimal models to train a supervised learning algorithm. We find that the machine is able to correctly predict the nature and the value of critical points of several strongly correlated spin models using only their energy spectra. This is in contrast to previous works that use machine learning to classify different phases of matter, but do not reveal the nature of the critical point between phases. Given that the ground-state entanglement Hamiltonian of certain topological phases of matter is also described by conformal field theories, we use supervised learning on Réyni entropies and find that the machine is able to identify which conformal field theory describes the entanglement Hamiltonian with only the lowest few Réyni entropies to a high degree of accuracy. Finally, using autoencoders, an unsupervised learning algorithm, we find a hidden variable that has a direct correlation with the central charge and discuss prospects for using machine learning to investigate other conformal field theories, including higher-dimensional ones. Our results highlight that machine learning can be used to find and characterize critical points and also hint at the intriguing possibility to use machine learning to learn about more complex conformal field theories.

UR - https://arxiv.org/abs/2106.13485 ER - TY - JOUR T1 - Device-independent Randomness Expansion with Entangled Photons JF - Nat. Phys. Y1 - 2021 A1 - Lynden K. Shalm A1 - Yanbao Zhang A1 - Joshua C. Bienfang A1 - Collin Schlager A1 - Martin J. Stevens A1 - Michael D. Mazurek A1 - Carlos Abellán A1 - Waldimar Amaya A1 - Morgan W. Mitchell A1 - Mohammad A. Alhejji A1 - Honghao Fu A1 - Joel Ornstein A1 - Richard P. Mirin A1 - Sae Woo Nam A1 - Emanuel Knill AB -

With the growing availability of experimental loophole-free Bell tests, it has become possible to implement a new class of device-independent random number generators whose output can be certified to be uniformly random without requiring a detailed model of the quantum devices used. However, all of these experiments require many input bits in order to certify a small number of output bits, and it is an outstanding challenge to develop a system that generates more randomness than is used. Here, we devise a device-independent spot-checking protocol which uses only uniform bits as input. Implemented with a photonic loophole-free Bell test, we can produce 24% more certified output bits (1,181,264,237) than consumed input bits (953,301,640), which is 5 orders of magnitude more efficient than our previous work [arXiv:1812.07786]. The experiment ran for 91.0 hours, creating randomness at an average rate of 3606 bits/s with a soundness error bounded by 5.7×10−7 in the presence of classical side information. Our system will allow for greater trust in public sources of randomness, such as randomness beacons, and the protocols may one day enable high-quality sources of private randomness as the device footprint shrinks.

UR - https://arxiv.org/abs/1912.11158 U5 - https://doi.org/10.1038/s41567-020-01153-4 ER - TY - JOUR T1 - Discovering hydrodynamic equations of many-body quantum systems Y1 - 2021 A1 - Yaroslav Kharkov A1 - Oles Shtanko A1 - Alireza Seif A1 - Przemyslaw Bienias A1 - Mathias Van Regemortel A1 - Mohammad Hafezi A1 - Alexey V. Gorshkov AB -

Simulating and predicting dynamics of quantum many-body systems is extremely challenging, even for state-of-the-art computational methods, due to the spread of entanglement across the system. However, in the long-wavelength limit, quantum systems often admit a simplified description, which involves a small set of physical observables and requires only a few parameters such as sound velocity or viscosity. Unveiling the relationship between these hydrodynamic equations and the underlying microscopic theory usually requires a great effort by condensed matter theorists. In the present paper, we develop a new machine-learning framework for automated discovery of effective equations from a limited set of available data, thus bypassing complicated analytical derivations. The data can be generated from numerical simulations or come from experimental quantum simulator platforms. Using integrable models, where direct comparisons can be made, we reproduce previously known hydrodynamic equations, strikingly discover novel equations and provide their derivation whenever possible. We discover new hydrodynamic equations describing dynamics of interacting systems, for which the derivation remains an outstanding challenge. Our approach provides a new interpretable method to study properties of quantum materials and quantum simulators in non-perturbative regimes.

UR - https://arxiv.org/abs/2111.02385 ER - TY - JOUR T1 - Destructive Error Interference in Product-Formula Lattice Simulation JF - Phys. Rev. Lett. Y1 - 2020 A1 - Minh C. Tran A1 - Su-Kuan Chu A1 - Yuan Su A1 - Andrew M. Childs A1 - Alexey V. Gorshkov AB -

Quantum computers can efficiently simulate the dynamics of quantum systems. In this paper, we study the cost of digitally simulating the dynamics of several physically relevant systems using the first-order product formula algorithm. We show that the errors from different Trotterization steps in the algorithm can interfere destructively, yielding a much smaller error than previously estimated. In particular, we prove that the total error in simulating a nearest-neighbor interacting system of n sites for time t using the first-order product formula with r time slices is O(nt/r+nt3/r2) when nt2/r is less than a small constant. Given an error tolerance ε, the error bound yields an estimate of max{O(n2t/ε),O(n2t3/2/ε1/2)} for the total gate count of the simulation. The estimate is tighter than previous bounds and matches the empirical performance observed in Childs et al. [PNAS 115, 9456-9461 (2018)]. We also provide numerical evidence for potential improvements and conjecture an even tighter estimate for the gate count. 

VL - 124 UR - https://arxiv.org/abs/1912.11047 CP - 220502 U5 - https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.220502 ER - TY - JOUR T1 - Discrete Time Crystals JF - Annual Review of Condensed Matter Physics Y1 - 2020 A1 - Dominic V. Else A1 - Christopher Monroe A1 - Chetan Nayak A1 - Norman Y. Yao AB -

Experimental advances have allowed for the exploration of nearly isolated quantum many-body systems whose coupling to an external bath is very weak. A particularly interesting class of such systems is those which do not thermalize under their own isolated quantum dynamics. In this review, we highlight the possibility for such systems to exhibit new non-equilibrium phases of matter. In particular, we focus on "discrete time crystals", which are many-body phases of matter characterized by a spontaneously broken discrete time translation symmetry. We give a definition of discrete time crystals from several points of view, emphasizing that they are a non-equilibrium phenomenon, which is stabilized by many-body interactions, with no analog in non-interacting systems. We explain the theory behind several proposed models of discrete time crystals, and compare a number of recent realizations, in different experimental contexts. 

VL - 11 U4 - 467-499 UR - https://arxiv.org/abs/1905.13232 U5 - https://doi.org/10.1146/annurev-conmatphys-031119-050658 ER - TY - JOUR T1 - Distinct Critical Behaviors from the Same State in Quantum Spin and Population Dynamics Perspectives Y1 - 2020 A1 - Christopher L. Baldwin A1 - S. Shivam A1 - S. L. Sondhi A1 - M. Kardar AB -

There is a deep connection between the ground states of transverse-field spin systems and the late-time distributions of evolving viral populations -- within simple models, both are obtained from the principal eigenvector of the same matrix. However, that vector is the wavefunction amplitude in the quantum spin model, whereas it is the probability itself in the population model. We show that this seemingly minor difference has significant consequences: phase transitions which are discontinuous in the spin system become continuous when viewed through the population perspective, and transitions which are continuous become governed by new critical exponents. We introduce a more general class of models which encompasses both cases, and that can be solved exactly in a mean-field limit. Numerical results are also presented for a number of one-dimensional chains with power-law interactions. We see that well-worn spin models of quantum statistical mechanics can contain unexpected new physics and insights when treated as population-dynamical models and beyond, motivating further studies. 

UR - https://arxiv.org/abs/2009.05064 ER - TY - JOUR T1 - Distributional property testing in a quantum world JF - Proceedings of ITCS 2020 Y1 - 2020 A1 - Andras Gilyen A1 - Tongyang Li AB -

A fundamental problem in statistics and learning theory is to test properties of distributions. We show that quantum computers can solve such problems with significant speed-ups. In particular, we give fast quantum algorithms for testing closeness between unknown distributions, testing independence between two distributions, and estimating the Shannon / von Neumann entropy of distributions. The distributions can be either classical or quantum, however our quantum algorithms require coherent quantum access to a process preparing the samples. Our results build on the recent technique of quantum singular value transformation, combined with more standard tricks such as divide-and-conquer. The presented approach is a natural fit for distributional property testing both in the classical and the quantum case, demonstrating the first speed-ups for testing properties of density operators that can be accessed coherently rather than only via sampling; for classical distributions our algorithms significantly improve the precision dependence of some earlier results.

VL - 25 U4 - 1-25 UR - https://arxiv.org/abs/1902.00814 CP - 19 U5 - http://dx.doi.org/10.4230/LIPIcs.ITCS.2020.25 ER - TY - JOUR T1 - Dynamical Purification Phase Transition Induced by Quantum Measurements JF - Physical Review X Y1 - 2020 A1 - Michael Gullans A1 - Huse, David A. AB -

Continuously monitoring the environment of a quantum many-body system reduces the entropy of (purifies) the reduced density matrix of the system, conditional on the outcomes of the measurements. We show that, for mixed initial states, a balanced competition between measurements and entangling interactions within the system can result in a dynamical purification phase transition between (i) a phase that locally purifies at a constant system-size-independent rate, and (ii) a "mixed" phase where the purification time diverges exponentially in the system size. The residual entropy density in the mixed phase implies the existence of a quantum error-protected subspace where quantum information is reliably encoded against the future non-unitary evolution of the system. We show that these codes are of potential relevance to fault-tolerant quantum computation as they are often highly degenerate and satisfy optimal tradeoffs between encoded information densities and error thresholds. In spatially local models in 1+1 dimensions, this phase transition for mixed initial states occurs concurrently with a recently identified class of entanglement phase transitions for pure initial states. The mutual information of an initially completely-mixed state in 1+1 dimensions grows sublinearly in time due to the formation of the error protected subspace. The purification transition studied here also generalizes to systems with long-range interactions, where conventional notions of entanglement transitions have to be reformulated. Purification dynamics is likely a more robust probe of the transition in experiments, where imperfections generically reduce entanglement and drive the system towards mixed states. We describe the motivations for studying this novel class of non-equilibrium quantum dynamics in the context of advanced quantum computing platforms and fault-tolerant quantum computation.

VL - 10 UR - https://arxiv.org/abs/1905.05195 CP - 4 J1 - Phys. Rev. X U5 - 10.1103/PhysRevX.10.041020 ER - TY - JOUR T1 - Development of Quantum InterConnects for Next-Generation Information Technologies Y1 - 2019 A1 - David Awschalom A1 - Karl K. Berggren A1 - Hannes Bernien A1 - Sunil Bhave A1 - Lincoln D. Carr A1 - Paul Davids A1 - Sophia E. Economou A1 - Dirk Englund A1 - Andrei Faraon A1 - Marty Fejer A1 - Saikat Guha A1 - Martin V. Gustafsson A1 - Evelyn Hu A1 - Liang Jiang A1 - Jungsang Kim A1 - Boris Korzh A1 - Prem Kumar A1 - Paul G. Kwiat A1 - Marko Lončar A1 - Mikhail D. Lukin A1 - David A. B. Miller A1 - Christopher Monroe A1 - Sae Woo Nam A1 - Prineha Narang A1 - Jason S. Orcutt AB -

Just as classical information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the interconnect, a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a quantum internet poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on Quantum Interconnects that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry and national laboratories is required. This document is a summary from a U.S. National Science Foundation supported workshop held on 31 October - 1 November 2019 in Alexandria, VA. Attendees were charged to identify the scientific and community needs, opportunities, and significant challenges for quantum interconnects over the next 2-5 years. 

UR - https://arxiv.org/abs/1912.06642 ER - TY - JOUR T1 - Dark state optical lattice with sub-wavelength spatial structure JF - Phys. Rev. Lett. Y1 - 2018 A1 - Yang Wang A1 - Sarthak Subhankar A1 - Przemyslaw Bienias A1 - Mateusz Lacki A1 - Tsz-Chun Tsui A1 - Mikhail A. Baranov A1 - Alexey V. Gorshkov A1 - Peter Zoller A1 - James V. Porto A1 - Steven L. Rolston AB -

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.

VL - 120 U4 - 083601 UR - https://link.aps.org/doi/10.1103/PhysRevLett.120.083601 U5 - 10.1103/PhysRevLett.120.083601 ER - TY - JOUR T1 - Demonstration of Bayesian quantum game on an ion trap quantum computer Y1 - 2018 A1 - Neal Solmeyer A1 - Norbert M. Linke A1 - Caroline Figgatt A1 - Kevin A. Landsman A1 - Radhakrishnan Balu A1 - George Siopsis A1 - Christopher Monroe AB -

We demonstrate a Bayesian quantum game on an ion trap quantum computer with five qubits. The players share an entangled pair of qubits and perform rotations on their qubit as the strategy choice. Two five-qubit circuits are sufficient to run all 16 possible strategy choice sets in a game with four possible strategies. The data are then parsed into player types randomly in order to combine them classically into a Bayesian framework. We exhaustively compute the possible strategies of the game so that the experimental data can be used to solve for the Nash equilibria of the game directly. Then we compare the payoff at the Nash equilibria and location of phase-change-like transitions obtained from the experimental data to the theory, and study how it changes as a function of the amount of entanglement.

UR - https://arxiv.org/abs/1802.08116 ER - TY - JOUR T1 - Diffusion Monte Carlo Versus Adiabatic Computation for Local Hamiltonians JF - Physical Review A Y1 - 2018 A1 - Jacob Bringewatt A1 - William Dorland A1 - Stephen P. Jordan A1 - Alan Mink AB -

Most research regarding quantum adiabatic optimization has focused on stoquastic Hamiltonians, whose ground states can be expressed with only real, nonnegative amplitudes. This raises the question of whether classical Monte Carlo algorithms can efficiently simulate quantum adiabatic optimization with stoquastic Hamiltonians. Recent results have given counterexamples in which path integral and diffusion Monte Carlo fail to do so. However, most adiabatic optimization algorithms, such as for solving MAX-k-SAT problems, use k-local Hamiltonians, whereas our previous counterexample for diffusion Monte Carlo involved n-body interactions. Here we present a new 6-local counterexample which demonstrates that even for these local Hamiltonians there are cases where diffusion Monte Carlo cannot efficiently simulate quantum adiabatic optimization. Furthermore, we perform empirical testing of diffusion Monte Carlo on a standard well-studied class of permutation-symmetric tunneling problems and similarly find large advantages for quantum optimization over diffusion Monte Carlo.

VL - 97 U4 - 022323 UR - https://journals.aps.org/pra/abstract/10.1103/PhysRevA.97.022323 CP - 2 U5 - 10.1103/PhysRevA.97.022323 ER - TY - JOUR T1 - Dissipation induced dipole blockade and anti-blockade in driven Rydberg systems JF - Phys. Rev. A Y1 - 2018 A1 - Jeremy T. Young A1 - Thomas Boulier A1 - Eric Magnan A1 - Elizabeth A. Goldschmidt A1 - Ryan M. Wilson A1 - Steven L. Rolston A1 - James V. Porto A1 - Alexey V. Gorshkov AB -

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'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'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.

VL - 97 U4 - 023424 UR - https://link.aps.org/doi/10.1103/PhysRevA.97.023424 U5 - 10.1103/PhysRevA.97.023424 ER - TY - JOUR T1 - Distributed Quantum Metrology and the Entangling Power of Linear Networks Y1 - 2018 A1 - Wenchao Ge A1 - Kurt Jacobs A1 - Zachary Eldredge A1 - Alexey V. Gorshkov A1 - Michael Foss-Feig AB -

We derive a bound on the ability of a linear optical network to estimate a linear combination of independent phase shifts by using an arbitrary non-classical but unentangled input state, thereby elucidating the quantum resources required to obtain the Heisenberg limit with a multi-port interferometer. Our bound reveals that while linear networks can generate highly entangled states, they cannot effectively combine quantum resources that are well distributed across multiple modes for the purposes of metrology: in this sense linear networks endowed with well-distributed quantum resources behave classically. Conversely, our bound shows that linear networks can achieve the Heisenberg limit for distributed metrology when the input photons are hoarded in a small number of input modes, and we present an explicit scheme for doing so. Our results also have implications for measures of non-classicality.

UR - https://arxiv.org/abs/1707.06655 U5 - https://doi.org/10.1103/PhysRevLett.121.043604 ER - TY - JOUR T1 - Distributed Quantum Metrology and the Entangling Power of Linear Networks JF - Phys. Rev. Lett. 121, 043604 Y1 - 2018 A1 - Wenchao Ge A1 - Kurt Jacobs A1 - Zachary Eldredge A1 - Alexey V. Gorshkov A1 - Michael Foss-Feig AB -

We derive a bound on the ability of a linear optical network to estimate a linear combination of independent phase shifts by using an arbitrary non-classical but unentangled input state, thereby elucidating the quantum resources required to obtain the Heisenberg limit with a multi-port interferometer. Our bound reveals that while linear networks can generate highly entangled states, they cannot effectively combine quantum resources that are well distributed across multiple modes for the purposes of metrology: in this sense linear networks endowed with well-distributed quantum resources behave classically. Conversely, our bound shows that linear networks can achieve the Heisenberg limit for distributed metrology when the input photons are hoarded in a small number of input modes, and we present an explicit scheme for doing so. Our results also have implications for measures of non-classicality. 

UR - https://arxiv.org/abs/1707.06655 U5 - https://doi.org/10.1103/PhysRevLett.121.043604 ER - TY - JOUR T1 - Dynamic suppression of Rayleigh light scattering in dielectric resonators Y1 - 2018 A1 - Seunghwi Kim A1 - J. M. Taylor A1 - Gaurav Bahl AB -

The ultimate limits of performance for any classical optical system are set by sub-wavelength fluctuations within the host material, that may be frozen-in or even dynamically induced. The most common manifestation of such sub-wavelength disorder is Rayleigh light scattering, which is observed in nearly all wave-guiding technologies today and can lead to both irreversible radiative losses as well as undesirable intermodal coupling. While it has been shown that backscattering from disorder can be suppressed by breaking time-reversal symmetry in magneto-optic and topological insulator materials, common optical dielectrics possess neither of these properties. Here we demonstrate an optomechanical approach for dynamically suppressing Rayleigh backscattering within dielectric resonators. We achieve this by locally breaking time-reversal symmetry in a silica resonator through a Brillouin scattering interaction that is available in all materials. Near-complete suppression of Rayleigh backscattering is experimentally confirmed through three independent measurements -- the reduction of the back-reflections caused by scatterers, the elimination of a commonly seen normal-mode splitting effect, and by measurement of the reduction in intrinsic optical loss. More broadly, our results provide new evidence that it is possible to dynamically suppress Rayleigh backscattering within any optical dielectric medium, for achieving robust light propagation in nanophotonic devices in spite of the presence of scatterers or defects.

UR - https://arxiv.org/abs/1803.02366 ER - TY - JOUR T1 - Dynamical phase transitions in sampling complexity JF - Phys. Rev. Lett. Y1 - 2018 A1 - Abhinav Deshpande A1 - Bill Fefferman A1 - Minh C. Tran A1 - Michael Foss-Feig A1 - Alexey V. Gorshkov AB -

We make the case for studying the complexity of approximately simulating (sampling) quantum systems for reasons beyond that of quantum computational supremacy, such as diagnosing phase transitions. We consider the sampling complexity as a function of time t due to evolution generated by spatially local quadratic bosonic Hamiltonians. We obtain an upper bound on the scaling of t with the number of bosons n for which approximate sampling is classically efficient. We also obtain a lower bound on the scaling of t with n for which any instance of the boson sampling problem reduces to this problem and hence implies that the problem is hard, assuming the conjectures of Aaronson and Arkhipov [Proc. 43rd Annu. ACM Symp. Theory Comput. STOC '11]. This establishes a dynamical phase transition in sampling complexity. Further, we show that systems in the Anderson-localized phase are always easy to sample from at arbitrarily long times. We view these results in the light of classifying phases of physical systems based on parameters in the Hamiltonian. In doing so, we combine ideas from mathematical physics and computational complexity to gain insight into the behavior of condensed matter, atomic, molecular and optical systems.

VL - 121 U4 - 12 pages, 4 figures. v3: published version UR - https://arxiv.org/abs/1703.05332 CP - 030501 U5 - https://doi.org/10.1103/PhysRevLett.121.030501 ER - TY - JOUR T1 - Development of a new UHV/XHV pressure standard (cold atom vacuum standard) JF - Metrologia Y1 - 2017 A1 - Julia Scherschligt A1 - James A Fedchak A1 - Daniel S Barker A1 - Stephen Eckel A1 - Nikolai Klimov A1 - Constantinos Makrides A1 - Eite Tiesinga AB -

The National Institute of Standards and Technology has recently begun a program to develop a primary pressure standard that is based on ultra-cold atoms, covering a pressure range of 1 x 10-6 to 1 x 10-10 Pa and possibly lower. These pressures correspond to the entire ultra-high vacuum range and extend into the extreme-high vacuum. This cold-atom vacuum standard (CAVS) is both a primary standard and absolute sensor of vacuum. The CAVS is based on the loss of cold, sensor atoms (such as the alkali-metal lithium) from a magnetic trap due to collisions with the background gas (primarily H2) in the vacuum. The pressure is determined from a thermally-averaged collision cross section, which is a fundamental atomic property, and the measured loss rate. The CAVS is primary because it will use collision cross sections determined from ab initio calculations for the Li + H2 system. Primary traceability is transferred to other systems of interest using sensitivity coefficients.

VL - 54 UR - https://arxiv.org/abs/1801.10120 CP - 6 U5 - https://doi.org/10.1088/1681-7575/aa8a7b ER - TY - JOUR T1 - Disorder induced transitions in resonantly driven Floquet Topological Insulators JF - Physical Review B Y1 - 2017 A1 - Paraj Titum A1 - Netanel H. Lindner A1 - Gil Refael AB -

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.

VL - 96 U4 - 054207 UR - https://arxiv.org/abs/1702.02956 CP - 5 U5 - 10.1103/PhysRevB.96.054207 ER - TY - JOUR T1 - Dispersive optical detection of magnetic Feshbach resonances in ultracold gases JF - Physical Review A Y1 - 2017 A1 - Bianca J. Sawyer A1 - Milena S. J. Horvath A1 - Eite Tiesinga A1 - Amita B. Deb A1 - Niels Kjærgaard AB -

Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of 87Rb, between the |F=1,mF=1 and |F=2,mF=0 states. Despite the loss features being 100 mG wide, we require only 21 experimental runs to explore a magnetic field range >18 G. The resonances consist of two known s-wave features in the vicinity of 9 G and 18 G and two previously unobserved p-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.

VL - 96 U4 - 022705 UR - https://arxiv.org/abs/1702.02216 CP - 2 U5 - 10.1103/PhysRevA.96.022705 ER - TY - JOUR T1 - Domination with decay in triangular matchstick arrangement graphs JF - Involve, a Journal of Mathematics Y1 - 2017 A1 - Jill Cochran A1 - Terry Henderson A1 - Aaron Ostrander A1 - Ron Taylor AB -

We provide results for the exponential dominating numbers and total exponential dominating numbers of a family of triangular grid graphs. We then prove inequalities for these numbers and compare them with inequalities that hold more generally for exponential dominating numbers of graphs.

VL - 10 U4 - 749 - 766 UR - http://msp.org/involve/http://msp.org/involve/2017/10-5/index.xhtmlhttp://msp.org/involve/2017/10-5/p03.xhtmlhttp://msp.org/involve/2017/10-5/involve-v10-n5-p03-s.pdf CP - 5 J1 - Involve U5 - 10.2140/involve10.2140/involve.2017.10-510.2140/involve.2017.10.749 ER - TY - JOUR T1 - Dynamically induced robust phonon transport and chiral cooling in an optomechanical system JF - Nature Communications Y1 - 2017 A1 - Seunghwi Kim A1 - Xunnong Xu A1 - J. M. Taylor A1 - Gaurav Bahl AB -

The transport of sound and heat, in the form of phonons, has a fundamental material limit: disorder-induced scattering. In electronic and optical settings, introduction of chiral transport - in which carrier propagation exhibits broken parity symmetry - provides robustness against such disorder by preventing elastic backscattering. Here we experimentally demonstrate a path for achieving robust phonon transport even in the presence of material disorder, by dynamically inducing chirality through traveling-wave optomechanical coupling. Using this approach, we demonstrate dramatic optically-induced chiral transport for clockwise and counterclockwise phonons in a symmetric resonator. This induced chirality also enhances isolation from the thermal bath and leads to gain-free reduction of the intrinsic damping of the phonons. Surprisingly, this passive mechanism is also accompanied by a chiral reduction in heat load leading to a novel optical cooling of the mechanics. This technique has the potential to improve upon the fundamental thermal limits of resonant mechanical sensor, which cannot be otherwise attained through conventional optomechanical cooling.

VL - 8 U4 - 205 UR - https://arxiv.org/abs/1609.08674 U5 - 10.1038/s41467-017-00247-7 ER - TY - JOUR T1 - Demonstration of a small programmable quantum computer with atomic qubits JF - Nature Y1 - 2016 A1 - S. Debnath A1 - N. M. Linke A1 - C. Figgatt A1 - K. A. Landsman A1 - K. Wright A1 - C. Monroe AB -

Quantum computers can solve certain problems more efficiently than any possible conventional computer. Small quantum algorithms have been demonstrated on multiple quantum computing platforms, many specifically tailored in hardware to implement a particular algorithm or execute a limited number of computational paths. Here, we demonstrate a five-qubit trapped-ion quantum computer that can be programmed in software to implement arbitrary quantum algorithms by executing any sequence of universal quantum logic gates. We compile algorithms into a fully-connected set of gate operations that are native to the hardware and have a mean fidelity of 98 %. Reconfiguring these gate sequences provides the flexibility to implement a variety of algorithms without altering the hardware. As examples, we implement the Deutsch-Jozsa (DJ) and Bernstein-Vazirani (BV) algorithms with average success rates of 95 % and 90 %, respectively. We also perform a coherent quantum Fourier transform (QFT) on five trappedion qubits for phase estimation and period finding with average fidelities of 62 % and 84 %, respectively. This small quantum computer can be scaled to larger numbers of qubits within a single register, and can be further expanded by connecting several such modules through ion shuttling or photonic quantum channels.

VL - 536 U4 - 63-66 UR - http://www.nature.com/nature/journal/v536/n7614/full/nature18648.html CP - 7614 U5 - 10.1038/nature18648 ER - TY - JOUR T1 - Detecting Consistency of Overlapping Quantum Marginals by Separability JF - Physical Review A Y1 - 2016 A1 - Jianxin Chen A1 - Zhengfeng Ji A1 - Nengkun Yu A1 - Bei Zeng AB - The quantum marginal problem asks whether a set of given density matrices are consistent, i.e., whether they can be the reduced density matrices of a global quantum state. Not many non-trivial analytic necessary (or sufficient) conditions are known for the problem in general. We propose a method to detect consistency of overlapping quantum marginals by considering the separability of some derived states. Our method works well for the $k$-symmetric extension problem in general, and for the general overlapping marginal problems in some cases. Our work is, in some sense, the converse to the well-known $k$-symmetric extension criterion for separability. VL - 93 U4 - 032105 UR - http://arxiv.org/abs/1509.06591 CP - 3 U5 - 10.1103/PhysRevA.93.032105 ER - TY - JOUR T1 - Double Quantum Dot Floquet Gain Medium JF - Physical Review X Y1 - 2016 A1 - J. Stehlik A1 - Y.-Y. Liu A1 - C. Eichler A1 - T. R. Hartke A1 - X. Mi A1 - Michael Gullans A1 - J. M. Taylor A1 - J. R. Petta AB -

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

VL - 6 U4 - 041027 UR - http://journals.aps.org/prx/abstract/10.1103/PhysRevX.6.041027 U5 - 10.1103/PhysRevX.6.041027 ER - TY - JOUR T1 - Demonstration of Robust Quantum Gate Tomography via Randomized Benchmarking JF - New Journal of Physics Y1 - 2015 A1 - Blake R. Johnson A1 - Marcus P. da Silva A1 - Colm A. Ryan A1 - Shelby Kimmel A1 - Jerry M. Chow A1 - Thomas A. Ohki AB - 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 `atomic' 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. VL - 17 U4 - 113019 UR - http://arxiv.org/abs/1505.06686 CP - 11 U5 - 10.1088/1367-2630/17/11/113019 ER - TY - JOUR T1 - Discontinuity of Maximum Entropy Inference and Quantum Phase Transitions JF - New Journal of Physics Y1 - 2015 A1 - Jianxin Chen A1 - Zhengfeng Ji A1 - Chi-Kwong Li A1 - Yiu-Tung Poon A1 - Yi Shen A1 - Nengkun Yu A1 - Bei Zeng A1 - Duanlu Zhou AB - In this paper, we discuss the connection between two genuinely quantum phenomena --- the discontinuity of quantum maximum entropy inference and quantum phase transitions at zero temperature. It is shown that the discontinuity of the maximum entropy inference of local observable measurements signals the non-local type of transitions, where local density matrices of the ground state change smoothly at the transition point. We then propose to use the quantum conditional mutual information of the ground state as an indicator to detect the discontinuity and the non-local type of quantum phase transitions in the thermodynamic limit. VL - 17 U4 - 083019 UR - http://arxiv.org/abs/1406.5046v2 CP - 8 J1 - New J. Phys. U5 - 10.1088/1367-2630/17/8/083019 ER - TY - JOUR T1 - Driving Rabi oscillations at the giant dipole resonance in xenon JF - Phys. Rev. A Y1 - 2015 A1 - Stefan Pabst A1 - Daochen Wang A1 - Robin Santra AB -

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

VL - 92 UR - https://arxiv.org/abs/1511.00058 CP - 053424 U5 - https://doi.org/10.1103/PhysRevA.92.053424 ER - TY - JOUR T1 - Different Strategies for Optimization Using the Quantum Adiabatic Algorithm Y1 - 2014 A1 - Elizabeth Crosson A1 - Edward Farhi A1 - Cedric Yen-Yu Lin A1 - Han-Hsuan Lin A1 - Peter Shor AB - We present the results of a numerical study, with 20 qubits, of the performance of the Quantum Adiabatic Algorithm on randomly generated instances of MAX 2-SAT with a unique assignment that maximizes the number of satisfied clauses. The probability of obtaining this assignment at the end of the quantum evolution measures the success of the algorithm. Here we report three strategies which consistently increase the success probability for the hardest instances in our ensemble: decreasing the overall evolution time, initializing the system in excited states, and adding a random local Hamiltonian to the middle of the evolution. UR - http://arxiv.org/abs/1401.7320v1 ER - TY - JOUR T1 - Discrete optimization using quantum annealing on sparse Ising models JF - Frontiers in Physics Y1 - 2014 A1 - Bian, Zhengbing A1 - Chudak, Fabian A1 - Israel, Robert A1 - Brad Lackey A1 - Macready, William G A1 - Roy, Aidan AB - 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. PB - Frontiers VL - 2 U4 - 56 ER - TY - JOUR T1 - Dissipative Many-body Quantum Optics in Rydberg Media JF - Physical Review Letters Y1 - 2013 A1 - Alexey V. Gorshkov A1 - Rejish Nath A1 - Thomas Pohl AB - We develop a theoretical framework for the dissipative propagation of quantized light in interacting optical media under conditions of electromagnetically induced transparency (EIT). The theory allows us to determine the peculiar spatiotemporal structure of the output of two complementary Rydberg-EIT-based light-processing modules: the recently demonstrated single-photon filter and the recently proposed single-photon subtractor, which, respectively, let through and absorb a single photon. In addition to being crucial for applications of these and other optical quantum devices, the theory opens the door to the study of exotic dissipative many-body dynamics of strongly interacting photons in nonlinear nonlocal media. VL - 110 UR - http://arxiv.org/abs/1211.7060v1 CP - 15 J1 - Phys. Rev. Lett. U5 - 10.1103/PhysRevLett.110.153601 ER - TY - JOUR T1 - Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence JF - New Journal of Physics Y1 - 2013 A1 - Michael Foss-Feig A1 - Kaden R A Hazzard A1 - John J Bollinger A1 - Ana Maria Rey A1 - Charles W Clark AB - 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. VL - 15 U4 - 113008 UR - http://arxiv.org/abs/1306.0172v1 CP - 11 J1 - New J. Phys. U5 - 10.1088/1367-2630/15/11/113008 ER - TY - JOUR T1 - Deciding Unitary Equivalence Between Matrix Polynomials and Sets of Bipartite Quantum States JF - Quantum Information and Computation Y1 - 2011 A1 - Chitambar, Eric A1 - Carl Miller A1 - Shi, Yaoyun KW - matrix polynomials KW - Schwartz-Zippel lemma KW - unitary transformations AB -

In this brief report, we consider the equivalence between two sets of m + 1 bipartite quantum states under local unitary transformations. For pure states, this problem corresponds to the matrix algebra question of whether two degree m matrix polynomials are unitarily equivalent; i.e. UAiV† = Bi for 0 ≤ i ≤ m where U and V are unitary and (Ai, Bi) are arbitrary pairs of rectangular matrices. We present a randomized polynomial-time algorithm that solves this problem with an arbitrarily high success probability and outputs transforming matrices U and V.

VL - 11 U4 - 813–819 UR - http://dl.acm.org/citation.cfm?id=2230936.2230942 CP - 9-10 ER - TY - JOUR T1 - Detecting paired and counterflow superfluidity via dipole oscillations JF - Physical Review A Y1 - 2011 A1 - Anzi Hu A1 - L. Mathey A1 - Eite Tiesinga A1 - Ippei Danshita A1 - Carl J. Williams A1 - Charles W. Clark AB - We suggest an experimentally feasible procedure to observe paired and counterflow superfluidity in ultra-cold atom systems. We study the time evolution of one-dimensional mixtures of bosonic atoms in an optical lattice following an abrupt displacement of an additional weak confining potential. We find that the dynamic responses of the paired superfluid phase for attractive inter-species interactions and the counterflow superfluid phase for repulsive interactions are qualitatively distinct and reflect the quasi long-range order that characterizes these states. These findings suggest a clear experimental procedure to detect these phases, and give an intuitive insight into their dynamics. VL - 84 UR - http://arxiv.org/abs/1103.3513v3 CP - 4 J1 - Phys. Rev. A U5 - 10.1103/PhysRevA.84.041609 ER - TY - JOUR T1 - A diagrammatic expansion of the Casimir energy in multiple reflections: theory and applications JF - Physical Review D Y1 - 2011 A1 - Mohammad F. Maghrebi AB - We develop a diagrammatic representation of the Casimir energy of a multibody configuration. The diagrams represent multiple reflections between the objects and can be organized by a few simple rules. The lowest-order diagrams (or reflections) give the main contribution to the Casimir interaction which proves the usefulness of this expansion. Among some applications of this, we find analytical formulae describing the interaction between "edges", i.e. semi-infinite plates, where we also give a first example of blocking in the context of the Casimir energy. We also find the interaction of edges with a needle and describe analytically a recent model of the repulsion due to the Casimir interaction. VL - 83 UR - http://arxiv.org/abs/1012.1060v1 CP - 4 J1 - Phys. Rev. D U5 - 10.1103/PhysRevD.83.045004 ER - TY - JOUR T1 - Direct Fidelity Estimation from Few Pauli Measurements JF - Physical Review Letters Y1 - 2011 A1 - Steven T. Flammia A1 - Yi-Kai Liu AB - We describe a simple method for certifying that an experimental device prepares a desired quantum state rho. Our method is applicable to any pure state rho, and it provides an estimate of the fidelity between rho and the actual (arbitrary) state in the lab, up to a constant additive error. The method requires measuring only a constant number of Pauli expectation values, selected at random according to an importance-weighting rule. Our method is faster than full tomography by a factor of d, the dimension of the state space, and extends easily and naturally to quantum channels. VL - 106 UR - http://arxiv.org/abs/1104.4695v3 CP - 23 J1 - Phys. Rev. Lett. U5 - 10.1103/PhysRevLett.106.230501 ER - TY - JOUR T1 - d-Wave Superfluidity in Optical Lattices of Ultracold Polar Molecules JF - Physical Review A Y1 - 2011 A1 - Kevin A. Kuns A1 - Ana Maria Rey A1 - Alexey V. Gorshkov AB - 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. VL - 84 UR - http://arxiv.org/abs/1110.5330v2 CP - 6 J1 - Phys. Rev. A U5 - 10.1103/PhysRevA.84.063639 ER - TY - JOUR T1 - Dynamics of Overhauser Field under nuclear spin diffusion in a quantum dot JF - New Journal of Physics Y1 - 2011 A1 - Zhe-Xuan Gong A1 - Zhang-qi Yin A1 - L. -M. Duan AB - The coherence of electron spin can be significantly enhanced by locking the Overhauser field from nuclear spins using the nuclear spin preparation. We propose a theoretical model to calculate the long time dynamics of the Overhauser field under intrinsic nuclear spin diffusion in a quantum dot. We obtain a simplified diffusion equation that can be numerically solved and show quantitatively how the Knight shift and the electron-mediated nuclear spin flip-flop affect the nuclear spin diffusion. The results explain several recent experimental observations, where the decay time of Overhauser field is measured under different configurations, including variation of the external magnetic field, the electron spin configuration in a double dot, and the initial nuclear spin polarization rate. VL - 13 U4 - 033036 UR - http://arxiv.org/abs/0912.4322v1 CP - 3 J1 - New J. Phys. U5 - 10.1088/1367-2630/13/3/033036 ER - TY - JOUR T1 - On the degeneracy of SU(3)k topological phases Y1 - 2010 A1 - Stephen P. Jordan A1 - Toufik Mansour A1 - Simone Severini AB -

The ground state degeneracy of an $SU(N)_k$ topological phase with $n$ quasiparticle excitations is relevant quantity for quantum computation, condensed matter physics, and knot theory. It is an open question to find a closed formula for this degeneracy for any $N > 2$. Here we present the problem in an explicit combinatorial way and analyze the case N=3. While not finding a complete closed-form solution, we obtain generating functions and solve some special cases.

UR - http://arxiv.org/abs/1009.0114v1 ER - TY - JOUR T1 - Dynamic Nuclear Polarization in Double Quantum Dots JF - Physical Review Letters Y1 - 2010 A1 - Michael Gullans A1 - J. J. Krich A1 - J. M. Taylor A1 - H. Bluhm A1 - B. I. Halperin A1 - C. M. Marcus A1 - M. Stopa A1 - A. Yacoby A1 - M. D. Lukin AB - We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called "dark states," and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process. VL - 104 UR - http://arxiv.org/abs/1003.4508v2 CP - 22 J1 - Phys. Rev. Lett. U5 - 10.1103/PhysRevLett.104.226807 ER - TY - JOUR T1 - Discrete-query quantum algorithm for NAND trees JF - Theory of Computing Y1 - 2009 A1 - Andrew M. Childs A1 - Richard Cleve A1 - Stephen P. Jordan A1 - David Yeung AB - Recently, Farhi, Goldstone, and Gutmann gave a quantum algorithm for evaluating NAND trees that runs in time O(sqrt(N log N)) in the Hamiltonian query model. In this note, we point out that their algorithm can be converted into an algorithm using O(N^{1/2 + epsilon}) queries in the conventional quantum query model, for any fixed epsilon > 0. VL - 5 U4 - 119 - 123 UR - http://arxiv.org/abs/quant-ph/0702160v1 CP - 1 J1 - Theory of Comput. U5 - 10.4086/toc.2009.v005a005 ER - TY - JOUR T1 - Decoherence in Quantum Walks on the Hypercube JF - Phys. Rev. A Y1 - 2005 A1 - Gorjan Alagic A1 - Alexander Russell AB -

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.

VL - 76 U4 - 062304 UR - https://arxiv.org/abs/quant-ph/0501169 CP - 6 U5 - https://doi.org/10.1103/PhysRevA.72.062304 ER - TY - JOUR T1 - Dephasing of quantum bits by a quasi-static mesoscopic environment Y1 - 2005 A1 - J. M. Taylor A1 - M. D. Lukin AB - We examine coherent processes in a two-state quantum system that is strongly coupled to a mesoscopic spin bath and weakly coupled to other environmental degrees of freedom. Our analysis is specifically aimed at understanding the quantum dynamics of solid-state quantum bits such as electron spins in semiconductor structures and superconducting islands. The role of mesoscopic degrees of freedom with long correlation times (local degrees of freedom such as nuclear spins and charge traps) in qubit-related dephasing is discussed in terms of a quasi-static bath. A mathemat- ical framework simultaneously describing coupling to the slow mesoscopic bath and a Markovian environment is developed and the dephasing and decoherence properties of the total system are investigated. The model is applied to several specific examples with direct relevance to current ex- periments. Comparisons to experiments suggests that such quasi-static degrees of freedom play an important role in current qubit implementations. Several methods of mitigating the bath-induced error are considered. UR - http://arxiv.org/abs/quant-ph/0512059v2 ER - TY - JOUR T1 - Designing Incentives for Peer-to-Peer Routing JF - Proc. INFOCOM Y1 - 2005 A1 - Alberto Blanc A1 - Yi-Kai Liu A1 - Amin Vahda AB - In a peer-to-peer network, nodes are typically required to route packets for each other. This leads to a problem of “free-loaders,” nodes that use the network but refuse to route other nodes’ packets. In this paper we study ways of designing incentives to discourage free-loading. We model the interactions between nodes as a “random matching game,” and describe a simple reputation system that provides incentives for good behavior. Under certain assumptions, we obtain a stable subgame-perfect equilibrium. We use simulations to investigate the robustness of this scheme in the presence of noise and malicious nodes, and we examine some of the design trade-offs. We also evaluate some possible adversarial strategies, and discuss how our results might apply to real peer-to-peer systems. U4 - 374-385 UR - http://cseweb.ucsd.edu/~vahdat/papers/infocom05.pdf ER -