Publications (32)85.76 Total impact
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ABSTRACT: We revisit the evidence for quantum annealing in the DWave One device (DW1) based on the study of random Ising instances. Using the probability distributions of finding the ground states of such instances, previous work found agreement with both simulated quantum annealing (SQA) and a classical rotor model. Thus the DW1 ground state success probabilities are consistent with both models, and a different measure is needed to distinguish the data and the models. Here we consider measures that account for ground state degeneracy and the distributions of excited states, and present evidence that for these new measures neither SQA nor the classical rotor model correlate perfectly with the DW1 experiments. We thus provide evidence that SQA and the classical rotor model, both of which are classically efficient algorithms, do not satisfactorily explain all the DW1 data. A complete model for the DW1 remains an open problem. Using the same criteria we find that, on the other hand, SQA and the classical rotor model correlate closely with each other. To explain this we show that the rotor model can be derived as the semiclassical limit of the spincoherent states path integral. We also find differences in which set of ground states is found by each method, though this feature is sensitive to calibration errors of the DW1 device and to simulation parameters.09/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We study the unitary time evolution of photons interacting with a dielectric resonator using coherent control pulses. We show that nonMarkovianity of transient photon dynamics in the resonator subsystem may be controlled to within a photonresonator transit time. In general, appropriate use of coherent pulses and choice of spatial subregion may be used to create and control a wide range of nonMarkovian transient dynamics in photon resonator systems.08/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate that the performance of a quantum annealer on hard random Ising optimization problems can be substantially improved using quantum annealing correction (QAC). Our error correction strategy is tailored to the DWave Two device. We find that QAC provides a statistically significant enhancement in the performance of the device over a classical repetition code, improving as a function of problem size as well as hardness. Moreover, QAC provides a mechanism for overcoming the precision limit of the device, in addition to correcting calibration errors. Performance is robust even to missing qubits. We present evidence for a constructive role played by quantum effects in our experiments by contrasting the experimental results with the predictions of a classical model of the device. Our work demonstrates the importance of error correction in appropriately determining the performance of quantum annealers.08/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Recently the question of whether the DWave processors exhibit largescale quantum behavior or can be described by a classical model has attracted significant interest. In this work we address this question by studying a 503 qubit DWave Two device as a "black box", i.e., by studying its inputoutput behavior. We examine three candidate classical models and one quantum model, and compare their predictions to experiments we have performed on the device using groups of up to 40 qubits. The candidate classical models are simulated annealing, spin dynamics, a recently proposed hybrid O(2) rotorMonte Carlo model, and three modified versions thereof. The quantum model is an adiabatic Markovian master equation derived in the weak coupling limit of an open quantum system. Our experiments realize an evolution from a transverse field to an Ising Hamiltonian, with a finaltime degenerate ground state that splits into two types of states we call "isolated" and "clustered". We study the population ratio of the isolated and clustered states as a function of the overall energy scale of the Ising term, and the distance between the final state and the Gibbs state, and find that these are sensitive probes that distinguish the classical models from one another and from both the experimental data and the master equation. The classical models are all found to disagree with the data, while the master equation agrees with the experiment without finetuning, and predicts mixed state entanglement at intermediate evolution times. This suggests that an open system quantum dynamical description of the DWave device is welljustified even in the presence of relevant thermal excitations and fast singlequbit decoherence.03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Quantum information processing offers dramatic speedups, yet is susceptible to decoherence, whereby quantum superpositions decay into mutually exclusive classical alternatives, thus robbing quantum computers of their power. This makes the development of quantum error correction an essential aspect of quantum computing. So far, little is known about protection against decoherence for quantum annealing, a computational paradigm aiming to exploit groundstate quantum dynamics to solve optimization problems more rapidly than is possible classically. Here we develop error correction for quantum annealing and experimentally demonstrate it using antiferromagnetic chains with up to 344 superconducting flux qubits in processors that have recently been shown to physically implement programmable quantum annealing. We demonstrate a substantial improvement over the performance of the processors in the absence of error correction. These results pave the way towards largescale noiseprotected adiabatic quantum optimization devices, although a threshold theorem such as has been established in the circuit model of quantum computing remains elusive.Nature Communications 02/2014; 5:3243. · 10.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present fluctuation theorems and moment generating function equalities for generalized thermodynamic observables and quantum dynamics described by completely positive trace preserving maps, with and without feedback control. Our results include the quantum Jarzynski equality and Crooks fluctuation theorem, and clarify the special role played by the thermodynamic work and thermal equilibrium states in previous studies. We show that for a specific class of generalized measurements, which include projective measurements, unitality replaces microreversibility as the condition for the physicality of the reverse process in our fluctuation theorems. We present an experimental application of our theory to the problem of extracting the systembath coupling magnitude, which we do for a system of pairs of coupled superconducting flux qubits undergoing quantum annealing.Physical Review E 09/2013; 88(31):032146. · 2.31 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Quantum annealing is a general strategy for solving difficult optimization problems with the aid of quantum adiabatic evolution. Both analytical and numerical evidence suggests that under idealized, closed system conditions, quantum annealing can outperform classical thermalizationbased algorithms such as simulated annealing. Current engineered quantum annealing devices have a decoherence timescale which is orders of magnitude shorter than the adiabatic evolution time. Do they effectively perform classical thermalization when coupled to a decohering thermal environment? Here we present an experimental signature which is consistent with quantum annealing, and at the same time inconsistent with classical thermalization. Our experiment uses groups of eight superconducting flux qubits with programmable spinspin couplings, embedded on a commercially available chip with >100 functional qubits. This suggests that programmable quantum devices, scalable with current superconducting technology, implement quantum annealing with a surprising robustness against noise and imperfections.Nature Communications 06/2013; 4:2067. · 10.02 Impact Factor 
Article: CoarseGraining Can Beat the Rotating Wave Approximation in Quantum Markovian Master Equations
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ABSTRACT: We present a firstprinciples derivation of the Markovian semigroup master equation without invoking the rotating wave approximation (RWA). Instead we use a time coarsegraining approach which leaves us with a free timescale parameter, which we can optimize. Comparing this approach to the standard RWAbased Markovian master equation, we find that significantly better agreement is possible using the coarsegraining approach, for a threelevel model coupled to a bath of oscillators, whose exact dynamics we can solve for at zero temperature. The model has the important feature that the RWA has a nontrivial effect on the dynamics of the populations. We show that the two different master equations can exhibit strong qualitative differences for the population of the energy eigenstates even for such a simple model. The RWAbased master equation misses an important feature which the coarsegraining based scheme does not. By optimizing the coarsegraining timescale the latter scheme can be made to approach the exact solution much more closely than the RWAbased master equation.Physical Review A 03/2013; 88(1). · 3.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present results on benchmarking the DWave One quantum optimizer chip using random 2D Ising spin problems. Finding the ground state of the 2D Ising model with randomly assigned local fields and couplings is NPhard. The chip attempts to find the ground state via quantum annealing, interpolating between a transverse field and the final Ising Hamiltonian. The experimentally obtained final states are checked against exact results and the performance of the chip is characterized by the probability of finding the ground state and the estimated annealing time for finding the ground state with high probability. By analyzing results for 8 to 108 spins, the scaling of the estimated annealing time as a function of the number of spins is compared with the computation time required by classical solvers. The correlation between classical and quantum hardness is also studied. Furthermore, we analyze the correlation between the experimental success probability and the minimum energy gap during the quantum annealing, as well as the interplay between the adiabatic condition and thermalization.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Working with a twoqubit Ising Hamiltonian as the target Hamiltonian of quantum annealing implemented on a DWave One chip, we study how the qubitqubit coupling strength affects the probability of finding the ground state. We solve the same problem analytically and numerically using classical thermalization models, and discuss conditions under which the classical prediction for the ground state probability, as a function of coupling strength, differs from the experimental results. For certain reasonable noise models this allows us to tell apart quantum annealing and classical thermalization.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Four dimensional gravity with a U(1) gauge field, coupled to various fields in asymptotically antide Sitter spacetime, provides a rich arena for the holographic study of the strongly coupled (2+1)dimensional dynamics of finite density matter charged under a global U(1). As a first step in furthering the study of the properties of fractionalized and partially fractionalized degrees of freedom in the strongly coupled theory, we construct electron star solutions at zero temperature in the presence of a background magnetic field. We work in EinsteinMaxwelldilaton theory. In all cases we construct, the magnetic source is cloaked by an event horizon. A key ingredient of our solutions is our observation that starting with the standard Landau level structure for the density of states, the electron star limits reduce the charge density and energy density to that of the free fermion result. Using this result we construct three types of solution: One has a star in the infrared with an electrically neutral horizon, another has a star that begins at an electrically charged event horizon, and another has the star begin a finite distance from an electrically charged horizon.Lecture Notes in Physics 07/2012;  [Show abstract] [Hide abstract]
ABSTRACT: We develop from first principles Markovian master equations suited for studying the time evolution of a system evolving adiabatically while coupled weakly to a thermal bath. We derive two sets of equations in the adiabatic limit, one using the rotating wave (secular) approximation that results in a master equation in Lindblad form, the other without the rotating wave approximation but not in Lindblad form. The two equations make markedly different predictions depending on whether or not the Lamb shift is included. Our analysis keeps track of the various time and energyscales associated with the various approximations we make, and thus allows for a systematic inclusion of higher order corrections, in particular beyond the adiabatic limit. We use our formalism to study the evolution of an Ising spin chain in a transverse field and coupled to a thermal bosonic bath, for which we identify four distinct evolution phases. While we do not expect this to be a generic feature, in one of these phases dissipation acts to increase the fidelity of the system state relative to the adiabatic ground state.New Journal of Physics 06/2012; 14(12). · 4.06 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We examine straininduced quantized Landau levels in graphene. Specifically, arcbend strains are found to cause nonuniform pseudomagnetic fields. Using an effective Dirac model which describes the lowenergy physics around the nodal points, we show that several of the key qualitative properties of graphene in a straininduced pseudomagnetic field are different compared to the case of an externally applied physical magnetic field. We discuss how using different strain strengths allows us to spatially separate the two components of the pseudospinor on the different sublattices of graphene. These results are checked against a tightbinding calculation on the graphene honeycomb lattice, which is found to exhibit all the features described. Furthermore, we find that introducing a Hubbard repulsion on the meanfield level induces a measurable polarization difference between the A and the B sublattices, which provides an independent experimental test of the theory presented here.Physical review. B, Condensed matter 05/2012; 86(12). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present the results of our studies of the entanglement entropy of a superconducting system described holographically as a fully backreacted gravity system, with a stable ground state. We use the holographic prescription for the entanglement entropy. We uncover the behavior of the entropy across the superconducting phase transition, showing the reorganization of the degrees of freedom of the system. We exhibit the behaviour of the entanglement entropy from the superconducting transition all the way down to the ground state at T=0. In some cases, we also observe a novel transition in the entanglement entropy at intermediate temperatures, resulting from the detection of an additional length scale.Journal of High Energy Physics 02/2012; 2012(5). · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Using holography, we study the entanglement entropy of strongly coupled field theories perturbed by operators that trigger an RG flow from a conformal field theory in the ultraviolet (UV) to a new theory in the infrared (IR). The holographic duals of such flows involve a geometry that has the UV and IR regions separated by a transitional structure in the form of a domain wall. We address the question of how the geometric approach to computing the entanglement entropy organizes the field theory data, exposing key features as the change in degrees of freedom across the flow, how the domain wall acts as a UV region for the IR theory, and a new area law controlled by the domain wall. Using a simple but robust model we uncover this organization, and expect much of it to persist in a wide range of holographic RG flow examples. We test our formulae in two known examples of RG flow in 3+1 and 2+1 dimensions that connect nontrivial fixed points.Journal of High Energy Physics 10/2011; · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the dynamics of quenched fundamental matter in supersymmetric large N SU(N) YangMills theory at zero temperature. Our tools for this study are probe D7branes in the holographically dual PilchWarner gravitational background. Previous work using D3brane probes of this geometry has shown that it captures the physics of a special slice of the Coulomb branch moduli space of the gauge theory, where the N constituent D3branes form a dense one dimensional locus known as the enhançon, located deep in the infrared. Our present work shows how this physics is supplemented by the physics of dynamical flavours, revealed by the D7branes embeddings we find. The PilchWarner background introduces new divergences into the D7branes free energy, which we are able to remove with a single counterterm. We find a family of D7brane embeddings in the geometry and discuss their properties. We study the physics of the quark condensate, constituent quark mass, and part of the meson spectrum. Notably, there is a special zero mass embedding that ends on the enhançon, which shows that while the geometry acts repulsively on the D7branes, it does not do so in a way that produces spontaneous chiral symmetry breaking.Journal of High Energy Physics  J HIGH ENERGY PHYS. 01/2011; 2011(4):122.  [Show abstract] [Hide abstract]
ABSTRACT: We study the dynamics of quenched fundamental matter in supersymmetric large N c SU(N c ) YangMills theory, extending our earlier work to finite temperature. We use probe D7branes in the holographically dual thermalized generalization of the PilchWarner gravitational background found by Buchel and Liu. Such a system provides an opportunity to study how key features of the dynamics are affected by being in a nonconformal setting where there is an intrinsic scale, set here by the mass, m H , of a hypermultiplet. Such studies are motivated by connections to experimental studies of the quarkgluon plasma at RHIC and LHC, where the microscopic theory of the constituents, QCD, has a scale, ΛQCD. We show that the binding energy of mesons in the theory is increased in the presence of the scale m H , and that subsequently the mesonmelting temperature is higher than for the conformal case.Journal of High Energy Physics  J HIGH ENERGY PHYS. 01/2011; 2011(7):116.  Physics. 01/2011;
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ABSTRACT: We study the evolution and scaling of the entanglement entropy after two types of quenches for a 2+1 field theory, using holographic techniques. We study a thermal quench, dual to the addition of a shell of uncharged matter to four dimensional Antide Sitter (AdS_4) spacetime, and study the subsequent formation of a Schwarzschild black hole. We also study an electromagnetic quench, dual to the addition of a shell of charged sources to AdS_4, following the subsequent formation of an extremal dyonic black hole. In these backgrounds we consider the entanglement entropy of two types of geometries, the infinite strip and the round disc, and find distinct behavior for each. Some of our findings naturally supply results analogous to observations made in the literature for lower dimensions, but we also uncover several new phenomena, such as (in some cases) a discontinuity in the time derivative of the entanglement entropy as it nears saturation, and for the electromagnetic quench, a logarithmic growth in the entanglement entropy with time for both the disc and strip, before settling to saturation.New Journal of Physics 08/2010; 13(4). · 4.06 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider the dynamics of a probe fermion charged under a U(1) Maxwell field and a two form potential $B_{(2)}$ in a five dimensional gravity background. The gravity background is constructed from a new solution we find of type IIB supergravity. This new solution is expected to be dual to noncommutative YangMills theory in the 't Hooft limit with global U(1) currents. We study the zero frequency, near horizon behavior of the fermion, where the equations of motion reduce to that of two interacting fermions in AdS$_2$ with an electric field. We show that the operator dimensions in the AdS$_2$ space are complex, leading to the two components of the retarded Green's function in the dual theory to be complex conjugates of each other. In order to preserve unitarity, this result implies there are no zero frequency quasinormal modes in our system. This has important implications for generalizations of recent holographic Fermi liquid setups with AdS$_2$ regions, as it suggests that infinite lifetime excitations can have energies above/below the chemical potential. Therefore, the Fermi energy may not be uniquely set by the chemical potential. Furthermore, since the gravity background breaks rotational symmetry along the spatial directions of the dual YangMills theory, we do not expect the Fermi surface to be spherical in shape in momentum space. Comment: 23 pages02/2010;
Publication Stats
547  Citations  
85.76  Total Impact Points  
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Institutions

2008–2014

University of Southern California
 Department of Physics and Astronomy
Los Angeles, California, United States 
University of California, Los Angeles
 Department of Physics and Astronomy
Los Angeles, CA, United States
