Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A27: Quantum Simulation of ManyBody PhysicsFocus

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Sponsoring Units: DQI Chair: Alicia Kollar Room: BCEC 160C 
Monday, March 4, 2019 8:00AM  8:12AM 
A27.00001: WITHDRAWN ABSTRACT

Monday, March 4, 2019 8:12AM  8:24AM 
A27.00002: Hybrid quantumclassical algorithm for variational coupled cluster method Sathyawageeswar Subramanian, Yudong Cao We present a hybrid quantum algorithm for the variational coupled cluster (vCC) method in quantum chemistry. We show that for a problem instance of $n$ electrons described by $m$ spin orbitals and a constant level $\ell$ of excitations, the energy expectation value of the vCC trial wavefunction can be estimated to precision $\epsilon$ in time proportional to $\widetilde{\O}(\ell m^\ell n^\ell /\epsilon^2)$ on a quantum computer. Classically, computing the same expectation generally incurs a cost exponential in $n$ and $m$, implying that our quantum algorithm can yield a significant speedup over known classical methods. We envision that such capability combined with the framework of the variational quantum eigensolver (VQE) will add to recent quantum algorithms for unitary coupled cluster methods, enriching the toolset of quantum chemistry calculations beyond what is feasible on classical computers. We also illustrate a method for calculating analytical gradients for the vCC method, which can be used with gradientfree directsearch optimisation methods (such as the NelderMead and COBYLA algorithms). 
Monday, March 4, 2019 8:24AM  8:36AM 
A27.00003: Optical manipulation of entanglement in plasmonically coupled quantum dot qubits Matthew Otten, Stephen K Gray, German Kolmakov We consider a system composed of two quantum dot qubits coupled with a common, damped surface plasmon mode; each quantum dot is also coupled to a separate photonic cavity mode. Cavity quantum electrodynamics calculations show that upon optical excitation by a femtosecond laser pulse, entanglement of the quantum dot excitons occurs, and the time evolution of the g^{(2) }pair correlation function of the cavity photons is an indicator of the entanglement. We also show that the degree of entanglement is conserved during the time evolution of the system. Furthermore, if coupling of the photonic cavity and quantum dot modes is large enough, the quantum dot entanglement can be transferred to the cavity modes to increase the overall entanglement lifetime. This latter phenomenon can be viewed as a signature of entangled, longlived quantum dot excitonpolariton formation. The preservation of total entanglement in the strong coupling limit of the cavity/quantum dot interactions suggests a novel means of entanglement storage and manipulation in highquality optical cavities. 
Monday, March 4, 2019 8:36AM  8:48AM 
A27.00004: Simulating the tJ Model on a Quantum Computer Brian Rost, James Freericks Quantum computers are expected to give an exponential speedup over classical computers for the simulation of strongly correlated quantum systems. Efficient implementation of these simulations is of great interest for many fields in the physical sciences. We consider algorithms for simulating the tJ model, a prominent model for high temperature superconductivity, on a quantum computer. Our approach focuses the computation on the lowenergy projected phase space instead of taking the large interaction limit of the Hubbard model. We investigate tradeoffs in noise, complexity, time and gate count in extracting the system's Green's function and selfenergy, from which a wide variety of interesting physical quantities can be computed. 
Monday, March 4, 2019 8:48AM  9:00AM 
A27.00005: Nonlinear sigma model approach to manybody quantum chaos: regularized and unregularized outoftimeordered correlators Yunxiang Liao, Victor Galitski In this work, we derive an extended version of Keldysh nonlinear sigma model to investigate manybody quantum chaos in a 2D interacting fermion system subject to quenched disorder. This model provides a framework in which the regularized and unregularized outoftimeordered correlators with different arrangements of the thermal factors can be studied. The two types of correlators grow exponentially with different growth rates which can be extracted from the mass of the “interworld” diffuson propagator. This should be compared with its “intraworld” counterpart that gives the dephasing rate corresponding to the phase relaxation of singleparticle states. Our result for regularized exponent is consistent with the previous one obtained from the diagrammatic perturbation approach. Furthermore, the regularized (unregularized) exponent obeys (violates) the chaos bound 2πT. We note that the regularized exponent is entirely due to the inelastic collisions between particles, while the unregularized exponent contains an additional contribution from elastic scattering of particles off of the Friedel oscillations of the particle density. Therefore, we believe that the unregularized exponent cannot measure manybody quantum chaos which originates from interactions. 
Monday, March 4, 2019 9:00AM  9:12AM 
A27.00006: Information spreading in manybody systems and the outoftimeordered correlator Shenglong Xu, Brian Swingle The information spreading in quantum manybody systems under unitary realtime evolution is related to the growth of simple Heisenberg operators, which is quantified by outoftimeorder correlation (OTOC) functions. Using a timedependent disordered Hamiltonian model, I will analytically show the emergence of hydrodynamics description of the OTOC arising from the unitary realtime dynamics. Corrections of quantum fluctuation are manifested as the diffusively broadened wavefront of the OTOC. This picture is supported by largescale (~200 spins) realtime dynamics simulation on realistic spin Hamiltonians using our newly developed tensornetwork method. 
Monday, March 4, 2019 9:12AM  9:24AM 
A27.00007: Graph theory and bounds on operator growth Andrew Lucas Motivated by recent developments in manybody quantum chaos, I will present bounds on the growth of operators in klocal quantum manybody systems. These bounds can be interpreted as simple combinatorics problems in graph theory. I will give explicit examples of how these bounds can be parametrically stronger than LiebRobinson bounds. 
Monday, March 4, 2019 9:24AM  9:36AM 
A27.00008: Quantum codes for quantum simulation of Fermions on a square lattice of qubits Mark Steudtner, Stephanie Wehner Quantum simulation of fermionic systems is a promising application of quantum computers, but in order to program them, we need to map fermionic states and operators to qubit states and quantum gates. While quantum processors may be built as twodimensional qubit networks with couplings between nearest neighbors, standard Fermiontoqubit mappings do not account for that kind of connectivity. In this work we concatenate the (onedimensional) JordanWigner transform with specific quantum codes defined under the addition of a certain number of auxiliary qubits. This yields a novel class of mappings with which any fermionic system can be embedded in a twodimensional qubit setup, fostering scalable quantum simulation. Our technique is demonstrated on the twodimensional FermiHubbard model, that we transform into a local Hamiltonian. What is more, we adapt the VerstraeteCirac transform and BravyiKitaev Superfast simulation to the square lattice connectivity and compare them to our mappings. 
Monday, March 4, 2019 9:36AM  9:48AM 
A27.00009: WITHDRAWN ABSTRACT

Monday, March 4, 2019 9:48AM  10:00AM 
A27.00010: Realizing quantum Ising models in tunable twodimensional arrays of single Rydberg atoms Invited Speaker: Antoine Browaeys tbd 
Monday, March 4, 2019 10:00AM  10:12AM 
A27.00011: Phase structure of Ising models at complex temperature Vadim Oganesyan, Sankhya Basu This work explores continuation of phases and phase transtions in statistical sums of planar Ising models to complex couplings  temperature and field. We use a combination of analytic and numerical techniques, based on tensor network formulations of renormalization group (TRG and SRG) to access the regimes of phase space displaying novel modulated correlations and entirely out of reach of standard monte carlo methods. We also study and improve convergence and stability properties of tensor renormalization methods. 
Monday, March 4, 2019 10:12AM  10:24AM 
A27.00012: Quantum Feedback Protocol for Calculating SingleParticle Green's Functions at Finite Temperature James Freericks, Jeffrey Cohn, Forest Yang, Khadijeh Najafi, Barbara Jones Drawing on the eigenstate thermalization hypothesis, we present a quantum feedback algorithm that approximates singleparticle Green's functions at finite temperature. Measuring a singleparticle Green's function from a thermal state on a quantum computer is a well known procedure, but the challenges and resources needed for full Gibbs state preparation are generally too expensive for practical simulation in the near future. We examine how sampling from more easily prepared states yields accurate approximations to singleparticle Green's functions at finite temperature. We employ an additional feedback mechanism, that tests for convergence and extracts the effective temperature of the system being simulated. We also compare the tradeoffs of different approaches to make these techniques applicable on near term devices. 
Monday, March 4, 2019 10:24AM  10:36AM 
A27.00013: Quantum Error Correcting Codes in Eigenstates of TranslationInvariant Spin Chains Fernando Brandao, Elizabeth Crosson, Burak Sahinoglu, John Bowen This work establishes new connections between quantum chaos and translationinvariance in manybody spin systems, on one hand, and approximate quantum error correcting codes (AQECC), on the other hand. We first observe that quantum chaotic systems exhibiting the Eigenstate Thermalization Hypothesis (ETH) have eigenstates forming approximate quantum error correcting codes. Then we show that AQECC can be obtained probabilistically from translationinvariant energy eigenstates of every translationinvariant spin chain, including integrable models. Applying this result to 1D classical systems, we describe a method for using local symmetries to construct parent Hamiltonians that embed these codes into the lowenergy subspace of gapless 1D quantum spin chains. As explicit examples we obtain local AQECC in the ground space of the 1D ferromagnetic Heisenberg model and the Motzkin spin chain model with periodic boundary conditions, thereby yielding nonstabilizer codes in the ground space and low energy subspace of physically plausible 1D gapless models. 
Monday, March 4, 2019 10:36AM  10:48AM 
A27.00014: A synthetic gauge field in a twodimensional timemultiplexed quantum random walk Hamidreza Chalabi, Sabyasachi Barik, Sunil Mittal, Mohammad Hafezi, Thomas E Murphy, Edo Waks Timemultiplexed quantum random walk provides an efficient method to realize a quantum random walk via time delays and beamsplitters. Previously, researchers have demonstrated the control of random walk evolution based on photon’s polarization degree of freedom. In this presentation, we propose adding synthetic gauge fields for controlling the evolution of the random walk which is important to simulate a broad class of physical effects. In this platform, varied lengths of optical fibers create the time delays and the gauge fields are implemented through phase modulations. We show how different gauge fields provide the possibility of opening band gaps in the band structure. The presence of these bandgaps leads to the pseudo magnetic confinement of the random walk distribution. We present the theoretical predictions and experimental observations of this confinement. We also demonstrate the possibility of creating edge states by applying different gauge fields to different regions of the synthetic space. The experimental results confirm the confinement of the evolution of the random walk around the boundary. 
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