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 Many-Body 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A27.00001: WITHDRAWN ABSTRACT
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Monday, March 4, 2019 8:12AM - 8:24AM |
A27.00002: Hybrid quantum-classical 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 gradient-free direct-search optimisation methods (such as the Nelder-Mead 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, long-lived quantum dot exciton-polariton 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 high-quality optical cavities. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A27.00004: Simulating the t-J Model on a Quantum Computer Brian Rost, James Freericks Quantum computers are expected to give an exponential speed-up 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 t-J model, a prominent model for high temperature superconductivity, on a quantum computer. Our approach focuses the computation on the low-energy projected phase space instead of taking the large interaction limit of the Hubbard model. We investigate trade-offs in noise, complexity, time and gate count in extracting the system's Green's function and self-energy, from which a wide variety of interesting physical quantities can be computed. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A27.00005: Non-linear sigma model approach to many-body quantum chaos: regularized and unregularized out-of-time-ordered correlators Yunxiang Liao, Victor Galitski In this work, we derive an extended version of Keldysh non-linear sigma model to investigate many-body quantum chaos in a 2D interacting fermion system subject to quenched disorder. This model provides a framework in which the regularized and unregularized out-of-time-ordered 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 “inter-world” diffuson propagator. This should be compared with its “intra-world” counterpart that gives the dephasing rate corresponding to the phase relaxation of single-particle 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 many-body quantum chaos which originates from interactions. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A27.00006: Information spreading in many-body systems and the out-of-time-ordered correlator Shenglong Xu, Brian Swingle The information spreading in quantum many-body systems under unitary real-time evolution is related to the growth of simple Heisenberg operators, which is quantified by out-of-time-order correlation (OTOC) functions. Using a time-dependent disordered Hamiltonian model, I will analytically show the emergence of hydrodynamics description of the OTOC arising from the unitary real-time dynamics. Corrections of quantum fluctuation are manifested as the diffusively broadened wavefront of the OTOC. This picture is supported by large-scale (~200 spins) real-time dynamics simulation on realistic spin Hamiltonians using our newly developed tensor-network 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 many-body quantum chaos, I will present bounds on the growth of operators in k-local quantum many-body 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 Lieb-Robinson 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 two-dimensional qubit networks with couplings between nearest neighbors, standard Fermion-to-qubit mappings do not account for that kind of connectivity. In this work we concatenate the (one-dimensional) Jordan-Wigner 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 two-dimensional qubit setup, fostering scalable quantum simulation. Our technique is demonstrated on the two-dimensional Fermi-Hubbard model, that we transform into a local Hamiltonian. What is more, we adapt the Verstraete-Cirac transform and Bravyi-Kitaev 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
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Monday, March 4, 2019 9:48AM - 10:00AM |
A27.00010: Realizing quantum Ising models in tunable two-dimensional 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 Single-Particle 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 single-particle Green's functions at finite temperature. Measuring a single-particle 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 single-particle 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 trade-offs 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 Translation-Invariant Spin Chains Fernando Brandao, Elizabeth Crosson, Burak Sahinoglu, John Bowen This work establishes new connections between quantum chaos and translation-invariance in many-body 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 translation-invariant energy eigenstates of every translation-invariant 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 low-energy 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 non-stabilizer 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 two-dimensional time-multiplexed quantum random walk Hamidreza Chalabi, Sabyasachi Barik, Sunil Mittal, Mohammad Hafezi, Thomas E Murphy, Edo Waks Time-multiplexed quantum random walk provides an efficient method to realize a quantum random walk via time delays and beam-splitters. 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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