Bulletin of the American Physical Society
86th Annual Meeting of the APS Southeastern Section
Volume 64, Number 19
Thursday–Saturday, November 7–9, 2019; Wrightsville Beach, North Carolina
Session A01: Quantum Information Theory |
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Chair: Ben Lawrie, Oak Ridge National Laboratory Room: Holiday Inn Resort Causeway/Masonboro |
Thursday, November 7, 2019 8:30AM - 9:00AM |
A01.00001: Physics of spin systems using quantum computers Invited Speaker: Alexander Kemper Quantum hardware has advanced to the point where it is now possible to perform simulations of physical systems and elucidate their ground states and excitation spectra. In this talk, I will overview some of our recent results on this topic, focusing mainly on systems of interacting spins and their properties as seen through a quantum computing lens. I will discuss the Heisenberg and Kitaev models, and how we might adapt concepts familiar from many-body theory to quantum hardware. [Preview Abstract] |
Thursday, November 7, 2019 9:00AM - 9:30AM |
A01.00002: Efficient Symmetry-Preserving State Preparation Circuits for the Variational Quantum Eigensolver Algorithm Invited Speaker: Bryan Gard The variational quantum eigensolver is one of the most promising approaches for performing chemistry simulations using noisy intermediate-scale quantum (NISQ) processors. The efficiency of this algorithm depends crucially on the ability to prepare multi-qubit trial states on the quantum processor that either include, or at least closely approximate, the actual energy eigenstates of the problem being simulated while avoiding states that have little overlap with them. Symmetries play a central role in determining the best trial states. Here, we present efficient state preparation circuits that respect particle number, total spin, spin projection, and time-reversal symmetries. These circuits contain the minimal number of variational parameters needed to fully span the appropriate symmetry subspace dictated by the chemistry problem while avoiding all irrelevant sectors of Hilbert space. We show how to construct these circuits for arbitrary numbers of orbitals, electrons, and spin quantum numbers, and we provide explicit decompositions and gate counts in terms of standard gate sets in each case. We test our circuits in quantum simulations of the $H_2$ and LiH molecules and find that they outperform standard state preparation methods in terms of both accuracy and circuit depth. [Preview Abstract] |
Thursday, November 7, 2019 9:30AM - 10:00AM |
A01.00003: Probing the quench dynamics of antiferromagnetic correlations in a 2D quantum Ising system of 200 spins Invited Speaker: Peter Schauss Simulating the real-time evolution of quantum spin systems far out of equilibrium poses a major theoretical challenge, especially in more than one dimension. Here, we experimentally explore quench dynamics in a two-dimensional Ising spin system with transverse and longitudinal fields - a system that is well-suited for scalable quantum computing. We prepare a near unit-occupancy atomic array of over 200 atoms by loading a spin-polarized band insulator of fermionic lithium into an optical lattice and induce nearest-neighbor interactions by direct excitation to a low-lying Rydberg state. Using site-resolved microscopy, we probe antiferromagnetic correlations in the system after a sudden quench from a paramagnetic state and compare our measurements to exact calculations in the regimes where it is possible. We achieve many-body states with longer-range antiferromagnetic correlations by implementing a near-adiabatic quench of the longitudinal field and study the buildup of correlations. The coherence is limited by anomalous dephasing that we attribute to motion of the atoms caused by the strong interactions. [Preview Abstract] |
Thursday, November 7, 2019 10:00AM - 10:12AM |
A01.00004: Quantum computation of magnon spectra Akhil Francis, James Freericks, Alexander Kemper We demonstrate quantum computation of two-point correlation functions for a Heisenberg spin chain. Using the IBM Q 20 Tokyo machine, we find that for two sites the correlation functions produce the exact results reliably. For four sites, results from the quantum computer are noisy due to readout errors and decoherence. Nevertheless, the correlation functions retain the correct spectral information. This is illustrated in the frequency domain by accurately extracting the magnon energies from peaks in the spectral function. [Preview Abstract] |
Thursday, November 7, 2019 10:12AM - 10:24AM |
A01.00005: Quantum-classical implementation of two-site dynamical mean-field theory using quantum computers Trevor Keen, Pavel Lougovski, Steven Johnston, Thomas Maier We report on a quantum-classical simulation of a two-site dynamical mean-field theory (DMFT) calculation of a Hubbard model. We employ IBM's superconducting qubit chip to compute the zero-temperature impurity Green's function in the time domain and utilize a classical computer to fit the measured Green's function and determine its frequency dependence. We find that Trotter errors lead to erroneous impurity parameters, which, along with noise from the quantum chip, prevent the DMFT algorithm from converging to the correct solution. To reduce this sensitivity to Trotter errors, we determine the impurity parameters by integrating the quasiparticle peaks in the spectral function. This allows us to iterate the DMFT loop to self-consistency for a strongly Mott insulating system at half-filling. [Preview Abstract] |
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