Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L32: Noisy Intermediate Scale Quantum Computers IIILive
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Sponsoring Units: DQI DCOMP Chair: Lindsay Bassman, Lawrence Berkeley National Lab |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L32.00001: Building efficient VQE ansatze with complete pools of operators. Vladyslav Shkolnykov, Nicholas J. Mayhall, Sophia Economou, Edwin Barnes In this talk we discuss the novel adapt-VQE algorithm [1] and show how to build an efficient ansatz for it. We found that a set of 2n-2 unitaries is sufficient to transform any real state to any other, and the generators of these unitaries we thus call a complete pool. We give a proof for the minimality of such pools, discuss their algebraic properties and present a technique to efficiently find all of them. We also discuss the performance of these pools in the presence of symmetries in the Hamiltonian, that exhibits nontrivial features. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L32.00002: Efficient VQE with the perturbation method Qingfeng Wang, Ming Li, Christopher Monroe, Yunseong Nam Quantum-classical hybrid approaches that harness computational power of early NISQ devices have recently been demonstrated on multiple platforms and VQE is perhaps one of the most widely known. In a typical VQE run, one approximates the ground state energy of a target molecular system by iteratively creating an ansatz ground state and evaluating its energy on a quantum computer. To boost the quality of the energy estimate and obtain faster convergence, it is crucial that one expends as little quantum resource as possible, since each quantum operation incurs computational error. In this talk, we present a perturbation-based method, namely, hybrid second-order Møllar-Plesset perturbation (HMP2), capable of guiding the development of a good ansatz state while significantly reducing the quantum resources required to estimate the energy. |
Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L32.00003: A variational method for quantum simulation of time evolution Samuel Wilkinson, Ludwig Nützel, Michael Josef Hartmann The simulation of quantum systems is one of the most promising applications of quantum computing. Typically, simulation of time evolution is achieved by decomposing the time evolution operator into a series of quantum gates through a process known as "Trotterization" [1], where the accuracy of the simulation can always be improved by adding more gates to the Trotter sequence. However, in the era of noisy, intermediate-scale quantum (NISQ) computing, applying many gates in sequence leads to compound errors that quickly make the simulation unreliable. Thus, it would be desirable to find a way to increase the accuracy of a given quantum simulation without increasing the number of gates. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L32.00004: Digital Quantum Simulation of Open Quantum Systems Using Quantum Imaginary Time Evolution Hirsh Kamakari, Mario Motta, Austin Minnich Quantum simulation on emerging quantum hardware is a topic of intense interest. While many studies focus on computing ground state properties or simulating unitary dynamics of closed systems, open quantum systems are a desirable target of study owing to their ubiquity and rich physical behavior. However, the non-unitary dynamics that makes these systems of interest is also difficult to simulate on near-term quantum hardware. Here, we report the digital quantum simulation of the dynamics of open quantum systems governed by a Lindblad equation using an adaptation of the quantum imaginary time evolution (QITE) algorithm. Our approach also allows the study of non-Markovian effects by simulation of time-convolutionless master equations and generalized master equations. We demonstrate our method through simulations of the spontaneous emission in the damped Jaynes-Cummings model on IBM quantum hardware. Our work demonstrates that the dynamics of open systems are accessible on near-term quantum hardware and without recourse to variational schemes. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L32.00005: Accurately computing electronic properties of materials using eigenenergies Charles Neill, Lev B Ioffe, Vadim Smelyanskiy A promising approach to study quantum materials is to synthesize them on an engineered platform. However, realizing the necessary parameter regimes with sufficient accuracy has been an outstanding challenge. Here, using superconducting qubits, we provide an experimental blueprint for a programmable and accurate quantum matter simulator. We illustrate the underlying method by resolving the single-particle band-structure of a one-dimensional wire. We demonstrate nearly complete mitigation of decoherence and readout errors and arrive at an unprecedented accuracy of 0.01 radians in energy eigenvalues. Insight into this result is provided by highlighting robust properties of a Fourier transform. Furthermore, we synthesize magnetic flux and disordered potentials, two key tenets of a condensed-matter system. When sweeping flux, we observe avoided level crossings in the spectrum, a signature of the spatial pattern of disorder. Finally, we reconstruct electronic properties of the eigenstates where we observe persistent currents and a strong suppression of conductance with added disorder. Our work outlines an accurate method for quantum simulation and paves the way to study novel quantum materials with superconducting qubits. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L32.00006: Efficient Preparation of Gutzwiller Ansatz on Noisy Intermediate-Scale Quantum Computers Bruno Murta, Joaquin Fernandez-Rossier Digital quantum computers are expected to solve instances of the quantum many-body problem that are intractable in classical hardware, notably those involving strongly-correlated electrons. The leading algorithm in Noisy Intermediate-Scale Quantum Computers (NISQCs) is the Variational Quantum Eigensolver, which attempts to find the ground state by minimizing the energy of a parameterized trial state. This ansatz is typically constructed by initializing a mean-field state, followed by either applying single- and double-electron excitations on this initial state or evolving it via a parameterized multi-step propagator in a similar spirit to adiabatic state preparation. However, as the system size increases, the mean-field state becomes an increasingly worse starting point due to the orthogonality catastrophe. Preparing a more educated guess of the exact ground state is therefore relevant to ease the classical optimization problem. To this end, we discuss how to efficiently prepare on a NISQC the Gutzwiller wavefunction, a simple, yet effective ansatz to approximate the ground state of the Fermi-Hubbard model, the reference model to describe correlated electrons in condensed matter. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L32.00007: Dynamical Self-energy mapping for Quantum Computing. Diksha Dhawan, Mekena Metcalf, Dominika Zgid For noisy intermediate-scale quantum (NISQ) devices only a moderate number of qubits with alimited coherence is available thus enabling only shallow circuits and a few time evolution stepsin the currently performed quantum computations. Here, we present how to bypass this challengein practical molecular chemistry simulations on NISQ devices by employing a quantum–classicalhybrid algorithm allowing us to produce a sparse Hamiltonian which contains onlyO(n2) terms in aGaussian orbital basis when compared to theO(n4) terms of a standard Hamiltonian, wherenis thenumber of orbitals in the system. Classical part of this hybrid entails parametrization of the sparse,fictitious Hamiltonian in such a way that it recovers the self-energy of the original molecular system.Quantum machine then uses this fictitious Hamiltonian to calculate the self-energy of the system.We show that the developed hybrid algorithm yields very good total energies for small moleculartest cases while reducing the depth of the quantum circuit by at least an order of magnitude whencompared with simulations involving a full Hamiltonian. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L32.00008: qubit-ADAPT-VQE: An adaptive algorithm for constructing hardware-efficient ansätze on a quantum processor Ho Lun Tang, Vladyslav Shkolnykov, George S Barron, Harper Grimsley, Nicholas J. Mayhall, Edwin Barnes, Sophia Economou Recently, an algorithm termed ADAPT-VQE used fermionic operators to iteratively build system-adapted ansätze with substantially fewer variational parameters compared to other approaches. However, deep state preparation circuits remain a challenge. Here, we present a hardware-efficient variant of this algorithm called qubit-ADAPT. By numerical simulations on H4, LiH and H6, we show that with a well-designed operator pool, qubit-ADAPT can reduce the circuit depth by one order of magnitude while maintaining the same accuracy as its fermionic counterpart. Addressing the high measurement cost which is proportional to the size of the operator pool, we present systematical ways to construct a sufficient pool with size growing linearly in the number of qubits. This result highlights the promise of adaptive simulation algorithms on near-term quantum devices. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L32.00009: Variational preparation of finite-temperature states on a quantum computer (Part 1: theory) Shavindra Premaratne, Ramiro Sagastizabal, Sonika Johri, Xiang Zou, Berend Klaver, Michiel Adriaan Rol, Victor Negîrneac, Miguel S Moreira, Nandini Muthusubramanian, Marc Beekman, Chris Zachariadis, Viacheslav Ostroukh, Nadia Haider, Alessandro Bruno, Leonardo DiCarlo, Anne Matsuura Simulating quantum phenomena is one of the most promising applications of Noisy Intermediate Scale Quantum (NISQ) computing systems. In particular, the ability to represent the dynamical evolution of many-body systems has been demonstrated1. However, the accuracy of these simulations depends on efficient initial state preparation in the quantum computer. Studying temperature-dependent phenomena, like high temperature superconductivity, requires the preparation of thermal equilibrium states called Gibbs states within the NISQ qubit ensembles. Here, we describe a procedure to generate finite-temperature Gibbs states for the transverse-field Ising chain Hamiltonian, via preparation of thermofield double (TFD) states. The TFD states are generated using a hybrid quantum-classical variational algorithm, whose advantages and limitations we discuss. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L32.00010: Variational preparation of finite-temperature states on a quantum computer (Part 2: experiment) Ramiro Sagastizabal, Shavindra Premaratne, Berend Klaver, Michiel Adriaan Rol, Victor Negîrneac, Miguel S Moreira, Xiang Zou, Sonika Johri, Nandini Muthusubramanian, Marc Beekman, Chris Zachariadis, Viacheslav Ostroukh, Nadia Haider, Alessandro Bruno, Anne Matsuura, Leonardo DiCarlo The preparation of thermal equilibrium states is important for the simulation of condensed-matter and cosmology systems using a quantum computer. Our method targets the generation of thermofield double states using a hybrid quantum-classical variational approach motivated by quantum-approximate optimization algorithms, without prior calculation of optimal variational parameters by numerical simulation. The fidelity of generated states to the thermal-equilibrium state varies smoothly from 99% at infinite temperature to 75% at near-zero temperature. We find quantitative agreement with a numerical simulation of our quantum processor with error parameters drawn from experiment. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L32.00011: Quantum simulations of materials on near-term quantum computers He Ma, Marco Govoni, Giulia Galli Quantum computers hold promise to enable efficient simulations of the properties of molecules and materials; however, at present they only permit ab initio calculations of a few atoms, due to a limited number of qubits. In order to harness the power of near-term quantum computers for simulations of larger systems, it is desirable to develop hybrid quantum-classical methods where the quantum computation is restricted to a small portion of the system. This is of particular relevance for molecules and solids where an active region requires a higher level of theoretical accuracy than its environment. We present a quantum embedding theory for the calculation of strongly-correlated electronic states of active regions, with the rest of the system described within density functional theory. We demonstrate the effectiveness of the approach by investigating spin-defects in semiconductors that are relevant for quantum information science. We discuss calculations on quantum computers and show that they yield results in agreement with those obtained with exact diagonalization on classical architectures1, paving the way to simulations of realistic materials on near-term quantum computers. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L32.00012: Hybrid Quantum-Classical Eigensolver Without Variation or Parametric Gates Pejman Jouzdani, Stefan A Bringuier The use of near-term quantum devices that lack quantum error correction, for addressing quantum |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L32.00013: Quantum-optimal-control-inspired ansätze for variational quantum algorithms Alexandre Choquette, Agustin Di Paolo, Panagiotis Barkoutsos, David Senechal, Ivano Tavernelli, Alexandre Blais A central component of variational quantum algorithms (VQA) is the state-preparation circuit, also known as ansatz. This circuit is most commonly designed to respect the symmetries of the problem Hamiltonian and, in this way, constrain the variational search to a subspace of interest. Here, we show that this approach is not always advantageous by introducing ansätze that incorporate symmetry-breaking unitaries. This class of ansätze, that we call Quantum-Optimal-Control-inspired Ansätze (QOCA), is inspired by the theory of quantum optimal control and leads to an improved convergence of VQAs for some important problems. Indeed, we benchmark QOCA against popular ansätze applied to the Fermi-Hubbard model at half-filling and show that our variational circuits can approximate the ground state of this model with significantly higher accuracy and for larger systems. We also show how QOCA can be used to find the ground state of the water molecule and compare the performance of our ansatz with other common choices used for chemistry problems. This work was recently published under arXiv:2008.01098. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L32.00014: Digital Quantum Simulation of Non-Equilibrium Quantum Many-Body Systems Benedikt Fauseweh, Jian-Xin Zhu Digital quantum simulations (DQS) is a driving force behind the development of universal quantum computers. DQS uses the capabilities of quantum computers, such as superposition and entanglement, to determine the dynamics of quantum systems, which are beyond the computability of modern classical computers. A notoriously challenging task in this field is the description of non-equilibrium dynamics in quantum many-body systems, because it defies the methods and principles of equilibrium physics. |
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