Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E42: Applications of Noisy Intermediate Scale Quantum Computers IIFocus

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Sponsoring Units: DQI Chair: Andras Gyenis, Princeton University Room: BCEC 210A 
Tuesday, March 5, 2019 8:00AM  8:12AM 
E42.00001: AlgorithmCentric Error Mitigation Ed Younis, Wim Lavrijsen, Koushik Sen, Aydin Buluc, Miroslav Urbanek, Wibe A De Jong, Costin Iancu We derive algorithmspecific error mitigation procedures for VQE. The core idea is to determine which part of a circuit contributes most to the final output error and then harden only these subcircuits. Using simulation with gate level noise injection, our results indicate that VQE is most sensitive to errors on the source qubit of CNOT gates, and relatively insensitive to errors on any other qubits or single gates, irrespective of the noise distribution. We then devise an algorithm that attempts to increase the fidelity of qubits that appear as sources of CNOT gates. Given a circuit, we determine the qubits that appear in most CNOT gates (contribute most to error) and use ancilla qubits to mimic their operations (same state) and periodically switch between the original and ancilla qubit during circuit operation. Simulation results indicate that the technique is able to improve the quality of the solution from the quantum circuit. This is confirmed by experiments on the IBM hardware, where for a 4qubit VQE with one ancilla qubit we observe fidelity improving from 46% to 52%. 
Tuesday, March 5, 2019 8:12AM  8:24AM 
E42.00002: Implementation of Grover’s quantum search algorithm with error mitigation at IBM Q computers Yulun Wang, Robert J Harrison, Predrag Krstic

Tuesday, March 5, 2019 8:24AM  8:36AM 
E42.00003: Fundamental limitations for measurements in quantum manybody systems Thomas Barthel, Jianfeng Lu Dynamical measurement schemes are an important tool for the investigation of quantum manybody systems, especially in the age of quantum simulation. Here, we address the question whether generic measurements can be implemented efficiently if we have access to a certain set of experimentally realizable measurements and can extend it through time evolution. For the latter, two scenarios are considered (a) evolution according to unitary circuits and (b) evolution due to Hamiltonians that we can control in a timedependent fashion. We find that the time needed to realize a certain measurement to a predefined accuracy scales in general exponentially with the system size – posing a fundamental limitation. The argument is based on the construction of εpackings for manifolds of observables with identical spectra and a comparison of their cardinalities to those of εcoverings for quantum circuits and unitary timeevolution operators. The former is related to the study of Grassmann manifolds. 
Tuesday, March 5, 2019 8:36AM  8:48AM 
E42.00004: Postponing the orthogonality catastrophe: efficient state preparation for electronic structure simulations on quantum devices Norm Tubman, Carlos Mejuto Zaera, Jeffrey Epstein, Diptarka Hait, Daniel Levine, William Huggins, Zhang Jiang, Jarrod McClean, Ryan Babbush, Martin HeadGordon, Birgitta K Whaley Many proposals for efficiently simulating eigenstates of physical systems on quantum computers require that one can easily initialize wavefunctions with nonvanishing overlap on eigenstates of interest. Though there is now a large body of work exploring the costs of simulating electronic structure systems on a quantum computer, 
Tuesday, March 5, 2019 8:48AM  9:00AM 
E42.00005: Testing, analysis, and refinement of the quantum Metropolis algorithm Jonathan Moussa The classical Metropolis algorithm has been adapted into a quantum algorithm [Temme et al., Nature 471, 87 (2011)] with three important drawbacks: (1) longtime Hamiltonian evolution is needed for precise phase estimation of system energies, (2) many repetitions are needed to successfully reject a Metropolis update with a low failure rate, and (3) thermalstate observables are not measured during the thermalization process. We present a revised quantum Metropolis algorithm that partially mitigates these drawbacks as evidenced by a combination of numerical experiments and theoretical analysis. 
Tuesday, March 5, 2019 9:00AM  9:12AM 
E42.00006: Simulating strongly interacting fermionic systems in a quantum computer Alexandre ChoquettePoitevin, Panagiotis Barkoutsos, Agustin Di Paolo, Alexandre Foley, David Senechal, Ivano Tavernelli, Alexandre Blais Noisy intermediatescale quantum computation has the potential to be useful for the quantum simulation of small fermionic systems using variational quantum algorithms (VQA). Applications of hybrid quantumclassical approaches provide proof that VQAs are robust against noise and can handle limited qubit connectivity. In this talk, we approach a class of strongly interacting fermionic Hamiltonians formulated in the variational cluster approach by means of VQAs. More precisely, we tackle the problem of a 1D lattice to study the Mott transition. This work is a first step towards quantum simulation of larger and higherdimensional strongly interacting electronic systems. 
Tuesday, March 5, 2019 9:12AM  9:24AM 
E42.00007: Endtoend quantum chemistry simulations with reduced errors Miroslav Urbanek, Wim Lavrijsen, Wibe A De Jong Error reduction through suppression, mitigation, or correction is essential to enable quantum chemistry applications on a larger number of qubits accessible in Noisy IntermediateScale Quantum (NISQ) computers. We focus on the development of quantum chemistry algorithms running within the hybrid classicalquantum Variational Quantum Eigensolver (VQE) approach. VQE has been successfully demonstrated on quantum computers with a small number of qubits, but its performance on larger systems is currently limited by various sources of experimental noise. Our target is to develop quantum algorithms and computational circuits that exploit various error mitigation and correction techniques for initialization, gate operations, and measurement. This is necessary to reduce errors introduced by experimental conditions in NISQ systems. We compare theoretical and simulation results with results obtained in experiments. 
Tuesday, March 5, 2019 9:24AM  9:36AM 
E42.00008: Trotter Error Scaling with System Size in Quantum Simulations Matthias Troyer, Natalie Pearson, David Poulin A main concern when implementing digital quantum simulation on quantum computers or simulators is the number of gate operations required. The most common implementation uses the Trotter decomposition to map an arbitrary Hamiltonian onto realisable gates. However, the proven upper bounds on the error introduced using this method grows with the system size. In order to maintain the same accuracy this would imply that the time step has to shrink with system size, leading to a sharp increase in the number of gates required. We show empirically that for local Hamiltonians the error for local observables and their correlation functions is independent of system size. This results is obtained by simulating large one dimensional quantum Ising model at and away from the critical point using a Trotter decomposition in imaginary time . We find that the Trotter errors saturate with increasing system size even at the critical points. We finally discuss longrange models and correlations in time. 
Tuesday, March 5, 2019 9:36AM  9:48AM 
E42.00009: Improved optimization algorithm for use in variational quantum eigensolvers Titus Morris Quantum algorithms for treating nuclear and electronic structure problems face a host of challenges in order to run successfully on both nearterm and future fault tolerant quantum computers. For variation quantum eigensolvers (VQE), one such challenge is how to optimize parameters dictating the wavefunction efficiently i.e. with few function evaluations in the presence of noisy energy evaluation. This has typically been done with classic optimizers, but these will begin to pose problems as the parameter sets required to treat larger realistic systems grow. Here I present an algorithm that, at the cost of extra Pauli expectation values, allows for a faithful estimation of the parameter gradient of several classes of wavefunctions. I apply this algorithm to the treatment of the deuteron and H_{2}, and show that it gives a much faster convergence in terms of overall function evaluations when compared to classic optimizers, and this advantage increases with parameter set size. This presents a promising step forward in pushing for treating realistic systems with quantum computers. 
Tuesday, March 5, 2019 9:48AM  10:00AM 
E42.00010: Characterization of Training Circuits for Hybrid QuantumClassical Algorithms Sukin Sim, Peter D. Johnson, Alan AspuruGuzik Performing useful computations with current and nearterm quantum computers is becoming increasingly viable due to rapid advances in both algorithms and hardware. A class of algorithms that are promising candidates for demonstrating the utility of nearterm quantum computers is the so called hybrid quantumclassical (HQC) algorithms. A common ingredient that plays a crucial role in the algorithmic performance of many HQC algorithms is the parametrized quantum circuit that is tuned to prepare quantum states relevant for (approximately) solving the problem of interest. Despite the importance of these circuits, they are often generated in the absence of a robust theoretical framework. In this work, we introduce several descriptors to characterize a set of parametrized circuits, including a measure of a circuit’s expressibility and how it correlates with algorithmic performance. Ultimately, having a deeper understanding of the qualities associated with an effective parametrized circuit can lead to significant improvements in the overall development of HQC algorithms. 
Tuesday, March 5, 2019 10:00AM  10:12AM 
E42.00011: Error mitigation by symmetry verification on a variational quantum eigensolver Ramiro Sagastizabal, Xavier BonetMonroig, Malay Singh, Thomas E O'Brien, Michiel Adriaan Rol, Cornelis Christiaan Bultink, Xiang Fu, Nandini Muthusubramanian, Alessandro Bruno, Leonardo DiCarlo Current efforts to increase the accuracy of quantum algorithms focus on the design and 
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