Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G40: Noisy Intermediate Scale Quantum Computers VFocus Recordings Available

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Sponsoring Units: DQI DCOMP Chair: Peter Johnson, Zapata Computing Room: McCormick Place W196B 
Tuesday, March 15, 2022 11:30AM  11:42AM 
G40.00001: NISQ Algorithm for Semidefinite Programming Kishor Bharti, Tobias Haug, Vlatko Vedral, LeongChuan Kwek Semidefinite programs (SDPs) are convex optimization programs with vast applications in control theory, quantum information and combinatorial optimization. Noisy intermediatescale quantum (NISQ) algorithms aim to efficiently use the current generation of quantum hardware. However, optimizing variational quantum algorithms is a challenge as it is a NPhard problem that in general requires an exponential time to solve and can contain many far from optimal local minima. 
Tuesday, March 15, 2022 11:42AM  11:54AM 
G40.00002: Testing Quantum Mechanics using Noisy Quantum Computers Kevin Slagle We outline a proposal to test quantum mechanics using noisy intermediatescale quantum (NISQ) devices in the highcomplexity quantum advantage regime. We are motivated by the possibility that quantum mechanics is not fundamental, but instead emerges from a theory with less computational power, such as classical mechanics. We show that our proposal can significantly rule out this possibility with 2000 logical qubits and a modest gate infidelity of 10^5, although useful experiments can already be conducted with e.g. 80 qubits and gate infidelity 10^3. Our procedure involves simulating a nonClifford random circuit, followed by its inverse, and then checking that the resulting state is the same as the initial state. We show that quantum mechanics predicts that the fidelity of this procedure decays exponentially with circuit depth (due to noise in NISQ computers). However, if quantum mechanics emerges from a theory with significantly less computational power, then we expect the fidelity to decay significantly more rapidly than the quantum mechanics prediction for sufficiently deep circuits, which is the experimental signature that we propose to search for. 
Tuesday, March 15, 2022 11:54AM  12:06PM 
G40.00003: Detecting the footprint of central charge using IBM quantum simulator Nazli U Koyluoglu, Khadijeh Najafi, Sarah Mostame Phase transitions are ubiquitous phenomena in physics. Physical systems close to phase transition are known to become scaleinvariant and manifest universality. In two dimensions, there is an infinitedimensional algebra of local conformal transformations which can be solved exactly and the corresponding field theory description is known as conformal field theory (CFT). While the central charge is one of the characterizations of the underlying symmetry of the corresponding CFT's, they have not been experimentally detected. In this work, we measure the central charge associated with CFT corresponding to various spin chains such as the transversefield Ising model and XXZ chain by preparing the IBM quantum system. We show that at and close to the critical point, Shannon entropy of measurable quantities such as formation probabilities manifest a footprint of central charge allowing us to extract the central charge. Leveraging on the power of variational algorithms such as Variational Quantum Eigensolver (VQE) and AdiabaticallyAssisted Variational Quantum Eigensolver (AAVQE), we prepare the ground state of the transversefield Ising model with periodic and open boundary conditions, at and near the critical point and perform our measurement for various system sizes. 
Tuesday, March 15, 2022 12:06PM  12:18PM 
G40.00004: Using NISQ Computing Devices to Simulate, Optimize, and Design NearTerm Quantum Communication Networks Brian Doolittle, Eric A Chitambar, Thomas Bromley, Nathan Killoran Computing the dynamics of quantum manybodied systems is a key challenge in developing applications for future quantum communication networks. Quantum computers show promise in their ability to efficiently simulate comlex quantum networks. We develop a hybrid quantumclassical computing framework that uses NISQ computing devices to simulate, optimize, and design nearterm quantum communication networks. We implement our framework in a publicly available python library that uses the PennyLane quantum machine learning framework to intergrate quantum computing APIs with machine learning libraries. We demonstrate our hybrid computing framework's ability to simulate and optimize small quantum networks and analyze its performance on both quantum hardware and classical simulator. For small networks with fewer than 20 qubits, we find that classical simulation is most efficent. For larger networks, we discuss how parallelization across many NISQ computing devices can yield efficient optimization and simulation. Finally, we explore how this software can be used to help design quantum internet applications. 
Tuesday, March 15, 2022 12:18PM  12:30PM 
G40.00005: Performing catbasis quantum logic on modulated coherent state pulses using a photonic NISQera processor Jack T Postlewaite, Brajesh K Gupt, Saikat Guha, Kaushik Seshadreesan Universal quantumlogic manipulation of modulated quantum light underlies several quantumenhanced classical communications and sensing applications. In quantumlimited optical communications with binary phase shift keyed (BPSK) coherent states, attaining quantumenhanced superadditive capacity hinges on the ability to perform quantum operations in the approximate qubit subspace spanned by cat states associated with the BPSK states. Such logic capability would enable the implementation of structured quantum joint detection receivers, e.g., based on the recently proposed belief propagation with quantum messages (BPQM) algorithm that can in principle discriminate codeword blocks of coherent state pulses quantum optimally. We consider alternative approaches to realizing photonic cat basis logic using linear optics, ancilla cat states, and photon number and homodyne measurements that have been previously proposed. We analyze the gate fidelities as a function of singlemode squeezing required in Gaussian boson samplingtype circuits that we use to generate the ancilla cat states involved in the gates and report thresholds on the squeezing levels necessary to retain quantum enhancement in discriminating communication codewords of small example codes using the BPQM receiver. 
Tuesday, March 15, 2022 12:30PM  12:42PM 
G40.00006: Realizing FractionalQuantumHall Gravitons on Quantum Computers Ammar Kirmani, Kieran Bull, ChangYu Hu, Zlatko Papic, Armin Rahmani, Pouyan Ghaemi

Tuesday, March 15, 2022 12:42PM  1:18PM 
G40.00007: Quantum simulation of quantum field theory in the lightfront formulation Invited Speaker: Peter J Love Quantum field theory is a natural target for simulation on future quantum computers. We will discuss such simulations in the lightfront formulation of quantum field theory. This formulation has several appealing features for such simulations foremost of which are its similarities to quantum chemistry where much effort has already been expended in developing and optimizing quantum simulation techiniques. In previous work (2002.04016, 2105.10941), we demonstrated this by developing quantum algorithms based on simulating time evolution and adiabatic state preparation. These algorithms are suitable for future largescale fault tolerant quantum computers. We also explain how to formulate the relativistic bound state problem as an instance of the Variational Quantum Eigensolver (VQE) algorithm using the Basis LightFront Quantization (BLFQ) technique (2011.13443, 2009.07885). The BLFQ formulation provides an ideal framework for benchmarking NISQ devices and testing existing algorithms on physically relevant problems such as the calculation of hadronic spectra and parton distribution functions. 
Tuesday, March 15, 2022 1:18PM  1:30PM 
G40.00008: Playing quantum nonlocal games with six noisy qubits on the cloud Emanuele G Dalla Torre, Daniel Azses, Meron Sheffer Nonlocal games are extensions of Bell inequalities, aimed at demonstrating quantum advantage. These games are well suited for noisy quantum computers because they only require the preparation of a shallow circuit, followed by the measurement of noncommuting observable. Here, we consider the minimal implementation of the nonlocal game proposed in Science 362, 308 (2018). We test this game by preparing a 6qubit cluster state using quantum computers on the cloud by IBM, Ionq, and Honeywell. Our approach includes several levels of optimization, such as circuit identities and error mitigation and allows us to cross the classical threshold and demonstrate quantum advantage in one quantum computer. We conclude by introducing a different inequality that allows us to observe quantum advantage in less accurate quantum computers, at the expense of probing a larger number of circuits. 
Tuesday, March 15, 2022 1:30PM  1:42PM Withdrawn 
G40.00009: Variational quantum algorithm with information sharing Chris N Self, Kiran E Khosla, Alistair W Smith, Frédéric Sauvage, Peter D Haynes, Johannes Knolle, Florian Mintert, Myungshik Kim We introduce an optimisation method for variational quantum algorithms and experimentally demonstrate a 100fold improvement in efficiency compared to naive implementations. The effectiveness of our approach is shown by obtaining multidimensional energy surfaces for small molecules and a spin model. Our method solves related variational problems in parallel by exploiting the global nature of Bayesian optimisation and sharing information between different optimisers. Parallelisation makes our method ideally suited to the next generation of variational problems with many physical degrees of freedom. This addresses a key challenge in scalingup quantum algorithms towards demonstrating quantum advantage for problems of realworld interest. 
Tuesday, March 15, 2022 1:42PM  1:54PM 
G40.00010: NoiseAssisted Variational Quantum Thermalization Jonathan Foldager, Arthur Pesah, Lars K Hansen Preparing thermal states on a quantum computer can have a variety of applications, from simulating manybody quantum systems to training machine learning models. Variational circuits have been proposed for this task on nearterm quantum computers, but several challenges remain such as finding a scalable costfunction, avoiding the need of purification, and mitigating noise effects. We propose a new algorithm for thermal state preparation that tackles those three challenges by exploiting the noise of quantum circuits. We consider a variational architecture containing a depolarizing channel after each gate, with the ability to directly control the level of noise. We derive a closedform approximation for the freeenergy of such circuit and use it as cost function for our variational algorithm. For a variety of Hamiltonians and system sizes, we show that the ability for our algorithm to learn the thermal state strongly depends on the temperature: while a high fidelity can be obtained for high and low temperatures, we identify a specific range of temperatures for which the problem becomes harder. We hope that this first study on noiseassisted thermal state preparation will spark interest in future research on exploiting noise in variational algorithms. 
Tuesday, March 15, 2022 1:54PM  2:06PM 
G40.00011: Quantum simulation of the spinboson model with noisy gatebased quantum computer Juha Leppäkangas, Kirsten Bark, Michael Marthaler, JanMichael Reiner We consider noisy gatebased quantum computers for the purpose of simulating the spinboson model. We establish a bosonic bath by an ensemble of qubits with finite coherence times. The energylevel broadening of qubits is mapped to broadening of the simulated bath spectral density. We study how desired forms of the spectral density can be constructed by optimizing simulated spinbath couplings and bath energies. We study the effect of different gate decompositions and system connectivity on the quality of the mapping to the desired form. In the ideal situation, the spinbath couplings can be decomposed using only variable angle twoqubit gates, such as a variable MølmerSørensen gate. In other cases, qubit noise can get mapped to twobody noise in the simulated spinbath system, which does not have exact correspondence in the original spinboson formulation. We show a numeric comparison of the quality of the mapping for various decompositions. Furthermore we compare the full inclusion of the twobody noise terms with an approximate mapping of the effects on the spectral density of the simulated spinboson problem. 
Tuesday, March 15, 2022 2:06PM  2:18PM 
G40.00012: Demonstrating scalable synthesis of multiqubit gates Ji Chu, Fei Yan, Xiaoyu He, Xiaoming Sun Multiqubit entangling gates are essential in many quantum applications. Although arbitrary quantum gates can be decomposed into sequences of universal single and twoqubit gates, the huge cost in circuit depth obstructs any meaningful operation within limited coherence time. We propose a scalable method for lowdepth synthesis of multiqubit controlled gates, based on a hardwareefficient construction of quantum AND logic. We successfully demonstrate Toffoli gates with up to 8 qubits on a fewIO, lowcrosstalk superconducting quantum processor. We also test our gates in Grover's search algorithm with multiple amplification cycles. 
Tuesday, March 15, 2022 2:18PM  2:30PM 
G40.00013: Classical Variational Optimization of Gate Sequences for Time Evolution of Translational Invariant Systems Refik Mansuroglu, Samuel A Wilkinson, Michael J Hartmann, Timo Eckstein, Ludwig Nützel The simulation of time evolution of large quantum systems is a classically challenging and often intractable task, making it a promising application for quantum computation. A TrotterSuzuki approximation yields an implementation thereof, where a certain desired accuracy can be achieved by raising the gate count adequately. In this work, we introduce a variational algorithm which uses solutions of classical optimizations to predict efficient quantum circuits for time evolution of translational invariant quantum systems. Our strategy improves on the TrotterSuzuki ansatz in accuracy by several orders of magnitude. Alternatively, one can trade the accuracy gain against a reduction of gate count. This is important in NISQapplications where the fidelity of the output state decays exponentially with the number of gates. We also propose an extension of our strategy to construct algorithms for the evolution of open boundary systems with translation symmetry in the bulk. 
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