Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session K38: Quantum Annealing and Optimization IIIFocus Recordings Available
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Sponsoring Units: DQI Chair: Ryan Levi, UIUC Room: McCormick Place W-195 |
Tuesday, March 15, 2022 3:00PM - 3:12PM |
K38.00001: Improving Quantum Annealing through microcanonical thermalization: a one-dimensional study Gianni Mossi, Eliot Kapit, Zhijie Tang, Vadim Oganesyan Random-Field Quantum Annealing (RFQA) is a recently-proposed analog protocol where a quantum system is coherently driven by oscillating an extensive number of single-spin operators at independent low frequencies, with the goal of quickly populating both energy levels involved in an avoided crossing ("microcanonical thermalization") so as to counter the exponential ground-state depletion encountered by standard QA in first-order phase transitions. |
Tuesday, March 15, 2022 3:12PM - 3:24PM |
K38.00002: Fully solvable example of an all-to-all coupling implemented in a local architecture Evgeny Mozgunov Both quantum simulation and quantum optimization tasks benefit greatly from the increase in connectivity of the underlying hardware graph. A complete graph, which we refer to as all-to-all coupling, is traditionally embedded via ferromagnetic chains. The length of these chains has to be proportional to the system size in any planar local architecture. Flipping a single chain by tunneling induced by a local transverse field takes time exponential in the chain length, thus also in the system size. This is known to slow down quantum optimization, making the scaling the time to solution of the embedded problems much worse than the corresponding scaling of the classical algorithms working with the complete graph directly. It also impedes quantum simulation by exponentially reducing the range of available effective transverse fields. We propose a way to improve from exponential to polynomial in both of those cases. This is achieved by replacing ferromagnetic chains by chains close to the critical point of the transverse field Ising model. The specific schedules for such embedding have at least 4 groups of terms with different dependences on the anneal parameter, compared to 2 in the traditional annealing. Next generation annealers are planned to be capable of implementing such schedules. |
Tuesday, March 15, 2022 3:24PM - 3:36PM |
K38.00003: Hardware-Efficient Quantum Optimization Layered Algorithms and Experiments Davide Venturelli, M. Sohaib Alam, Matthew J Reagor, Bram Evert, Shon Grabbe, Benjamin P Hall, Mark Hodson, Ryan M LaRose, P. Aaron Lott, Eleanor G Rieffel, James Sud, Zhihui Wang, Filip A Wudarski Quantum optimization algorithms, such as QAOA, that implement parametrized stochastic optimization solvers attempt to identify low-energy solutions of Ising systems by exploiting available quantum effects in noisy-intermediate scale machines. Engineering a well-performing parametrized quantum optimization circuit is indeed an exercise in balancing the trade-off between expressivity and implementation complexity. We show that, for MaxCut QAOA circuits defined on native hardware topology (Rigetti’s Aspen Quantum Processors), error-mitigation techniques recover simulated features of the noiseless theory. Moreover, we explore a design space for QAOA-like ansatze that perform well in theory as well as in hardware for fully-connected problems [1]. We also discuss how efficient coherence and entanglement detection methods that could be coupled with quantum optimization experiments require only linear overhead in benchmarking time [2]. |
Tuesday, March 15, 2022 3:36PM - 4:12PM |
K38.00004: Implementation of a 25-Qubit System for Analog Quantum Computing Invited Speaker: Steven M Disseler Quantum annealing, and more generalized extensions, have long been studied as a potential route to achieving quantum speedup, particularly in classes of problems relating to optimization and sampling. Experimental advancements in this area, however, have been limited in part by the small number of available systems which allow exploration of novel algorithms and control paradigms. In particular, though there exist commercial systems with several thousand annealing qubits, the control mechanisms used in these systems necessitate a restricted set of annealing schedules. |
Tuesday, March 15, 2022 4:12PM - 4:24PM |
K38.00005: Computational phase transition in Quantum Approximate Optimization Algorithm -- the difference between hard and easy Bingzhi Zhang, Akira N Sone, Quntao Zhuang Quantum Approximate Optimization algorithm (QAOA) is one of the candidates to achieve a near-term quantum advantage. As QAOA seems only capable of solving optimization problems, there is a folklore that QAOA cannot see the difference between easy problems such as 2-SAT and hard problems such as 3-SAT—although 2-SAT is in the polynomial-time (P) class, its optimization version is also nondeterministic polynomial-time (NP)-hard. In this paper, we show that the folklore is not true. We find a computational phase transition in QAOA when solving a variant of 3-SAT— the amplitude of gradient and the success probability achieve their minimum at the well-known SAT-UNSAT phase transition. On the contrary, for 2-SAT, such a phenomenon is absent at SATUNSAT phase transition and the success probability is unity for a reasonable circuit depth. We connect the gradient transition to the dynamical Lie algebra of the QAOA circuit. In solving the NP-hard optimization versions of SAT, we identify quantum advantages over a classical approximate algorithm at quite a shallow depth of p = 4 for the problem size of n = 10. |
Tuesday, March 15, 2022 4:24PM - 4:36PM |
K38.00006: The power of density: Suppression of quantum phase transitions via dense driver Hamiltonians in adiabatic quantum computation Matthias Werner, Marta P Estarellas, Artur Garcia-Saez In the context of adiabatic quantum computation (AQC), it has been argued that first order quantum phase transitions (QPTs) due to localisation phenomena will always cause adiabatic quantum computation (AQC) to fail by exponentially decreasing the minimal spectral gap of the Hamiltonian along the annealing path. However this notion has been subject to some debate in the community, since more recent findings suggest the existence of methods to avoid this by carefully designing the involved Hamiltonians. It remains a challenge to formulate a comprehensive theory on the effect of the various parameters and the conditions under which QPTs make the AQC algorithm fail. In this work we investigate the conditions under which localisation causes first order QPTs. As a consequence of this analysis and using methods from spectral graph theory, we here examine both analytically and numerically the role of the connectivity of the driver Hamiltonian in the mitigation of such effects in different AQC algorithms and show that in the limiting case of full connectivity, first order QPTs due to localisation are avoided entirely. |
Tuesday, March 15, 2022 4:36PM - 4:48PM |
K38.00007: Dissipative Landau Zener transition beyond the weak coupling limit with a superconducting flux qubit Xi Dai, Robbyn Trappen, Huo Chen, Rui Yang, Denis Melanson, Antonio J Martinez, Muhammet Ali Yurtalan, Yongchao Tang, Rabindra Das, David K Kim, Alexander Melville, Bethany M Niedzielski, Cyrus F Hirjibehedin, Kyle Serniak, Steven J Weber, Jonilyn L Yoder, William D Oliver, Evgeny Mozgunov, Daniel A Lidar, Adrian Lupascu The Landau-Zener problem for a two-level system is a suitable toy problem for studying quantum tunnelling in an annealer. Coupling to the environment can influence the tunnelling probability and theoretical understanding is only available for specific coupling limits or noise models. We present experimental results on Landau-Zener measurements on a capacitively-shunted flux qubit. The result shows crossover from weak to strong coupling to the environment, consistent with different master equations in different limits. The result gives insight into the scaling of tunnelling probability in a large-scale quantum annealer. |
Tuesday, March 15, 2022 4:48PM - 5:00PM |
K38.00008: Effects of XX-Catalysts on Annealing Spectra with a Scalable Weighted Maximum-Independent-SetProblem Natasha J Feinstein, Louis Fry-Bouriaux, Sougato Bose, P. A Warburton Quantum annealing is an algorithm designed to solve optimisation problems by initialising a system in the ground state of a simple driver Hamiltonian and evolving to a Hamiltonian whose ground state encodes the solution to the problem. To prevent excitations, the evolution time must scale inversely with the square of the minimum gap between the instantaneous ground and first excited states - which generally closes exponentially with the problem size. A promising solution is the introduction of a catalyst Hamiltonian that modifies the evolution, either softening the scaling of the minimum gap, or producing a spectrum that enables diabatic annealing. To controllably manipulate annealing spectra, it is vital to study the impact of catalyst Hamiltonians in various settings. Using the weighted MIS problem, we examine how catalysts containing carefully chosen XX-couplings can be introduced to predictably alter the annealing spectrum. We show how, depending on the properties of the original spectrum, the same catalyst will either increase the gap or introduce an additional, tuneable, small gap. This additional gap shows potential for exploitation in diabatic annealing protocols. |
Tuesday, March 15, 2022 5:00PM - 5:12PM |
K38.00009: Evidence of coherent quantum annealing Andrew D King, Daniel A Lidar, Mohammad Amin, Hidetoshi Nishimori, Sei Suzuki, Jack Raymond, Alex Zucca, Richard G Harris, Trevor Lanting, Andrew J Berkley, Fabio Altomare Recent experiments have demonstrated the ability to simulate programmable quantum spin systems at finite temperature using quantum annealers. In this work we examine the 1D quantum Ising chain, probing anneal times down to the 10 nanosecond range. For fast anneals we see signatures of closed-system quantum annealing: In large systems we see quantum Kibble-Zurek scaling, with theoretically predicted kink statistics up to the third cumulant, independent of system temperature. In small systems we see Landau-Zener scaling characteristic of an excitation mechanism dominated by a single anticrossing. |
Tuesday, March 15, 2022 5:12PM - 5:24PM |
K38.00010: Error Suppression in Continuous-time Quantum Computing Jemma Bennett, Viv Kendon, Nicholas Chancellor, Adam Callison, Mia West, Tom O'Leary In the quantum optimisation setting, we build on [Young et al., PRA 88,062314, 2013] where logical qubits in multiple copies, constituting an Ising spin system, are linked together to increase the logical system's robustness to error. We introduce several refinements that improve the scheme significantly. First we note that only one copy needs to be correct by the end of computation, since solution quality can be checked efficiently. Second, we find that ferromagnetic links do not help in the "one correct copy" situation, but low strength anti-ferromagnetic links do help sometimes. Third, we developed a protocol based on local field and coupling strengths in the problem Hamiltonian, to decide whether logical qubits should be connected anti-ferromagnetically, or left disconnected. We have tested our method on small instances of spin glasses from [Callison et al., NJP 21, 123022, 2019] and similar small spin chain instances, and we find improved error tolerance for three or more copies in configurations that include frustration. |
Tuesday, March 15, 2022 5:24PM - 5:36PM |
K38.00011: Many-body localization enables iterative quantum optimization Hanteng Wang, Hsiu-Chung Yeh, Alex Kamenev We suggest an iterative quantum protocol, allowing to solve optimization problems with a glassy energy landscape. It is based on a periodic cycling around the tricritical point of the many-body localization transition. This ensures that each iteration leads to a non-exponentially small probability to find a lower local energy minimum. The other key ingredient is to tailor the cycle parameters to a currently achieved optimal state (the "reference" state) and to reset them once a deeper minimum is found. We show that, if the position of the tricritical point is known, the algorithm allows to approach the absolute minimum with any given precision in a polynomial time. |
Tuesday, March 15, 2022 5:36PM - 5:48PM |
K38.00012: Testing Hardness in the Transverse-Field Ising Model using the Perturbed Ferromagnetic Chain Daniel T O'Connor, Louis Fry-Bouriaux, Paul A Warburton Probing the role of coherence in quantum annealing relies on the use of “toy” Hamiltonians, whereby the distribution of computational states at the end of an anneal depends on the degree of coherence in the experimental implementation. Increasing the hardness generally relies on increasing the system size, and therefore the solution cannot be computed after the problem exceeds a few tens of qubits. |
Tuesday, March 15, 2022 5:48PM - 6:00PM |
K38.00013: Efficient criteria of quantumness for a large system of qubits Shohei Watabe, Michael Serikow, Shiro Kawabata, Alexandre M Zagoskin In order to model and evaluate large-scale quantum systems, e.g. quantum computer and quantum annealer, it is necessary to quantify the "quantumness" of such systems. In this talk, I will discuss the dimensionless combinations of basic parameters of large, partially quantum coherent systems, which could be used to characterize their degree of quantumness. Based on model analytical and numerical calculations, we suggest one such number for a system of qubits undergoing adiabatic evolution. Applying it to the case of D-Wave One superconducting quantum annealing device, we find that its operation as described falls well within the quantum domain. |
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