Discovering novel quantum dynamics with NISQ simulators

Abstract

Major technological advances of the past century are rooted in our understanding of quantum physics in the non-interacting limit. A central challenge today is to understand the behavior of complex quantum many-body systems, where interactions play an essential role. About four decades ago, Richard Feynman proposed using controllable quantum systems to efficiently simulate complex physics and chemistry problems, envisioning quantum orreries, highly tunable quantum devices built to emulate less understood quantum systems. Here we ask whether quantum simulators have already uncovered new physical phenomena-and, if so, in which areas and with what impact. We find that, in several notable instances, they have advanced our understanding of many-body quantum dynamics. Although many of these insights could in principle have been obtained theoretically or numerically, they were nevertheless first achieved using quantum processors. While a broad landscape of problems beyond non-equilibrium dynamics still awaits exploration, it is encouraging that quantum simulators are already beginning to challenge and refine our conventional wisdom.

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• Figure 1: Instances of NISQ discoveries.(A) A 1D chain of 9 Rb atoms is initialized in a spatially alternating pattern of low and high energy Rydberg states bernien2017probing. Single-atom excitation probabilities show an oscillatory pattern of entanglement and disentanglement, rather than monotonically equilibrating. (B) In a ring of 24 superconducting qubits and for a certain parameter range, the system is integrable, and initially adjacent excitations (microwave photons) stay together at all times morvan2022formation (left, green interference pattern). Introducing an integrabilty-breaking perturbation is anticipated to break these bound states; however, even for a sizable perturbations the photons stay together, in contrast to a simple application of Fermi-golden rule. (C) Two-point correlation measurement in a chain of 11 $^{171}$Yb ions Richerme, which simulates long-range XY model, shows faster-than-linear (white line) growth of the light-cone boundary. (D) Using a a chain of 50 $^{87}$Rb atoms, Wei et al.wei2022quantum measured the spin states of a Heisenberg chain (top) by removing one spin species (center) and imaged the atomic site occupation (bottom), and computed the probability distribution of magnetic domain-wall relaxation across a 1D chain's center. The first, second (not shown) and third moment (filled circle markers) of transferred magnetization are in the KPZ universality class. Using a chain of 46 superconducting qubits (square markers), it was found that skewness and kurtosis (fourth moments) increasingly tend toward zero vs. time as the initial states are taken closer to infinite temperature Eliott2024. (E) Direct measurement of spin diffusion coefficient of the half-filled Fermi-Hubbard system nichols2019spin. (F) Fluorescence image of the average density of spin polarization is used to extract resistivity data brown2019bad. Initially the system is in thermal equilibrium with a spatially modulated density. Immediately after a sinusoidal potential is turned off, the system is no longer in equilibrium but the density has not yet changed. The rate of relaxation toward equilibrium enables extraction of resistivity.
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