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Journal Club for Condensed Matter Physics

Quantum scars and partial breaking of ergodicity

Published Apr 30, 2020• 0 citations
Publisher

Abstract

1. Probing many-body dynamics on a 51-atom quantum simulator Authors: Hannes Bernien, Sylvain Schwartz, Alexander Keesling, Harry Levin, Ahmed Omran, Hannes Pichler, Soonwon Choi, Alexander S. Zibrov, Manuel Endres, Markus Greiner, Vladan Vuletić and Mikhail D. Lukin Nature 551, 579-584 (2017); DOI: 10.1038/nature24622 arXiv:1707.04344 2. Weak ergodicity breaking from quantum many body scars Authors: C. […]

DOI
10.36471/jccm_april_2020_01

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  1. Probing many-body dynamics on a 51-atom quantum simulator Authors: Hannes Bernien, Sylvain Schwartz, Alexander Keesling, Harry Levin, Ahmed Omran, Hannes Pichler, Soonwon Choi, Alexander S. Zibrov, Manuel Endres, Markus Greiner, Vladan Vuletić and Mikhail D. Lukin Nature 551, 579-584 (2017); DOI: 10.1038/nature24622 arXiv:1707.04344 2. Weak ergodicity breaking from quantum many body scars Authors: C. […]
The recent discovery of persistent revivals in the Rydberg-atom quantum simulator has revealed a weakly ergodicity-breaking mechanism dubbed quantum many-body scars, which are a set of nonthermal states embedded in otherwise thermal spectra. Until now, such a mechanism has been mainly studied in Hermitian systems. Here, we establish the non-Hermitian quantum many-body scars and systematically characterize their nature from dynamic revivals, entanglement entropy, physical observables, and energy level statistics. Notably, we find the non-Hermitian quantum many-body scars exhibit significantly enhanced coherent revival dynamics when approaching the exceptional point. The signatures of non-Hermitian scars switch from the real-energy axis to the imaginary-energy axis after a real-to-complex spectrum transition driven by increasing non-Hermiticity, where an exceptional point and a quantum tricritical point emerge simultaneously. We further examine the stability of non-Hermitian quantum many-body scars against external fields, reveal the non-Hermitian quantum criticality and eventually set up the whole phase diagram. The possible connection to the open quantum many-body systems is also explored. Our findings offer insights for realizing long-lived coherent states in non-Hermitian many-body systems.
Quantum many-body scarring is a recently discovered weak ergodicity-breaking phenomenon in quantum many-body systems. It was first seen in experiments with Rydberg atom array simulators, and characterized by nonthermalizing periodic revivals of certain initial-state configurations. Here, the authors extend our understanding of the physical origins of this phenomenon by showing through an explicit construction that scars can arise through the presence of entangled virtual degrees of freedom undergoing periodic dynamics. This demonstrates a novel analytic scenario, whereby intrinsically entangled states can exhibit periodic revivals in an otherwise chaotic many-body system, enriching the phenomenology of quantum many-body scarring.
The recent discovery of persistent revivals in the Rydberg-atom quantum simulator has revealed a weakly ergodicity-breaking mechanism dubbed quantum many-body scars, which are a set of nonthermal states embedded in otherwise thermal spectra. Until now, such a mechanism has been mainly studied in Hermitian systems. Here, we establish the non-Hermitian quantum many-body scars and systematically characterize their nature from dynamic revivals, entanglement entropy, physical observables, and energy level statistics. Notably, we find the non-Hermitian quantum many-body scars exhibit significantly enhanced coherent revival dynamics when approaching the exceptional point. The signatures of non-Hermitian scars switch from the real-energy axis to the imaginary-energy axis after a real-to-complex spectrum transition driven by increasing non-Hermiticity, where an exceptional point and a quantum tricritical point emerge simultaneously. We further examine the stability of non-Hermitian quantum many-body scars against external fields, reveal the non-Hermitian quantum criticality and eventually set up the whole phase diagram. The possible connection to the open quantum many-body systems is also explored. Our findings offer insights for realizing long-lived coherent states in non-Hermitian many-body systems.
The discovery of quantum many-body scars (QMBS) both in Rydberg atom simulators and in the Affleck-Kennedy-Lieb-Tasaki spin-1 chain model, have shown that a weak violation of ergodicity can still lead to rich experimental and theoretical physics. In this review, we provide a pedagogical introduction to and an overview of the exact results on weak ergodicity breaking via QMBS in isolated quantum systems with the help of simple examples such as the fermionic Hubbard model. We also discuss various mechanisms and unifying formalisms that have been proposed to encompass the plethora of systems exhibiting QMBS. We cover examples of equally-spaced towers that lead to exact revivals for particular initial states, as well as isolated examples of QMBS. Finally, we review Hilbert space fragmentation, a related phenomenon where systems exhibit a richer variety of ergodic and non-ergodic behaviors, and discuss its connections to QMBS.