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Routledge eBooks

Quantum Scarring

Paul Thomas

Published Oct 20, 2025• 0 citations
Publisher

Abstract

Quantum scarring, or many-body scarring, first captured the attention of physicists in 1984. The evocative name was coined by physicist Eric Heller in 1984, based on his interpretation of scientific data visualisation. This phenomenon carries a strange ephemerality—a conundrum that reveals a poetic jitterbug dance of atoms. Interest in quantum scarring has resurfaced again today. In a recent experiment, researchers took 51 atoms at different energy states and held them in place with a laser beam. When released, the 51 atoms began to thermalise, entering a chaotic state, but then—unexpectedly—retraced their chaotic steps, repeatedly returning to their original order. This hesitation or stuttering lasts only for a femtosecond before they resume their chaotic journey. Like most quantum phenomena, the repetition is inexplicable: we can observe the effects, but the cause remains elusive.

DOI
10.4324/9781003683285-12

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Quantum scarring, or many-body scarring, first captured the attention of physicists in 1984. The evocative name was coined by physicist Eric Heller in 1984, based on his interpretation of scientific data visualisation. This phenomenon carries a strange ephemerality—a conundrum that reveals a poetic jitterbug dance of atoms. Interest in quantum scarring has resurfaced again today. In a recent experiment, researchers took 51 atoms at different energy states and held them in place with a laser beam. When released, the 51 atoms began to thermalise, entering a chaotic state, but then—unexpectedly—retraced their chaotic steps, repeatedly returning to their original order. This hesitation or stuttering lasts only for a femtosecond before they resume their chaotic journey. Like most quantum phenomena, the repetition is inexplicable: we can observe the effects, but the cause remains elusive.
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.
According to quantum statistical mechanics, generic many-body systems decohere and thermalize regardless of their initial state. A recent experiment found a surprising exception to this rule, wherein an otherwise thermalizing system maintains its coherence only for a particular initial condition, a phenomenon dubbed ``quantum many-body scarring.'' Here, the authors propose and provide numerical evidence for a physical mechanism underlying the observed phenomena: Bose-Einstein condensation, wherein macroscopic numbers of quasiparticles occupy a single quantum state. While this phenomenon is well known at equilibrium, its emergence in a strongly interacting, far-from-equilibrium system is striking and unexpected. The physical principle explored in this work can be applied to other systems to reveal new examples of quantum many-body scars.
We introduce the concept of ergodicity and explore its deviation caused by quantum scars in an isolated quantum system, employing a pedagogical approach based on a toy model. Quantum scars, originally identified as traces of classically unstable orbits in certain wavefunctions of chaotic systems, have recently regained interest for their role in non-ergodic dynamics, as they retain memory of their initial states. We elucidate these features of quantum scars within the same framework of this toy model. The integrable part of the model consists of two large spins, with a classical counterpart, which we combine with a random matrix to induce ergodic behavior. Scarred states can be selectively generated from the integrable spin Hamiltonian by protecting them from the ergodic states using a projector method. Deformed projectors mimic the ‘quantum leakage’ of scarred states, enabling tunable mixing with ergodic states and thereby controlling the degree of scarring. In this simple model, we investigate various properties of quantum scarring and shed light on different aspects of many-body quantum scars observed in more complex quantum systems. Notably, the underlying classicality can be revealed through the entanglement spectrum and the dynamics of ‘out-of-time-ordered correlators’.
Quantum many-body scars (QMBSs) represent a weak ergodicity-breaking phenomenon that defies the common scenario of thermalization in closed quantum systems. They are often regarded as a many-body analog of quantum scars (QSs)—a single-particle phenomenon in quantum chaos—due to their superficial similarities. However, unlike QSs, a clear connection between QMBSs and classical chaos has remained elusive. It has nevertheless been speculated that in an appropriate semiclassical limit, QMBSs should have a correspondence to weakly unstable periodic orbits. In this paper, I present a counterexample to this conjecture by studying a bosonic model with a large number of flavors. The dynamics of out-of-time-ordered correlators (OTOCs) suggest that QMBS do not display chaotic behavior in the semiclassical limit. In contrast, chaotic dynamics are expected for initial states not associated with QMBS. Interestingly, the anomalous OTOC dynamics persist even under weak perturbations that eliminate the scarred eigenstates, suggesting a certain robustness in the phenomenon.