Abstract Generic closed quantum many-body systems tend to thermalize, a phenomenon known as quantum ergodicity, where initial local information scrambles into the full space. A paradigmatic exception is many-body localization (MBL), where disorder prevents thermalization and breaks ergodicity strongly. While environmental dissipation is generally expected to delocalize such systems, this consequence has recently been challenged. Here we show that local dissipative operators can instead stabilize weak ergodicity breaking in systems with quantum many-body scars (QMBS). These operators drive the system into steady states dominated by scarred eigenstates. Furthermore, we show that this tailored dissipation induces robust dynamic revivals, even in the presence of local dephasing noise. We propose an experimental realization using a cold atomic setup. Our findings make ergodicity-breaking signatures directly observable in dissipative dynamics and open avenues for preparing long-lived QMBS states with coherent dynamics.
Abstract Generic closed quantum many-body systems tend to thermalize, a phenomenon known as quantum ergodicity, where initial local information scrambles into the full space. A paradigmatic exception is many-body localization (MBL), where disorder prevents thermalization and breaks ergodicity strongly. While environmental dissipation is generally expected to delocalize such systems, this consequence has recently been challenged. Here we show that local dissipative operators can instead stabilize weak ergodicity breaking in systems with quantum many-body scars (QMBS). These operators drive the system into steady states dominated by scarred eigenstates. Furthermore, we show that this tailored dissipation induces robust dynamic revivals, even in the presence of local dephasing noise. We propose an experimental realization using a cold atomic setup. Our findings make ergodicity-breaking signatures directly observable in dissipative dynamics and open avenues for preparing long-lived QMBS states with coherent dynamics.
A generic closed quantum many-body system will inevitably tend to thermalization, whose local information encoded in the initial state eventually scrambles into the full space, known as quantum ergodicity. A paradigmatic exception in closed quantum systems for strong ergodicity breaking is known as many-body localization, where strong disorder-induced localization prevents the occurrence of thermalization. It is generally recognized that a localized quantum system would be delocalized under dissipation induced by the environment. However, this consequence recently has received challenges where an exotic dissipation-induced localization mechanism is proposed, and transitions between localized and extended phases are found. In this Letter, we promote this mechanism to systems for weak ergodicity breaking hosting quantum many-body scars (QMBS). We find that the system relaxes to a steady state dominated by QMBS, and the dissipative dynamics exhibit dynamic revivals by suitably preparing an initial state. We point out an experimental realization of the controlled dissipation with a cold atomic setup. This makes the signature of ergodicity breaking visible over dissipative dynamics and offers potential possibilities for experimentally preparing stable QMBS with associated coherent dynamics.
The success of statistical mechanics in describing complex quantum systems rests upon typicality properties such as ergodicity. Both integrable systems and the recently discovered many-body localisation show that these assumptions can be strongly violated in either finely tuned cases, or in the presence of quenched disorder. In this thesis, we uncover a qualitatively different form of ergodicity breaking, wherein a small number of atypical eigenstates are embedded throughout an otherwise thermalising spectrum. We call this a many-body quantum scar, in analogy to quantum scars in single-particle quantum chaos, where quantum scarred eigenfunctions concentrate around associated periodic classical trajectories. \nWe demonstrate that many-body quantum scars can be found in an unusual model recently realised in a 51 Rydberg atom quantum simulator. The observed coherent oscillations following in a certain quench experiment are a consequence of the quantum scar. At the same time, the level statistics rules out conventional explanations such as integrability and many-body localisation. We develop an approximate method to construct scarred eigenstates, in order to describe their structure and physical properties. Additionally, we find a local perturbation which makes these non-equilibrium properties much more pronounced, with near perfect quantum revivals. At the same time the other eigenstates remain thermal. Our results suggest that many-body quantum scars forms a new class of quantum dynamics with unusual properties, which are realisable in current experiments.
Abstract Quantum many-body scars (QMBSs) are atypical eigenstates in otherwise nonintegrable systems that weakly break ergodicity, exhibiting nonthermal dynamics and long-lived periodic revivals. However, implementing QMBS in non-Hermitian systems remains a significant challenge, as it demands a deep understanding of the influence of exceptional points (EPs) on quantum many-body dynamics, especially higher-order EPs. Here, we use single-photon interference to demonstrate that experimentally controlled non-Hermitian dissipation stabilizes and enhances scarred dynamics, realizing nearly perfect, high-fidelity revival. These ideas can be exemplified in quantum systems that support scarred dynamics. In the non-Hermitian regime, symmetry-promoted exceptional points confine the dynamics to robust nonthermal subspaces, enabling high-fidelity scarred revivals over an extended parameter range. These results establish non-Hermitian quantum engineering as a versatile strategy for creating and sustaining perfect quantum scars, offering pathways for quantum-enhanced metrology, coherent state preparation, and fault-tolerant quantum information processing.
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 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 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.