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arXiv:2604.12296 · Nature Communications

Dynamical signatures of conventional and asymptotic quantum many-body scars on a trapped ion simulator

Leonard Logarić, John Goold, Shane Dooley

quant-phSubmitted Apr 14, 2026• Announced Apr 15, 2026• 0 citations
arXivHTMLPDF

Abstract

One of the promising applications of digital quantum processors is the simulation of many-body quantum systems. They have been already used to investigate several ergodicity violating mechanisms, which were initially discovered in synthetic quantum matter, such as many-body localisation, Hilbert space fragmentation and quantum many-body scars (QMBS). In addition to conventional QMBS, a recently discovered mechanism for ergodicity violation are the so-called asymptotic quantum many-body scars (AQMBS). These become more stable as system size is increased, leading to progressively longer thermalisation timescales. In this work, we show a connection between gapless excitations and AQMBS in certain qudit-based models. We then consider a 2-local model, hosting both conventional and asymptotic scars, in which the AQMBS states are gapless excitations of a ground state localisation transition. By exploiting the structure of the found AQMBS states and the all-to-all connectivity of the Quantinuum H1-1 quantum processor, we prepare these states in logarithmic circuit depth, and probe their thermalisation behaviour under Floquet dynamics. Performing simulations on up to N = 20 qubits and up to 418 entangling ZZ gates, we find slower thermalisation times as the system size is increased, providing the first experimental signatures of asymptotic scars.

DOI
10.1038/s41467-026-77411-5
Comments
7 + 17 pages; 2 + 7 figures; Comments welcome!

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One of the promising applications of digital quantum processors is the simulation of many-body quantum systems. They have been already used to investigate several ergodicity violating mechanisms, which were initially discovered in synthetic quantum matter, such as many-body localisation, Hilbert space fragmentation and quantum many-body scars (QMBS). In addition to conventional QMBS, a recently discovered mechanism for ergodicity violation are the so-called asymptotic quantum many-body scars (AQMBS). These become more stable as system size is increased, leading to progressively longer thermalisation timescales. In this work, we show a connection between gapless excitations and AQMBS in certain qudit-based models. We then consider a 2-local model, hosting both conventional and asymptotic scars, in which the AQMBS states are gapless excitations of a ground state localisation transition. By exploiting the structure of the found AQMBS states and the all-to-all connectivity of the Quantinuum H1-1 quantum processor, we prepare these states in logarithmic circuit depth, and probe their thermalisation behaviour under Floquet dynamics. Performing simulations on up to N = 20 qubits and up to 418 entangling ZZ gates, we find slower thermalisation times as the system size is increased, providing the first experimental signatures of asymptotic scars.
Many-body localisation, Hilbert space fragmentation and quantum many-body scars (QMBS) are well-established mechanisms of ergodicity breaking in quantum systems. Recently, asymptotic quantum many-body scars (AQMBS), non-thermal states whose stability grows with system size, have been proposed theoretically and probed experimentally. Here, we show that AQMBS arise naturally as remnants of certain quantum phase transitions. For a broad class of frustration-free Hamiltonians, we prove that their gapless excitations survive deformation into a scarred Hamiltonian as states whose energy variance is bounded by the excitation gap. Consequently, the gapless excitations at a quantum critical point become AQMBS of the scarred Hamiltonian. Their fidelity relaxation time is then set by the critical scaling of the gap, determined by the dynamic critical exponent. Modelling the critical Fredkin and Motzkin chains and the two-dimensional PVBS model, we obtain AQMBS whose variances directly reflect the dynamic exponents of the underlying transitions. Where the gap of the Motzkin chain closes super-exponentially, we find exponentially stable AQMBS, for which we also observe numerical evidence of volume-law entanglement.
We report, to our knowledge, the first observation of asymptotic quantum many-body scars (AQMBS) on a superconducting processor. Implementing the periodically-driven PVBS model of Logarić, Goold, and Dooley at g = 1 as a digital Floquet brickwork circuit on IBM's 156-qubit ibm_fez, we prepare the scar-adjacent state |A₁⟩ on N = 8, 12, 16, 20 qubits and measure the normalized magnetization M_Z(t)/M_Z(0) over ten Floquet steps. The decay slows dramatically with system size — the defining AQMBS signature — with pairwise size-gap significances of 7.5σ, 4.1σ, and 5.8σ at t = 10 (the last from a pre-registered combined analysis of two independent runs), against a pre-registered 3σ criterion per gap. The thermalizing (|1⟩^⊗N) control behaves as predicted at every size; the stationary (|V⟩ = |0⟩^⊗N) control is flat across N but decays further than first modeled — a miscalibrated bar documented and corrected openly in §3. AQMBS were previously observed only once, on a trapped-ion simulator; no superconducting-qubit realization existed in the literature. All three hardware runs were pre-registered before execution, with frozen success criteria; two initial runs missed the bar (2.2σ and 2.98σ on the tightest gap) and were reported as misses before a powered top-up run closed the detection.
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.
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 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.
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.