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.
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.
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 a pre-registered observation of quantum many-body scar revivals in the one-dimensional PXP model, executed as Trotterized digital quantum simulation on a 156-qubit superconducting processor (IBM ibm_fez). Preparing the Neel-ordered product states |Z2>, |Z3>, and |Z4> on N=12 qubits with periodic boundary conditions and evolving under a first-order Trotter decomposition of the PXP Hamiltonian, we measure the return probability P(t) at five times up to t = 4.8. The scarred state |Z2> shows a return probability of 0.0710 ± 0.0041 at t = 4.8, while the thermalizing control |Z4>, evolved through an identical-depth circuit, shows 0.0008 ± 0.0004. The differential signal constitutes a 17.2-sigma observation of selective revival, exceeding our pre-registered 3-sigma criterion. Because both states traverse identical circuit depths, hardware noise affects them equally and the difference isolates the physical scar signature.
Construction of asymptotic quantum many-body scar states in the SU(N) Hubbard model
We construct asymptotic quantum many-body scars (AQMBS) in one-dimensional SU(N) Hubbard chains (N≥3) by embedding the scar subspace into an auxiliary Hilbert subspace HP and identifying a parent Hamiltonian within it, together with a corresponding extension of the restricted spectrum-generating algebra to the multi-ladder case. Unlike previous applications of the parent-Hamiltonian scheme, we show that the parent Hamiltonian becomes the SU(N) ferromagnetic Heisenberg model rather than the spin-1/2 case, so that its gapless magnons realize explicit AQMBS of the original model. Working in the doublon-holon subspace, we derive this mapping, obtain the one-magnon dispersion for periodic and open boundaries, and prove (i) orthogonality to the scar states, (ii) vanishing energy variance in the thermodynamic limit, and (iii) subvolume entanglement entropy with rigorous MPS/MPO bounds. Our results broaden the parent-Hamiltonian family for AQMBS beyond spin-1/2 and provide analytic, low-entanglement excitations in SU(N)-symmetric systems.
Quantum many-body scars are energy eigenstates which fail to reproduce thermal expectation values of local observables, in systems where the rest of the many-body spectrum fulfils eigenstate thermalization. Experimental observation of quantum many-body scars has so far been limited to models with multiple scar states evenly spaced in energy. It is thus an interesting question whether even single isolated scars, which theoretically embody the weakest possibile violation of eigenstate thermalization and may be thought to have no detectable impact in experiments, can leave a trace in measurable quantities. Moreover, single scars offer an interesting scenario for exploring the connection between quantum many-body scars and the original notion of scarring in quantum dynamical systems theory. Here we propose protocols to observe single scars in architectures of fixed-frequency, fixed-coupling superconducting qubits. We first adapt known models possessing the desired features into a form particularly suited for the experimental platform. We develop protocols for the implementation of these models, through trotterized sequences of two-qubit cross-resonance interactions, and verify the existence of the approximate scar state in the stroboscopic effective Hamiltonian. Since a single scar cannot be detected from coherent revivals in the dynamics, differently from towers of scar states, we propose and numerically investigate alternative and experimentally-accessible signatures. These include the dynamical response of the scar to local state deformations, to controlled noise, and to the resolution of the Lie-Suzuki-Trotter digitization.
Periodically driven interacting quantum many-body systems can exhibit long-lived prethermal dynamics, where local observables retain coherent structure even as entanglement and operator complexity grow. Accessing this regime at the system sizes and times needed to determine physical properties of the prethermal state remains a central challenge: state-of-the-art classical methods become unreliable, while noise in quantum hardware degrades observable expectation values. Here we overcome these limitations for a Floquet Ising magnet realized on a heavy-hex lattice. Using the advanced error mitigation software QESEM on an IBM Heron r3 superconducting quantum processor, we measure magnetization dynamics with percent-level precision and resolve long-lived subharmonic prethermal oscillations in systems of up to 74 qubits. These experiments reach regimes for which leading tensor-network simulations fail to converge, while sparse Pauli-path simulations remain strongly truncation dependent despite extensive computations on advanced GPUs and the Fugaku supercomputer. Leveraging this quantum-accessible regime, we extend finite-size scaling to larger systems and find an unexpectedly slow decrease of the oscillation amplitude with system size, providing strong evidence that this oscillatory response persists in the thermodynamic limit of heavy-hex ladders. A hierarchy of mitigation and validation tests, including unbiased error mitigation, agreement between independent mitigation estimators, noise-model validation on the superconducting hardware, and cross-platform corroboration at selected Floquet cycles on Quantinuum System Model H2 and Quantinuum Helios trapped-ion hardware, supports the reliability of these findings. Our work establishes error-mitigated quantum processors as quantitative scientific instruments for discovering new physics in non-equilibrium quantum matter.