Quantum Fragility Spotted in 14-Atom Chain Could Unlock Better Quantum Tech

Scientists are increasingly interested in understanding how fragile conservation laws impact quantum systems. Cheng Chen (Institute of Physics, Chinese Academy of Sciences), Luca Capizzi and Alice Marché (Université Paris-Saclay, CNRS, Laboratoire de Physique Théorique et Modèles Statistiques) et al. demonstrate that even tiny disturbances can dramatically change the behaviour of these laws, leaving a detectable trace in quantum spin chains. Their research, utilising a system of just 14 Rydberg atoms, reveals that weak disruptions from interatomic interactions cause anomalous growth in magnetization fluctuations, a phenomenon they both observe experimentally and replicate through numerical simulations. This work establishes a new method for probing fragile conservation laws and highlights Rydberg-atom arrays as a promising platform for testing the dynamics of weakly integrable systems.

Experimental observation of integrability breaking in a Rydberg atom spin chain reveals novel many-body dynamics

Researchers have demonstrated the detection of weakly broken conservation laws within a quantum spin chain comprised of just 14 Rydberg atoms. This breakthrough establishes a clear experimental fingerprint of integrability breaking, a phenomenon where even minimal disturbances can drastically alter a system’s behaviour.
The work centres on a one-dimensional chain of Rydberg atoms, meticulously controlled to model a quantum spin chain with integrable characteristics perturbed by weak, longer-range dipolar couplings. These couplings, though subtle, are directly detectable within experimentally achievable timescales through the observation of non-local observables.

Specifically, the study reveals that magnetization fluctuations are exceptionally sensitive to the disruption of fragile conservation laws, exhibiting anomalous growth not predicted by strictly integrable models. This anomalous growth was observed experimentally, alongside similar signatures in a semilocal string observable, confirming the theoretical predictions.

Numerical simulations, performed on significantly larger chains, and a simplified classical stochastic model corroborate these findings, strengthening the evidence for the impact of weak integrability breaking. The research establishes non-local observables as a powerful tool for probing fragile conservation laws in quantum spin chains.

Furthermore, Rydberg-atom arrays are positioned as a promising platform for testing perturbative descriptions of quantum many-body dynamics when faced with weak integrability breaking. By studying the dynamics of an initial inhomogeneous domain-wall state, where half the atoms are polarized in one direction and half in the opposite, the team observed how the system deviates from the behaviour expected of a perfectly integrable system. This deviation manifests as a departure from ballistic transport, hinting at the emergence of more complex dynamics.

Domain-wall dynamics and magnetization propagation in a Rydberg atom chain are actively researched topics

A 14-atom Rydberg array serves as the experimental platform for this work, enabling the investigation of integrability breaking in a one-dimensional spin chain. Researchers fabricated this array and initialized it in a domain-wall state, preparing a configuration with differing magnetization on either side of a central junction.

Dynamics were then induced using a time-evolving Hamiltonian comprising both nearest-neighbor interactions (Hnn) and a perturbative next-nearest-neighbor term (Hnnn), representing weak integrability breaking. Magnetization profiles, specifically the z-component of magnetization σz j, were measured as a function of position and time to observe the propagation of the initial magnetic order.

To mitigate experimental errors, data post-selection enforced total magnetization conservation, discarding measurement outcomes deviating from the initial value. Numerical simulations, performed on longer 48-spin chains using matrix-product-states implemented within the ITensors Julia package, corroborated the experimental findings.

These simulations restricted evolution to Jtf = 12 to minimize finite-size effects and allowed for a detailed comparison with the observed dynamics. The research team rescaled the magnetization data using variables (j −1/2)/(Jt)1/z, where z represents the dynamical exponent, to characterize the transport regime.

Figures were generated displaying space-time maps of the magnetization, alongside ballistic (z=1) and diffusive (z=2) rescalings, revealing a sharpening increase in slope around the center of the magnetization profile. This emergent diffusive broadening is attributed to the Hnnn term, which breaks fragile conservation laws within the system, and suggests a tendency towards a piecewise-constant plateau. The study establishes non-local observables as a sensitive probe of these fragile conservation laws and demonstrates the Rydberg-atom array’s capability to test perturbative descriptions of dynamics with weak integrability breaking.

Integrability breakdown evidenced by anomalous fluctuations in Rydberg atom chains suggests emergent many-body behavior

Researchers demonstrate that a one-dimensional spin chain comprising 14 Rydberg atoms exhibits detectable integrability breaking due to weak dipolar couplings. Magnetization fluctuations, quantified as variance, reveal anomalous growth, indicating sensitivity to the disruption of fragile conservation laws.

A semilocal string observable also displays similar signatures, confirming the impact of these subtle interactions. Numerical simulations, performed on chains of 48 spins and extending to a time of 12Jt, reproduce the observed features, validating the experimental findings. The experimental setup utilizes 87Rb atoms trapped in optical tweezers with a uniform interatomic spacing of 10.8μm.

Encoding a pseudo-spin 1/2 using the |60S1/2, mJ = 1/2⟩ and |60P1/2, mJ = −1/2⟩ Rydberg states allows for the creation of a dipolar XX Hamiltonian with an interaction strength of 2π × 1.2MHz. Applying a magnetic field of approximately 45 G ensures isotropic interactions within the system. Initializing the atoms in a magnetic domain-wall state |ΨDW⟩, the study measures the evolution of magnetization σz j governed by the XX Hamiltonian.

Under the integrable nearest-neighbor Hamiltonian, the initial domain-wall state relaxes ballistically, with net magnetization transfer growing linearly with time. The local magnetization develops a light-cone structure, representing the coherent melting of magnetic order. Simulations on longer chains reveal that the addition of the next-to-nearest-neighbor dipole-dipole term, Hnnn, generates interactions between quasiparticles, breaking the infinite set of local conservation laws present in the integrable model.

This perturbation, with a prefactor of 1/8 at a distance of 2a, establishes a controlled regime of weak integrability breaking. Analysis of the magnetization profile, both numerically and experimentally, demonstrates a deviation from the ballistic scaling expected for an integrable system. The data, when rescaled, shows a clear departure from the predicted arcsin(j/(2Jt)) behavior, indicating the influence of the Hnnn perturbation. This work establishes non-local observables as a sensitive probe of fragile conservation laws in spin chains and Rydberg-atom arrays as a platform for testing perturbative descriptions of dynamics with weak integrability breaking.

Rydberg atom arrays reveal anomalous magnetization growth linked to integrability breaking in driven, disordered systems

Researchers have demonstrated the experimental detection of weak integrability breaking in a one-dimensional spin chain comprised of Rydberg atoms. The study reveals that even subtle perturbations, stemming from interatomic dipolar couplings, can significantly alter the system’s dynamics and leave a discernible signature in non-local observables.

Specifically, the variance of subsystem magnetization exhibits anomalous growth, indicative of the disruption of fragile conservation laws, and this behaviour has been observed and corroborated by numerical simulations. This finding establishes Rydberg atom arrays as a promising platform for investigating perturbative descriptions of dynamics with weak integrability breaking, and highlights the sensitivity of non-local observables as probes of fragile conservation laws in spin chains.

The observed anomalous growth in magnetization fluctuations provides a direct link between theoretical predictions of integrability breaking and experimental measurements. Finite size effects were identified as the dominant limitation, with numerical simulations closely matching experimental data obtained from a 14-atom chain.

The authors acknowledge that diffusive broadening in their experimental results is likely due to imperfections in the setup, and that their observations are limited to a specific system size. Future research could explore the dynamics in longer chains to better understand the crossover between initial quadratic growth and slower trends in the variance of magnetization. Further investigation into the behaviour of semilocal string observables could also provide additional insights into the nature of integrability breaking and the associated conservation laws.