Anomalous spin transport in integrable random quantum circuits

TL;DR

This paper demonstrates that integrability in quantum spin circuits can be maintained even without time-translation symmetry, leading to diverse spin transport behaviors and enabling a generalized hydrodynamics framework for analysis.

Contribution

It constructs and analyzes integrable random quantum circuits with inhomogeneous R-matrices, revealing persistent integrability and complex transport regimes despite broken time-translation symmetry.

Findings

Identifies ballistic, superdiffusive, and diffusive transport regimes controlled by R-matrix parameters.

Develops a generalized hydrodynamics framework for time-dependent integrable circuits.

Numerically confirms the theory with large-scale simulations of spin correlations.

Abstract

High-temperature spin transport in integrable quantum spin chains exhibits a rich dynamical phase diagram, including ballistic, superdiffusive, and diffusive regimes. While integrability is known to survive in static and periodically driven systems, its fate in the complete absence of time-translation symmetry, particularly in interacting random quantum circuits, has remained unclear. Here we construct integrable random quantum circuits built from inhomogeneous XXZ R-matrices. Remarkably, integrability is preserved for arbitrary sequences of gate layers, ranging from quasiperiodic to fully random, thereby explicitly breaking both continuous and discrete time-translation symmetry. Using large-scale time-dependent density-matrix renormalization group simulations at infinite temperature and half filling, we map out the resulting spin-transport phase diagram and identify ballistic, superdiffusive, and diffusive regimes controlled by the spectral parameters of the R-matrices. The spatiotemporal structure of spin correlations within each regime depends sensitively on the inhomogeneity, exhibiting spatial asymmetry and sharp peak structures tied to near-degenerate quasiparticle velocities. To account for these findings, we develop a generalized hydrodynamics framework adapted to time-dependent integrable circuits, yielding Euler-scale predictions for correlation functions, Drude weights, and diffusion bounds. This approach identifies the quasiparticles governing transport and quantitatively captures both the scaling exponents and fine structures of the correlation profiles observed numerically. Our results demonstrate that exact Yang-Baxter integrability is compatible with stochastic quantum dynamics and establish generalized hydrodynamics as a predictive framework for transport in time-dependent integrable systems.