Hilbert space fragmentation and slow dynamics in particle-conserving quantum East models

TL;DR

This paper introduces a new class of particle-conserving quantum models exhibiting Hilbert space fragmentation, leading to slow dynamics and non-thermal eigenstates, revealing novel universality classes in constrained quantum systems.

Contribution

The authors present a novel family of kinetically constrained models with particle conservation and inversion symmetry breaking, demonstrating quantum Hilbert space fragmentation and its effects on dynamics.

Findings

Eigenstates with zero entanglement entropy across bipartitions.

Faster-than-diffusive particle spreading at high densities.

Logarithmic relaxation dynamics at lower densities.

Abstract

Quantum kinetically constrained models have recently attracted significant attention due to their anomalous dynamics and thermalization. In this work, we introduce a hitherto unexplored family of kinetically constrained models featuring a conserved particle number and strong inversion-symmetry breaking due to facilitated hopping. We demonstrate that these models provide a generic example of so-called quantum Hilbert space fragmentation, that is manifested in disconnected sectors in the Hilbert space that are not apparent in the computational basis. Quantum Hilbert space fragmentation leads to an exponential in system size number of eigenstates with exactly zero entanglement entropy across several bipartite cuts. These eigenstates can be probed dynamically using quenches from simple initial product states. In addition, we study the particle spreading under unitary dynamics launched from the domain wall state, and find faster than diffusive dynamics at high particle densities, that crosses over into logarithmically slow relaxation at smaller densities. Using a classically simulable cellular automaton, we reproduce the logarithmic dynamics observed in the quantum case. Our work suggests that particle conserving constrained models with inversion symmetry breaking realize so far unexplored universality classes of dynamics and invite their further theoretical and experimental studies.