Activity-induced phase transition in a quantum many-body system

Abstract

A crowd of nonequilibrium entities can show phase transition behaviours that are prohibited in conventional equilibrium setups. One of the simplest and well-studied phenomena observed in such active matter is the motility-induced phase separation (MIPS), where self-propelled particles spontaneously aggregate. An interesting question is whether a similar activity-driven phase transition occurs in a pure quantum system. Here we introduce a non-Hermitian quantum many-body model that undergoes quantum phase transitions, which includes the analogue of the classical MIPS within its parameter space. The phase diagram, which can be experimentally tested in an open quantum system, indicates that the addition of spin-dependent asymmetric hopping to a simple model of hard-core bosons is sufficient to induce flocking as well as phase separation. Moreover, we find that the quantum phase transitions in the model are equivalent to the transitions of dynamical paths in classical kinetics upon the application of biasing fields. This example demonstrates that quantum systems can indeed show activity-induced phase transitions, and sheds light on the rich connection between classical nonequilibrium kinetics and non-Hermitian quantum physics.