Exceptional Dynamical Quantum Phase Transitions in Periodically Driven Systems

TL;DR

This paper reveals that spontaneous symmetry breaking and universal dynamical quantum phase transitions can occur in short-time regimes of periodically driven systems, linked to nonunitary physics and exceptional points.

Contribution

It introduces the concept of dynamical quantum phase transitions driven by antiunitary symmetry breaking in short-time regimes, using space-time duality and nonunitary operators.

Findings

Existence of distinct dynamical phases in periodically driven systems.

Unconventional singularities in dynamical free energy.

Detection of phases via quasilocal operators.

Abstract

Extending notions of phase transitions to nonequilibrium realm is a fundamental problem for statistical mechanics. While it was discovered that critical transitions occur even for transient states before relaxation as the singularity of a dynamical version of free energy, their nature is yet to be elusive. Here, we show that spontaneous symmetry breaking can occur at a short-time regime and causes universal dynamical quantum phase transitions in periodically driven unitary dynamics. Unlike conventional phase transitions, the relevant symmetry is antiunitary: its breaking is accompanied by a many-body exceptional point of a nonunitary operator obtained by space-time duality. Using a stroboscopic Ising model, we demonstrate the existence of distinct phases and unconventional singularity of dynamical free energy, whose signature can be accessed through quasilocal operators. Our results open up research for hitherto unknown phases in short-time regimes, where time serves as another pivotal parameter, with their hidden connection to nonunitary physics.