Non-thermal quantum phase transitions

TL;DR

This paper introduces a novel type of quantum phase transition occurring in isolated systems with non-thermal states and an additional symmetry, characterized by a critical energy and observable symmetry-breaking behavior.

Contribution

It identifies and characterizes non-thermal quantum phase transitions driven by work in isolated systems with symmetry, a phenomenon not observed in thermal equilibrium.

Findings

Critical energy separates symmetry-broken and symmetric phases.

Transition occurs when sufficient work is performed on the system.

Numerical validation in three two-level systems.

Abstract

We report a kind of quantum phase transition which takes place in isolated quantum systems with non-thermal equilibrium states and an extra symmetry that commutes with the Hamiltonian for any values of the system parameters. A critical energy separates two different phases, one in which the symmetry is broken. This critical behavior is ruled out as soon as the system is put in contact with a thermal bath. The critical point is crossed when a sufficent amount of work is performed on the system, keeping it isolated from the environment. Different phases are identified by means of an order parameter, which is only different from zero in the symmetry-breaking phase. The behavior of the system near the critical point is determined by a set of critical exponents. We illustrate this phenomenon by means of numerical calculations in three different two-level systems.