# Dissipation driven quantum phase transitions and symmetry breaking

## TL;DR

This paper explores a solvable driven-dissipative quantum model showing that continuous phase transitions can occur without spontaneous symmetry breaking, driven by the interplay of Hamiltonian and dissipative dynamics, differing from equilibrium transitions.

## Contribution

It introduces a model demonstrating second order phase transitions without symmetry breaking, highlighting a new mechanism distinct from equilibrium phase transitions.

## Key findings

- Transitions are non-analytic in the steady state in the thermodynamic limit.
- Local observables are continuous with discontinuous first derivatives.
- Mean-field results do not fully capture the transition dynamics.

## Abstract

By considering a solvable driven-dissipative quantum model, we demonstrate that continuous second order phase transitions in dissipative systems may occur without an accompanying spontaneous symmetry breaking. As such, the underlying mechanism for this type of transition is qualitatively different from that of continuous equilibrium phase transitions. In our model, the transition is solely a result of the interplay between Hamiltonian and dissipative dynamics and manifests as a non-analyticity in the steady state $\hat\rho_\mathrm{ss}$ in the thermodynamic limit. Expectations of local observables are continuous but typically with discontinuous first order derivatives in agreement with second order phase transitions. While the model, a large number of driven two-level systems under collective dissipation, is in some sense fully connected, mean-field results are incapable of capturing the full picture.