Non-Hermitian phase transition from a polariton Bose-Einstein condensate to a photon laser

TL;DR

This paper introduces a non-Hermitian driven-dissipative phase transition mechanism in polariton systems, revealing a first-order-like transition between polariton condensates and photon lasers, challenging traditional interpretations of experimental thresholds.

Contribution

It uncovers a novel non-Hermitian phase transition in polariton condensates, identifying exceptional points as critical markers, and broadens understanding of driven-dissipative condensate behavior beyond population inversion.

Findings

Exceptional point marks the phase boundary endpoint.

First-order-like phase transition between polariton states.

Second threshold not necessarily due to population inversion.

Abstract

We propose a novel mechanism for a nonequilibrium phase transition in a $U(1)$-broken phase of an electron-hole-photon system, from a Bose-Einstein condensate of polaritons to a photon laser, induced by the non-Hermitian nature of the condensate. We show that a (uniform) steady state of the condensate can always be classified into two types, namely, arising either from lower or upper-branch polaritons. We prove (for a general model) and demonstrate (for a particular model of polaritons) that an exceptional point where the two types coalesce marks the endpoint of a first-order-like phase boundary between the two types, similar to a critical point in a liquid-gas phase transition. Since the phase transition found in this paper is not in general triggered by population inversion, our result implies that the second threshold observed in experiments is not necessarily a strong-to-weak-coupling transition, contrary to the widely-believed understanding. Although our calculation mainly aims to clarify polariton physics, our discussion is applicable to general driven-dissipative condensates composed of two complex fields.