Observation of a non-Hermitian phase transition in an optical quantum gas

TL;DR

This paper reports the experimental observation of a non-Hermitian phase transition in an optical quantum gas, revealing a new dissipative phase characterized by biexponential decay, linked to an exceptional point, and distinct from traditional lasing.

Contribution

It demonstrates a non-Hermitian phase transition in a photon Bose-Einstein condensate, introducing a new dissipative phase with unique coherence decay properties, expanding understanding of dissipative quantum phases.

Findings

Observation of a biexponential decay in condensate coherence

Identification of an exceptional point in the quantum gas

Distinct separation from lasing and oscillatory regimes

Abstract

Quantum gases of light, as photons or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from e.g. cavity loss. This makes them a tool to study dissipative phases, an emerging subject in quantum manybody physics. Here we experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a new dissipative phase, characterized by a biexponential decay of the condensate's second-order coherence. The phase transition occurs due to the emergence of an exceptional point in the quantum gas. While Bose-Einstein condensation is usually connected to ordinary lasing by a smooth crossover, the observed phase transition separates the novel, biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our findings pave the way for studies of a wide class of dissipative quantum phases, for instance in topological or lattice systems.