Observation of a Non-Hermitian Phase Transition in an Optical Quantum Gas

Quantum gases of light, such as photon or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from, for example, cavity loss. This makes them valuable tools to create and understand phases of systems, which are dissipatively coupled to the environment. Bose-Einstein condensates of photons, realized in dye-filled microcavities, provide a platform to study the dynamics emerging in an open, grand canonical situation where the condensate particles are coupled to reservoirs. The steady-state particle flux from the pump beam through the condensate and out to the environment induces a novel behavior of the particle number fluctuations.

We experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a dissipative dynamical phase characterized by a biexponential decay of the condensate’s second-order coherence. The stochastic driving induced by the grand canonical condensate number fluctuations makes the system dynamics observable in stationary-state operation. In contrast to closed systems, the dissipative coupling to the environment is described by a non-Hermitian time-evolution operator with complex eigenvalues. The phase transition occurs at an exceptional point of the quantum gas dynamics, separating the biexponential dynamical phase from both lasing and an intermediate, oscillatory condensate regime. Our approach opens ways for studies of new dissipative phases in lattice systems.