Open-system dynamics and fluctuation-dissipation relation in a photon Bose-Einstein condensate*

The tunable openness of optical quantum gases, such as photon or polariton condensates in optical microcavities, enables the exploration of new system states and phases, which would not be accessible under closed system conditions. Here, we experimentally demonstrate a non-Hermitian phase transition in a photon Bose-Einstein condensate in an open dye-filled microcavity. The transition separates a phase of biexponential photon number correlations from both lasing and an intermediate, oscillatory regime, as characterised by the second-order correlation dynamics of the BEC [1]. By studying the magnitude of the condensate number fluctuations and relating them to a response function, we verify a fluctuation-dissipation relation for a BEC coupled to a molecular reservoir [2]. In more recent work, we have extended these studies to the time domain, establishing a connection between the fluctuation dynamics and the response of the condensate population to an external pulse-like perturbation of the molecular reservoir.