Coherence of a non-equilibrium polariton condensate across the interaction-mediated phase transition

The emergence of spatial coherence in a confined two-dimensional Bose gas of exciton polaritons with tuneable interactions offers a unique opportunity to explore the role of interactions in a phase transition in a driven-dissipative quantum system, where both the phase transition and thermalisation are mediated by interactions. We investigate, experimentally and numerically, the spatial coherence and steady-state properties of the gas over a wide range of interaction strengths by varying the photonic/excitonic fraction of the polaritons and their density. We find that the first order spatial correlation function exhibits algebraic decay consistent with the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. The exponent of the algebraic decay is inversely proportional to the coherent fraction of polaritons, in analogy to superfluid fraction of equilibrium quantum gases above the BKT transition, but with a different proportionality constant. Our work paves the way for future investigations of the phenomenon of phase transitions and superfluidity in a driven-dissipative setting.