  Bosonic condensation and lasing of exciton-polaritons in microcavities is a
fascinating solid-state phenomenon. It provides a versatile platform to study
out-of-equilibrium many-body physics and has recently appeared at the forefront
of quantum technologies. Here, we study the photon statistics via the
second-order temporal correlation function of polariton lasing emerging from an
optical microcavity integrated with an atomically thin MoSe2 crystal.
Furthermore, we investigate the macroscopic polariton phase transition for
varying excitation powers and temperatures. The lower-polariton exhibits photon
bunching below the threshold, implying a dominant thermal distribution of the
emission, while above the threshold, the second-order correlation transits
towards unity, which evidences the formation of a coherent state. Our findings
are in agreement with a microscopic numerical model, which explicitly includes
scattering with phonons on the quantum level.
