  The emergence of spatial and temporal coherence in optical light-fields
emitted from solid-state systems is a fundamental phenomenon, rooting in a
plethora of microscopic processes and it is intrinsically aligned with the
control of light-matter coupling. Optical coherence is canonical for lasing
systems. It also emerges in the superradiance of multiple, phase-locked
emitters. More recently, coherence and long-range order have been investigated
in bosonic condensates of thermalized light, as well as in exciton-polaritons
driven to a ground state via stimulated scattering. Here, we experimentally
show that the interaction between cavity photons with an atomically thin layer
of material is sufficient to create strong light-matter coupling and coherent
light emission at ambient conditions. Our experiments are conducted on a
microcavity embedding a monolayer of WSe$_2$. We measure spatial and temporal
coherence of the emitted light. The coherence build-up is accompanied by a
threshold-like behaviour of the emitted light intensity, which is a fingerprint
of a polariton laser effect. Our findings are of high application relevance, as
they confirm the possibility to use atomically thin crystals as simple and
versatile components of coherent light-sources at room temperature.
