  The emergence of spatial and temporal coherence of light emitted from
solid-state systems is a fundamental phenomenon, rooting in a plethora of
microscopic processes. It is intrinsically aligned with the control of
light-matter coupling, and canonical for laser oscillation. However, it also
emerges in the superradiance of multiple, phase-locked emitters, and 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 photons in a Fabry-Perot microcavity and excitons in an
atomically thin WSe$_2$ layer is sufficient such that the system enters the
hybridized regime of strong light-matter coupling at ambient conditions. Via
Michelson interferometry, we capture clear evidence of increased spatial and
temporal coherence of the emitted light from the spatially confined system
ground-state. 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. Valley-physics is manifested in the presence of an external
magnetic field, which allows us to manipulate K and K' polaritons via the
Valley-Zeeman-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, and in valleytronic applications at room
temperature.
