  Condensation of bosons into a macroscopic quantum state belongs to the most
intriguing phenomena in nature. It was first realized in quantum gases of
ultra-cold atoms, but more recently became accessible in open-dissipative,
exciton-based solid-state systems at elevated temperatures. Semiconducting
monolayer crystals have emerged as a new platform for studies of strongly bound
excitons in ultimately thin materials. Here, we demonstrate the formation of a
bosonic condensate driven by excitons hosted in an atomically thin layer of
MoSe2, strongly coupled to light in a solid-state resonator. The structure is
operated in the regime of collective strong coupling, giving rise to hybrid
exciton-polariton modes composed of a Tamm-plasmon resonance, GaAs quantum well
excitons and two-dimensional excitons confined in a monolayer of MoSe2.
Polariton condensation in a monolayer crystal manifests by a superlinear
increase of emission intensity from the hybrid polariton mode at injection
powers as low as 4.8 pJ/pulse, as well as its density-dependent blueshift and a
dramatic collapse of the emission linewidth as a hallmark of temporal
coherence. Importantly, we observe a significant spin-polarization in the
injected polariton condensate, a fingerprint of the core property of monolayer
excitons subject to spin-valley locking. The observed effects clearly underpin
the perspective of building novel highly non-linear valleytronic devices based
on light-matter fluids, coherent bosonic light sources based on atomically thin
materials, and paves the way towards studying materials with unconventional
topological properties in the framework of bosonic condensation.
