  Solid-state single photon sources are central building blocks in quantum
communication networks and on-chip quantum information processing. Atomically
thin crystals were established as possible candidates to emit non-classical
states of light, however, the performance of monolayer-based single photon
sources has so far been lacking behind state-of-the-art devices based on volume
crystals. Here, we implement a single photon source based on an atomically thin
sheet of WSe2 coupled to a spectrally tunable optical cavity. It is
characterized by a high single photon purity with a $g^{(2)}(0)$ value as low
as $4.7 \pm 0.7 \%$ and a record-high first lens brightness of linearly
polarized photons as large as $65 \pm 4 \%$. Interestingly, the high
performance of our devices allows us to observe genuine quantum interference
phenomena in a Hong-Ou-Mandel experiment. Our results demonstrate that open
cavities and two-dimensional materials constitute an excellent platform for
ultra-bright quantum light sources: the unique properties of such
two-dimensional materials and the versatility of open cavities open an
inspiring avenue for novel quantum optoelectronic devices.
