  Single quantum emitters (SQEs) are at the heart of quantum optics and
photonic quantum information technologies. To date, all demonstrated
solid-state single-photon sources are confined in three-dimensional materials.
Here, we report a new class of SQEs based on excitons that are spatially
localized by defects in two-dimensional tungsten-diselenide monolayers. The
optical emission from these SQEs shows narrow linewidths of ~0.13 meV, about
two orders of magnitude smaller than that of delocalized valley excitons.
Second-order correlation measurements reveal strong photon anti-bunching,
unambiguously establishing the single photon nature of the emission. The SQE
emission shows two non-degenerate transitions, which are cross-linearly
polarized. We assign this fine structure to two excitonic eigen-modes whose
degeneracy is lifted by a large ~0.71 meV coupling, likely due to the
electron-hole exchange interaction in presence of anisotropy. Magneto-optical
measurements also reveal an exciton g-factor of ~8.7, several times larger than
that of delocalized valley excitons. In addition to their fundamental
importance, establishing new SQEs in 2D quantum materials could give rise to
practical advantages in quantum information processing, such as efficient
photon extraction and high integratability and scalability.
