  Monolayers of transition metal dichalcogenide materials emerged as a new
material class to study excitonic effects in solid state, since they benefit
from enormous coulomb correlations between electrons and holes. Especially in
WSe2, sharp emission features have been observed at cryogenic temperatures,
which act as single photon sources . Tight exciton localization has been
assumed to induce an anharmonic excitation spectrum, however, the evidence of
the hypothesis, namely the demonstration of a localized biexciton, is elusive.
Here, we unambiguously demonstrate the existence of a localized biexciton in a
monolayer of WSe2, which triggers an emission cascade of single photons. The
biexciton is identified by its time-resolved photoluminescence, superlinearity
and distinct polarization in micro-photoluminescence experiments. We evidence
the cascaded nature of the emission process in a cross-correlation experiment,
which yields a strong bunching behavior. Our work paves the way to a new
generation of quantum optics experiments with two-dimensional semiconductors.
