  Resonant optical excitation of few-level quantum systems enables coherent
quantum control, resonance fluorescence, and direct characterization of
dephasing mechanisms. Experimental demonstrations have been achieved in a
variety of atomic and solid-state systems. An alternative but intriguing
quantum photonic platform is based on single layer transition metal
chalcogenide semiconductors, which exhibit a direct band-gap with optically
addressable exciton valley-pseudospins in a uniquely two-dimensional form. Here
we perform resonance and near-resonance excitation of three-dimensionally
confined excitons in monolayer WSe$_2$ to reveal near ideal single photon
fluorescence with count rates up to 3 MHz and uncover a weakly-fluorescent
exciton state ~ 5 meV blue-shifted from the ground-state exciton. We perform
high-resolution photoluminescence excitation spectroscopy of the localized
excitons, providing important information to unravel the precise nature of the
quantum states. Successful demonstration of resonance fluorescence paves the
way to probe the localized exciton coherence. Moreover, these results yield a
route for investigations of the spin and valley coherence of confined excitons
in two-dimensional semiconductors.
