  Emitter dephasing is one of the key issues in the performance of solid-state
single photon sources. Among the various sources of dephasing, acoustic phonons
play a central role in adding decoherence to the single photon emission. Here,
we demonstrate, that it is possible to tune and engineer the coherence of
photons emitted from a single WSe$_2$ monolayer quantum dot via selectively
coupling it to a spectral cavity resonance. We utilize an open cavity to
demonstrate spectral enhancement, leveling, and suppression of the highly
asymmetric phonon sideband, finding excellent agreement with a microscopic
description of the exciton-phonon dephasing in a truly two-dimensional system.
Moreover, the impact of cavity tuning on the dephasing is directly assessed via
optical interferometry, which points out the capability to utilize light-matter
coupling to steer and design dephasing and coherence of quantum emitters in
atomically thin crystals.
