Monolayer transition metal dichalcogenides feature Coulomb-bound
electron-hole pairs (excitons) with exceptionally large binding energy and
coupled spin and valley degrees of freedom. These unique attributes have been
leveraged for electrical and optical control of excitons for atomically-thin
optoelectronics and valleytronics. The development of such technologies relies
on understanding and quantifying the fundamental properties of the exciton. A
key parameter is the intrinsic exciton homogeneous linewidth, which reflects
irreversible quantum dissipation arising from system (exciton) and bath (vacuum
and other quasiparticles) interactions. Using optical coherent two-dimensional
spectroscopy, we provide the first experimental determination of the exciton
homogeneous linewidth in monolayer transition metal dichalcogenides,
specifically tungsten diselenide (WSe2). The role of exciton-exciton and
exciton-phonon interactions in quantum decoherence is revealed through
excitation density and temperature dependent linewidth measurements. The
residual homogeneous linewidth extrapolated to zero density and temperature is
~1.5 meV, placing a lower bound of approximately 0.2 ps on the exciton
radiative lifetime. The exciton quantum decoherence mechanisms presented in
this work are expected to be ubiquitous in atomically-thin semiconductors
