  While valleys (energy extrema) are present in all band structures of solids,
their preeminent role in determining exciton resonances and dynamics in
atomically thin transition metal dichalcogenides (TMDC) is unique. Using
two-dimensional coherent electronic spectroscopy, we find that exciton
decoherence occurs on a much faster time scale in MoSe$_2$ bilayers than that
in the monolayers. We further identify two population relaxation channels in
the bilayer, a coherent and an incoherent one. Our microscopic model reveals
that phonon-emission processes facilitate scattering events from the $K$ valley
to other lower energy $\Gamma$ and $\Lambda$ valleys in the bilayer. Our
combined experimental and theoretical studies unequivocally establish different
microscopic mechanisms that determine exciton quantum dynamics in TMDC
monolayers and bilayers. Understanding exciton quantum dynamics provides
critical guidance to manipulation of spin/valley degrees of freedom in TMDC
bilayers.
