  In crystals, energy band extrema in momentum space can be identified by their
valley index. The internal quantum degree of freedom associated with valley
pseudospin indices can act as a useful information carrier analogous to
electronic charge or spin. Interest in valleytronics has been revived in recent
years following the discovery of atomically thin materials such as graphene and
transition metal dichalcogenides. However, the valley coherence time, a key
quantity for manipulating the valley pseudospin, has never been measured in any
material. In this work, we use a sequence of laser pulses to resonantly
generate a coherent superposition of excitons (Coulomb-bound electron-hole
pairs) in opposite valleys of monolayer WSe$_2$. The imposed valley coherence
persists for approximately one hundred femtoseconds. We propose that the
electron-hole exchange interaction provides an important decoherence mechanism
in addition to exciton population decay. Our work provides critical insight
into the requirements and strategies for optical manipulation of the valley
pseudospin for future valleytronics applications.
