  Semiconducting transition metal dichalcogenide monolayers have emerged as
promising candidates for future valleytronics-based quantum information
technologies. Two distinct momentum-states of tightly-bound electron-hole pairs
in these materials can be deterministically initialized via irradiation with
circularly polarized light. Here, we investigate the ultrafast dynamics of such
a valley polarization in monolayer tungsten diselenide by means of
time-resolved Kerr reflectometry. The observed Kerr signal in our sample stems
exclusively from charge-neutral excitons. Our findings support the picture of a
fast decay of the valley polarization of bright excitons due to radiative
recombination, intra-conduction-band spin-flip transitions,
intervalley-scattering processes, and the formation of long-lived
valley-polarized dark states.
