  The coherent manipulation of spin and pseudospin underlies existing and
emerging quantum technologies, including NMR, quantum communication, and
quantum computation. Valley polarization, associated with the occupancy of
degenerate, but quantum mechanically distinct valleys in momentum space,
closely resembles spin polarization and has been proposed as a pseudospin
carrier for the future quantum electronics. Valley exciton polarization has
been created in the transition metal dichalcogenide (TMDC) monolayers using
excitation by circularly polarized light and has been detected both optically
and electrically. In addition, the existence of coherence in the valley
pseudospin has been identified experimentally. The manipulation of such valley
coherence has, however, remained out of reach. Here we demonstrate an
all-optical control of the valley coherence by means of the pseudomagnetic
field associated with the optical Stark effect. Using below-bandgap circularly
polarized light, we experimentally rotate the valley exciton pseudospin in
monolayer WSe2 on the femtosecond time scale. Both the direction and speed of
the rotation can be optically manipulated by tuning the dynamic phase of
excitons in opposite valleys. This study completes the
generation-manipulation-detection paradigm for valley pseudospin, enabling the
platform of excitons in 2D materials for the control of this novel degree of
freedom in solids.
