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The coherent manipulation of spin and pseudospin underlies existing andemerging quantum technologies, including NMR, quantum communication, andquantum computation. Valley polarization, associated with the occupancy ofdegenerate, but quantum mechanically distinct valleys in momentum space,closely resembles spin polarization and has been proposed as a pseudospincarrier for the future quantum electronics. Valley exciton polarization hasbeen created in the transition metal dichalcogenide (TMDC) monolayers usingexcitation by circularly polarized light and has been detected both opticallyand electrically. In addition, the existence of coherence in the valleypseudospin has been identified experimentally. The manipulation of such valleycoherence has, however, remained out of reach. Here we demonstrate anall-optical control of the valley coherence by means of the pseudomagneticfield associated with the optical Stark effect. Using below-bandgap circularlypolarized light, we experimentally rotate the valley exciton pseudospin inmonolayer WSe2 on the femtosecond time scale. Both the direction and speed ofthe rotation can be optically manipulated by tuning the dynamic phase ofexcitons in opposite valleys. This study completes thegeneration-manipulation-detection paradigm for valley pseudospin, enabling theplatform of excitons in 2D materials for the control of this novel degree offreedom in solids.

Ultrafast Manipulation of Valley Pseudospin