  Increasing the speed limits of conventional electronics requires innovative
approaches to manipulate other quantum properties of electrons besides their
charge. An alternative approach utilizes the valley degree of freedom in
low-dimensional semiconductors. Here we demonstrate that the valley degeneracy
of exciton energies in transition metal dichalcogenide monolayers may be lifted
by coherent optical interactions on timescales corresponding to few tens of
femtoseconds. The optical Stark and Bloch-Siegert effects generated by strong
nonresonant circularly-polarized light induce valley-selective blue shifts of
exciton quantum levels by more than 30 meV. We show these phenomena by studying
the two most intensive exciton resonances in transiton metal dichalcogenide
monolayers and compare the results to a theoretical model, which properly
includes the Coulomb interaction and exciton dispersion. These results open the
door for ultrafast valleytronics working at multiterahertz frequencies.
