  Strong spin-orbit coupling and inversion symmetry breaking in transition
metal dichalcogenide monolayers yield the intriguing effects of
valley-dependent optical selection rules. As such, it is possible to
substantially polarize valley excitons with chiral light and furthermore create
coherent superpositions of K and K- polarized states. Yet, at ambient
conditions dephasing usually becomes too dominant, and valley coherence
typically is not observable. Here, we demonstrate that valley coherence is,
however, clearly observable for a single monolayer of WSe2, if it is strongly
coupled to the optical mode of a high quality factor microcavity. The azimuthal
vector, representing the phase of the valley coherent superposition, can be
directly manipulated by applying magnetic fields, and furthermore, it sensibly
reacts to the polarization anisotropy of the cavity which represents an
artificial magnetic field. Our results are in qualitative and quantitative
agreement with our model based on pseudospin rate equations, accounting for
both effects of real and pseudo-magnetic fields.
