  Coherence is a crucial requirement to realize quantum manipulation through
light-matter interactions. Here we report the observation of anomalously robust
valley polarization and valley coherence in bilayer WS2. The polarization of
the photoluminescence from bilayer WS2 inherits that of the excitation source
with both circularly and linearly polarized and retains even at room
temperature. The near unity circular polarization of the luminescence reveals
the coupling of spin, layer and valley degree of freedom in bilayer system,
while the linear polarized photoluminescence manifests quantum coherence
between the two inequivalent band extrema in momentum space, namely, the valley
quantum coherence in atomically thin bilayer WS2. This observation opens new
perspectives for quantum manipulation in atomically thin semiconductors.
