  Monolayer photonic materials offer a tremendous potential for on-chip
optoelectronic devices. Their realization requires knowledge of optical
coherence properties of excitons and trions that have so far been limited to
nonlinear optical experiments carried out with strongly inhomogenously
broadened material. Here we employ h-BN encapsulated and electrically gated
MoSe2 to reveal coherence properties of trion-species directly in the linear
optical response. Autocorrelation measurements reveal long dephasing times up
to T2=1.16+-0.05 ps for positively charged excitons. Gate dependent
measurements provide evidence that the positively-charged trion forms via
spatially localized hole states making this trion less prone to dephasing in
the presence of elevated hole carrier concentrations. Quantum beat signatures
demonstrate coherent coupling between excitons and trions that have a dephasing
time up to 0.6 ps, a two-fold increase over those in previous reports. A key
merit of the prolonged exciton/trion coherences is that they were achieved in a
linear optical experiment, and thus are directly relevant to applications in
nanolasers, coherent control, and on-chip quantum information processing
requiring long photon coherence.
