  The emerging field of valleytronics aims to exploit the valley pseudospin of
electrons residing near Bloch band extrema as an information carrier. Recent
experiments demonstrating optical generation and manipulation of exciton valley
coherence (the superposition of electron-hole pairs at opposite valleys) in
monolayer transition metal dichalcogenides (TMDs) provide a critical step
towards control of this quantum degree of freedom. The charged exciton (trion)
in TMDs is an intriguing alternative to the neutral exciton for control of
valley pseudospin because of its long spontaneous recombination lifetime, its
robust valley polarization, and its coupling to residual electronic spin. Trion
valley coherence has however been unexplored due to experimental challenges in
accessing it spectroscopically. In this work, we employ ultrafast
two-dimensional coherent spectroscopy to resonantly generate and detect trion
valley coherence in monolayer MoSe$_2$ demonstrating that it persists for a
few-hundred femtoseconds. We conclude that the underlying mechanisms limiting
trion valley coherence are fundamentally different from those applicable to
exciton valley coherence. Based on these observations, we suggest possible
strategies for extending valley coherence times in two-dimensional materials.
