  Optimizing valley dynamics is an effective instrument towards precisely
manipulating qubit in the context of two-dimensional semiconductor. In this
work, we construct a comprehensive model, involving both intra- and intervalley
channels of excitons in monolayer WSe$_2$, and simultaneously takes the
light-matter interaction into account, to investigate the optimal control of
valley dynamics with an initial coherent excitonic state. Based on the quantum
speed limit (QSL) theory, we propose two optimal control schemes aiming to
reduce the evolution time of valley dynamics reaching the target state, along
with to boost the evolution speed over a period of time. Further, we emphasize
that the implementation of dynamical optimization is closely related to the
detuning difference -- the difference of exciton-laser field detunings between
the K and K' valleys -- which is determined by the optical excitation mode and
magnetically-induced valley splitting. In particular, we reveal that a small
detuning difference drives the actual dynamical path to converge towards the
geodesic length between the initial and final states, allowing the system to
evolve with the least time. Especially, in the presence of valley coherence,
the actual evolution time and the calculated QSL time almost coincide,
facilitating high fidelity in information transmission based on the valley
qubit. Remarkably, we demonstrate an intriguing enhancement in evolution speed
of valley dynamics, by adopting a large detuning difference, which induces an
emerging valley polarization even without initial polarization. Our work opens
a new paradigm for optically tuning excitonic physics in valleytronic
applications, and may also offer solutions to some urgent problems such as
speed limit of information transmission in qubit.
