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Optimizing valley dynamics is an effective instrument towards preciselymanipulating qubit in the context of two-dimensional semiconductor. In thiswork, we construct a comprehensive model, involving both intra- and intervalleychannels of excitons in monolayer WSe$_2$, and simultaneously takes thelight-matter interaction into account, to investigate the optimal control ofvalley dynamics with an initial coherent excitonic state. Based on the quantumspeed limit (QSL) theory, we propose two optimal control schemes aiming toreduce the evolution time of valley dynamics reaching the target state, alongwith to boost the evolution speed over a period of time. Further, we emphasizethat the implementation of dynamical optimization is closely related to thedetuning difference -- the difference of exciton-laser field detunings betweenthe K and K' valleys -- which is determined by the optical excitation mode andmagnetically-induced valley splitting. In particular, we reveal that a smalldetuning difference drives the actual dynamical path to converge towards thegeodesic length between the initial and final states, allowing the system toevolve 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 valleyqubit. Remarkably, we demonstrate an intriguing enhancement in evolution speedof valley dynamics, by adopting a large detuning difference, which induces anemerging valley polarization even without initial polarization. Our work opensa new paradigm for optically tuning excitonic physics in valleytronicapplications, and may also offer solutions to some urgent problems such asspeed limit of information transmission in qubit.

Laser-field detuning assisted optimization of exciton valley dynamics in monolayer WSe$_2$: Geometric quantum speed limit