  The lowest energy states in transition metal dichalcogenide (TMD) monolayers
follow valley selection rules, which have attracted vast interest due to the
possibility of encoding and processing of quantum information. However, these
quantum states are strongly affected by the temperature-dependent intervalley
scattering causing complete valley depolarization, which is hampering any
practical applications of TMD monolayers at room temperature. Therefore, for
achieving clear and robust valley polarization in TMD monolayers one needs to
suppress parasitic depolarization processes, which is the central challenge in
the growing field of valleytronics. Here, in electron-doping experiments on TMD
monolayers, we demonstrate that strong doping levels beyond $10^{13}$~cm$^{-2}$
can induce 61\% and 37\% valley contrast at room temperature in tungsten
diselenide and molybdenum diselenide monolayers, respectively. Our results
indicate that charged excitons in TMD monolayers can be utilized as quantum
units in designing of practical valleytronic devices operating at 300 K.
