  Together with charge and spin degrees of freedom, many new 2D materials also
permit information to be encoded in an electron's valley degree of freedom -
that is, in particular momentum states in the material's Brillouin zone. With a
view towards future generations of valley-based (opto)electronic technologies,
the intrinsic timescales of scattering and relaxation between valleys therefore
represent fundamental parameters of interest. Here we introduce and demonstrate
an entirely passive, noise-based approach for exploring intrinsic valley
dynamics in atomically-thin transition-metal dichalcogenide (TMD)
semiconductors. Exploiting the valley-specific optical selection rules in
monolayer TMDs, we use optical Faraday rotation to detect, under conditions of
strict thermal equilibrium, the stochastic thermodynamic fluctuations of the
valley polarization in a Fermi sea of resident carriers. Frequency spectra of
this spontaneous "valley noise" reveal narrow Lorentzian lineshapes and
therefore long exponentially-decaying intrinsic valley relaxation. Moreover,
the valley noise signals are shown to validate both the relaxation times and
the spectral dependence of conventional (perturbative) pump-probe measurements.
These results provide a viable route toward quantitative measurements of
intrinsic valley dynamics, free from any external perturbation, pumping, or
excitation.
