Together with charge and spin, many novel two-dimensional 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 toward valley-based (opto)electronic technologies, the intrinsic time scales of valley scattering are therefore of fundamental interest. Here, we demonstrate an entirely noise-based approach for exploring valley dynamics in monolayer transition-metal dichalcogenide

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... emiconductors. Exploiting their valley-specific optical selection rules, we use optical Faraday rotation to passively detect the thermodynamic fluctuations of valley polarization in a Fermi sea of resident carriers. This spontaneous "valley noise" reveals narrow Lorentzian line shapes and, therefore, long exponentially-decaying intrinsic valley relaxation. Moreover, the noise signatures 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.