Optical Generation of Excitonic Valley Coherence in Monolayer WSe2

TL;DR

This paper demonstrates the first optical generation and detection of SU(2) valley coherence in monolayer WSe2, enabling coherent manipulation of valley pseudospins for quantum technologies.

Contribution

It introduces a method to generate and detect valley coherence in monolayer WSe2, advancing the field of valleytronics and quantum manipulation.

Findings

Valley excitons can be coherently superposed, evidenced by linearly polarized luminescence.

Circularly polarized light selectively addresses individual valley excitons and trions.

Valley coherence is confirmed through polarization-resolved photoluminescence measurements.

Abstract

Due to degeneracies arising from crystal symmetries, it is possible for electron states at band edges ("valleys") to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that routinely implemented using true spin, in the quest for quantum technologies. Here we show for the first time that SU(2) valley coherence can indeed be generated and detected. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then reveal coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation always coincides with that of any linearly polarized excitation. Since excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valleys. In contrast, the corresponding photoluminescence from trions is not linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, adds a critical dimension to the quantum manipulation of valley index necessary for coherent valleytronics.