Ultrafast Manipulation of Valley Pseudospin

TL;DR

This paper demonstrates ultrafast all-optical control of valley pseudospin in monolayer WSe2 using the optical Stark effect, enabling rapid manipulation of valley coherence for quantum technology applications.

Contribution

It introduces a method to rotate valley pseudospin using below-bandgap circularly polarized light, achieving femtosecond control of valley coherence in 2D materials.

Findings

Valley pseudospin can be optically rotated on femtosecond timescales.

The direction and speed of pseudospin rotation are tunable via the optical Stark effect.

Complete control over valley pseudospin enables new quantum information processing possibilities.

Abstract

The coherent manipulation of spin and pseudospin underlies existing and emerging quantum technologies, including NMR, quantum communication, and quantum computation. Valley polarization, associated with the occupancy of degenerate, but quantum mechanically distinct valleys in momentum space, closely resembles spin polarization and has been proposed as a pseudospin carrier for the future quantum electronics. Valley exciton polarization has been created in the transition metal dichalcogenide (TMDC) monolayers using excitation by circularly polarized light and has been detected both optically and electrically. In addition, the existence of coherence in the valley pseudospin has been identified experimentally. The manipulation of such valley coherence has, however, remained out of reach. Here we demonstrate an all-optical control of the valley coherence by means of the pseudomagnetic field associated with the optical Stark effect. Using below-bandgap circularly polarized light, we experimentally rotate the valley exciton pseudospin in monolayer WSe2 on the femtosecond time scale. Both the direction and speed of the rotation can be optically manipulated by tuning the dynamic phase of excitons in opposite valleys. This study completes the generation-manipulation-detection paradigm for valley pseudospin, enabling the platform of excitons in 2D materials for the control of this novel degree of freedom in solids.