Using a spatially resolved optical pump-probe experiment, we measure the lateral transport of spin-valley polarized electrons over very long distances (tens of micrometers) in a single ${\mathrm{WSe}}_2$ monolayer. By locally pumping the Fermi sea of 2D electrons to a high degree of spin-valley polarization (up to 75%) using circularly polarized light, the lateral diffusion of the electron polarization can be mapped out via the photoluminescence induced by a spatially separated and linearly polarized probe laser. Up to 25% spin-valley polarization is observed at pump-probe separations up to $20\mu \mathrm{m}$. Characteristic spin-valley diffusion lengths of $18\pm 3\mu \mathrm{m}$ are revealed at low temperatures. The dependence on temperature, pump helicity, pump intensity, and electron density highlight the key roles played by spin relaxation time and pumping efficiency on polarized electron transport in monolayer semiconductors possessing spin-valley locking.

• Received 27 January 2022
• Accepted 18 May 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.027402