  The twist degree of freedom provides a powerful new tool for engineering the
electrical and optical properties of van der Waals heterostructures. Here, we
show that the twist angle can be used to control the spin-valley properties of
transition metal dichalcogenide bilayers by changing the momentum alignment of
the valleys in the two layers. Specifically, we observe that the interlayer
excitons in twisted WSe$_2$/WSe$_2$ bilayers exhibit a high (>60%) degree of
circular polarization (DOCP) and long valley lifetimes (>40 ns) at zero
electric and magnetic fields. The valley lifetime can be tuned by more than
three orders of magnitude via electrostatic doping, enabling switching of the
DOCP from ~80% in the n-doped regime to <5% in the p-doped regime. These
results open up new avenues for tunable chiral light-matter interactions,
enabling novel device schemes that exploit the valley degree of freedom.

Mesoscale and Nanoscale Physics;Materials Science;Optics