  Excitons are composite bosons that can feature spin singlet and triplet
states. In usual semiconductors, without an additional spin-flip mechanism,
triplet excitons are extremely inefficient optical emitters. Large spin-orbit
coupling in transition metal dichalcogenides (TMDs) couples circularly
polarized light to excitons with selective valley and spin. Here, we
demonstrate electrically controlled brightening of spin-triplet interlayer
excitons in a MoSe$_2$/WSe$_2$ TMD van der Waals (vdW) heterostructure. The
atomic registry of vdW layers in TMD heterostructures provides a quasi-angular
momentum to interlayer excitons, enabling emission from otherwise dark
spin-triplet excitons. Employing magnetic field, we show that photons emitted
by triplet and singlet excitons in the same valley have opposite chirality. We
also measure effective exciton g-factors, presenting direct and quantitative
evidence of triplet interlayer excitons. We further demonstrate gate tuning of
the relative photoluminescence intensity between singlet and triplet charged
excitons. Electrically controlled emission between singlet and triplet excitons
enables a route for optoelectronic devices that can configure excitonic chiral,
spin, and valley quantum states.
