Optical initialization of a single spin-valley in charged WSe quantum dots.

Control and manipulation of single charges and their internal degrees of freedom, such as spin, may enable applications in quantum information technology, spintronics and quantum sensing. Recently, atomically thin semiconductors with a direct bandgap such as group VI-B transition-metal dichalcogenide monolayers have emerged as a platform for valleytronics-the study of the valley degree of freedom of charge carriers to store and control information. They offer optical, magnetic and electrical control of the valley index, which, with the spin, is locked into a robust spin-valley index. However, because recombination lifetimes of photogenerated excitations in transition-metal dichalcogenides are of the order of a few picoseconds, optically generated valley excitons possess similar lifetimes. On the other hand, the valley polarization of free holes has a lifetime of microseconds. Whereas progress has been made in optical control of the valley index in ensembles of charge carriers, valley control of individual charges, which is crucial for valleytronics, remains unexplored. Here we provide unambiguous evidence for localized holes with a net spin in optically active WSe quantum dots and we initialize their spin-valley state with the helicity of the excitation laser under small magnetic fields. Under such conditions, we estimate a lower bound of the valley lifetime of a single charge in a quantum dot from the recombination time to be of the order of nanoseconds. Remarkably, neutral quantum dots do not exhibit such spin-valley initialization, which illustrates the role of the excess charge in prolonging the valley lifetime. Our work extends the field of two-dimensional valleytronics to the level of single spin- valleys, with implications for quantum information and sensing applications.