Optically Controlled Switching of the Charge State of a Single Nitrogen-Vacancy Center in Diamond at Cryogenic Temperatures

In this Letter, the photoinduced switching of the single nitrogen-vacancy (NV) center between two different charge states, negative (NV^-) and neutral (NV⁰), is studied under resonant excitation at liquid helium temperature. We show that resonant conversion of NV⁰ to NV^- significantly improves spectral stability of the NV^- defect and allows high fidelity initialization of the spin qubit. Based on density functional theory calculations a novel mechanism involving an Auger ionization of NV^- and charge transfer of an electron from the valence band to NV⁰ is discussed. This study provides further insight into the charge dynamics of the NV center, which is relevant for quantum information processing based on an NV^- defect in diamond.