  Group IV vacancy color centers in diamond are promising spin-photon
interfaces with strong potential for applications for photonic quantum
technologies. Reliable methods for controlling and stabilizing their charge
state are urgently needed for scaling to multi-qubit devices. Here, we
manipulate the charge state of silicon vacancy (SiV) ensembles by combining
luminescence and photo-current spectroscopy. We controllably convert the charge
state between the optically active SiV$^-$ and dark SiV$^{2-}$ with MHz rates
and 90% contrast by judiciously choosing the local potential applied to
in-plane surface electrodes and the laser excitation wavelength. We observe
intense SiV$^-$ photoluminescence under hole-capture, measure the intrinsic
conversion time from the dark SiV$^{2-}$ to the bright SiV$^-$ to be
36.4(6.7)ms and demonstrate how it can be enhanced by a factor of $10^5$ via
optical pumping. Moreover, we obtain new information on the defects that
contribute to photo-conductivity, indicating the presence of substitutional
nitrogen and divacancies.
