  Charge state instabilities have been a bottleneck for the implementation of
solid-state spin systems and pose a major challenge to the development of
spin-based quantum technologies. Here we investigate the stabilization of
negatively charged nitrogen-vacancy (NV$^-$) centers in phosphorus-doped
diamond at liquid helium temperatures. Photoionization of phosphorous donors in
conjunction with charge diffusion at the nanoscale enhances NV$^0$ to NV$^-$
conversion and stabilizes the NV$^-$ charge state without the need for an
additional repump laser. The phosphorus-assisted stabilization is explored and
confirmed both with experiments and our theoretical model. Stable
photoluminescence-excitation spectra are obtained for NV$^-$ centers created
during the growth. The fluorescence is continuously recorded under resonant
excitation to real-time monitor the charge state and the ionization and
recombination rates are extracted from time traces. We find a linear laser
power dependence of the recombination rate as opposed to the conventional
quadratic dependence, which is attributed to the photo-ionization of phosphorus
atoms.
