  We report electrical tuning by the Stark effect of the excited-state
structure of single nitrogen-vacancy (NV) centers located less than ~100 nm
from the diamond surface. The zero-phonon line (ZPL) emission frequency is
controllably varied over a range of 300 GHz. Using high-resolution emission
spectroscopy, we observe electrical tuning of the strengths of both cycling and
spin-altering transitions. Under resonant excitation, we apply dynamic feedback
to stabilize the ZPL frequency. The transition is locked over several minutes
and drifts of the peak position on timescales greater than ~100 ms are reduced
to a fraction of the single-scan linewidth, with standard deviation as low as
16 MHz (obtained for an NV in bulk, ultra-pure diamond). These techniques
should improve the entanglement success probability in quantum communications
protocols.
