  The last decade has seen an explosive growth in the use of color centers for
metrology applications, the paradigm example arguably being the
nitrogen-vacancy (NV) center in diamond. Here, we focus on the regime of
cryogenic temperatures and examine the impact of spin-selective, narrow-band
laser excitation on NV readout. Specifically, we demonstrate a more than
four-fold improvement in sensitivity compared to that possible with
non-resonant (green) illumination, largely due to a boost in readout contrast
and integrated photon count. We also leverage nuclear spin relaxation under
resonant excitation to polarize the 14N host, which we then prove beneficial
for spin magnetometry. These results open opportunities in the application of
NV sensing to the investigation of condensed matter systems, particularly those
exhibiting superconducting, magnetic, or topological phases selectively present
at low temperatures.
