Resonant versus non-resonant spin readout of a nitrogen-vacancy center in diamond under cryogenic conditions*

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, where the NV's zero-phonon line splits into a set of narrow, spin-selective optical transitions and investigate alternative (non-ionizing) spin read-out strategies. In the low power limit, narrow-band laser excitation resonant with a cycling transition has shown high fidelity, non-destructive spin read-out, at the cost of a strong overhead. Here, we use a similar pathway at higher power to demonstrate more than four-fold improvement in sensitivity compared to off-resonant green excitation, largely due to a boost in readout contrast and integrated photon count. We also leverage nuclear spin relaxation to polarize the Nitrogen-14 host, which we prove beneficial for spin magnetometry.