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.