  The nitrogen-vacancy (NV) defect in diamond is an efficient quantum sensor of
randomly fluctuating signals via relaxometry measurements. In particular, the
longitudinal spin relaxation of the NV defect accelerates in the presence of
magnetic noise with a spectral component at its electron spin resonance
frequency. We look into this effect quantitatively by applying a calibrated and
tunable magnetic noise on a single NV defect. We show that an increase of the
longitudinal spin relaxation rate translates into a reduction of the
photoluminescence (PL) signal emitted under continuous optical illumination,
which can be explained using a simplified three-level model of the NV defect.
This PL quenching mechanism offers a simple, all-optical method to detect
magnetic noise sources at the nanoscale.
