We report on the use of a single nitrogen-vacancy (NV) center to probe fluctuating ac
magnetic fields. Using engineered currents to induce random changes in the field amplitude
and phase, we show that stochastic fluctuations reduce the NV center sensitivity and, in
general, make the NV response field-dependent. We also introduce two modalities to
determine the field spectral composition, unknown a priori in a practical application. One
strategy capitalizes on the generation of ac-field-induced coherence “revivals” while the …