  Detection of AC magnetic fields at the nanoscale is critical in applications
ranging from fundamental physics to materials science. Isolated quantum spin
defects, such as the nitrogen-vacancy center in diamond, can achieve the
desired spatial resolution with high sensitivity. Still, vector AC magnetometry
currently relies on using different orientations of an ensemble of sensors,
with degraded spatial resolution, and a protocol based on a single NV is
lacking. Here we propose and experimentally demonstrate a protocol that
exploits a single NV to reconstruct the vectorial components of an AC magnetic
field by tuning a continuous driving to distinct resonance conditions. We map
the spatial distribution of an AC field generated by a copper wire on the
surface of the diamond. The proposed protocol combines high sensitivity, broad
dynamic range, and sensitivity to both coherent and stochastic signals, with
broad applications in condensed matter physics, such as probing spin
fluctuations.
