  Sensing static or slowly varying magnetic fields with high sensitivity and
spatial resolution is critical to many applications in fundamental physics,
bioimaging and materials science. Several versatile magnetometry platforms have
emerged over the past decade, such as electronic spins associated with Nitrogen
Vacancy (NV) centers in diamond. However, their high sensitivity to external
fields also makes them poor sensors of DC fields. Indeed, the usual method of
Ramsey magnetometry leaves them prone to environmental noise, limiting the
allowable interrogation time to the short dephasing time T2*. Here we introduce
a hybridized magnetometery platform, consisting of a sensor and ancilla, that
allows sensing static magnetic fields with interrogation times up to the much
longer T2 coherence time, allowing significant potential gains in field
sensitivity. While more generally applicable, we demonstrate the method for an
electronic NV sensor and a nuclear ancilla. It relies on frequency upconversion
of transverse DC fields through the ancilla, allowing quantum lock-in detection
with low-frequency noise rejection. In our experiments, we demonstrate
sensitivities better than 6uT/vHz, comparable to the Ramsey method, and
narrow-band signal noise filtering better than 64kHz. With technical
optimization, we expect more than an one order of magnitude improvement in each
of these parameters. Since our method measures transverse fields, in
combination with the Ramsey detection of longitudinal fields, it ushers in a
compelling technique for sensitive vector DC magnetometry at the nanoscale.
