Microwave-Free Vector Magnetometry with Nitrogen-Vacancy Centers along a Single Axis in Diamond

Sensing vector magnetic fields is critical to many applications in fundamental physics, bioimaging, and material science. Magnetic field sensors exploiting nitrogen-vacancy (N-V) centers are particularly compelling as they offer high sensitivity and spatial resolution even at the nanoscale. Achieving vector magnetometry, however, often requires applying microwaves sequentially or simultaneously, limiting the sensors' applications under cryogenic temperature. Here, we propose and demonstrate a microwave-free vector magnetometer that simultaneously measures all Cartesian components of a magnetic field using N-V ensembles in diamond. In particular, the present magnetometer leverages the level anticrossing in the triplet ground state at 102.4 mT, allowing the measurement of both longitudinal and transverse fields with a wide bandwidth from zero to the megahertz range. Full vector sensing capability is proffered by modulating fields along the preferential N-V axis and in the transverse plane and subsequent demodulation of the signal. This sensor exhibits a root-mean-square noise floor that approximately equals 300 pT/Hz in all directions. The present technique is broadly applicable to both ensemble sensors and potentially also to single-N-V sensors, extending the vector capability to nanoscale measurements under ambient temperatures.