Zero-field magnetometry based on nitrogen-vacancy ensembles in diamond

Ensembles of nitrogen-vacancy (N-V) centers in diamonds are widely utilized for magnetometry, magnetic field imaging, and magnetic resonance detection. At zero ambient field, Zeeman sublevels in the N-V centers lose first-order sensitivity to magnetic fields, as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a C-13-depleted crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of 250 pT/root Hz. This technique opens the door to practical applications of N-V sensors for ZF magnetic sensing, such as ZF nuclear magnetic resonance and investigation of magnetic fields in biological systems.