  Ensembles of nitrogen-vacancy (NV) centers in diamonds are widely utilized
for magnetometry, magnetic-field imaging and magnetic-resonance detection. They
have not been used for magnetometry at zero ambient field because Zeeman
sublevels 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
12C-enriched HPHT 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/Hz^(1/2). This technique
opens the door to practical applications of NV sensors for ZF magnetic sensing,
such as ZF nuclear magnetic resonance, and investigation of magnetic fields in
biological systems.
