Detection of magnetic field polarity with magnetic field modulation ODMR on NV– centers in diamond

Diamond with negatively charged nitrogen-vacancy (NV–) colour centers is a promising material system for quantum sensing applications and in particular for magnetometry. The sensing protocol leverages on optically detected magnetic resonance (ODMR) technique which provides high sensitivity to magnetic field magnitude and orientation from the analysis of the resonance frequencies. Direct detection of the magnetic field polarity would be appealing for the study of physical phenomena and applications where real-time detection of static and dynamic stray field from samples is required. Unfortunately, ODMR resonance lines are inherently insensitive to the magnetic field polarity. Here, an effect has been observed where the ODMR spectra recorded for an ensemble of NV– centers by demodulating a small magnetic field (Bmod-ODMR) directly correlates the symmetry of the first-derivative spectral lines to the magnetic field polarity. A model is introduced to explain the mechanism underlying for Bmod-ODMR along with a proof-of-concept for real-time detection of magnetic field switching.