The negatively charged nitrogen-vacancy color center in diamond has been identified as a sensitive magnetic field sensor based on the optically detected magnetic resonance (ODMR). However, it requires knowledge of the crystal axes and it needs an external magnetic bias field to measure the field’s orientation, or the use of single centers. Recently, by combining ensembles of color centers with polarimetry, we have been able to determine the magnitude and direction of an unknown magnetic field. The out-of-plane polarization components of the excitation laser create asymmetry in the polarization resolved ODMR spectrum. This provides the necessary conditions to reconstruct the three-dimensional magnetic field vector without a bias field. Our approach is general for other spin-1 color centers with the same symmetry, and it is compatible with standard microscopy methods, such as scanning probe, super-resolution, confocal, and wide-field imaging.
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