We demonstrate an absolute magnetometer immune to temperature fluctuation and strain inhomogeneity, based on quantum beats in the ground state of nitrogen-vacancy centers in diamond. We apply this technique to measure low-frequency magnetic field noise using a single nitrogen-vacancy center located within 500 nm of the surface of an isotopically-pure (99.99% C12) diamond. The photon-shot-noise limited sensitivity achieves 38 nT/Hz^1/2 for 4.45 s acquisition time, a factor of 2^1/2 better than the implementation which uses only two spin levels. For long acquisition times (>10 s), we realize up to a factor of 15 improvement in magnetic sensitivity, which demonstrates the robustness of our technique against thermal drifts. Applying our technique to nitrogen-vacancy center ensembles, we eliminate dephasing from longitudinal strain inhomogeneity, resulting in a factor of 2.3 improvement in sensitivity.
@article{arxiv.1212.1495,
title = {Absolute magnetometry based on quantum beats in diamond nitrogen-vacancy centers},
author = {Kejie Fang and Victor M. Acosta and Charles Santori and Zhihong Huang and Kohei M. Itoh and Hideyuki Watanabe and Shinichi Shikata and Raymond G. Beausoleil},
journal= {arXiv preprint arXiv:1212.1495},
year = {2015}
}