DC magnetometry below the Ramsey limit with rapidly rotating diamonds

Magnetometers based on the nitrogen-vacancy (NV) center in diamond provide μT to nT Hz^-1/2 sensitivity for single centers at ambient temperature and mm-to-sub-μm length scales, making ideal for studying magnetic phenomena in challenging, real-world sensing environments. Intense effort has been devoted to improving dc sensitivity due to the vast range of applications requiring high sensitivity magnetometers operating at low frequency. Many if not all of these approaches to improving the measurement signal are frustrated by a commensurate increase in noise. I will report on quantum sensing of dc magnetic fields exceeding the sensitivity of conventional T₂*-limited dc Ramsey magnetometry by more than an order of magnitude. We used NV centers in a diamond rotating at periods comparable to the spin coherence time T₂ to convert dc magnetic fields to ac, and characterized the dependence of magnetic sensitivity on measurement time and rotation speed. Our method up-converts only the dc field of interest, eliminates in-diamond noise and and preserves the quantum coherence of the sensor. These results make a definitive improvement to the sensitivity of a quantum magnetometer to dc fields, demonstrating that sensitivity below the T₂* limit is possible and can be applied to any diamond magnetometer where T₂>>T₂* to yield an order of magnitude or more sensitivity improvement.