DC Quantum Magnetometry Below the Ramsey Limit

Abstract

We demonstrate quantum sensing of dc magnetic fields that exceeds the sensitivity of conventional T * 2 -limited dc magnetometry by more than an order of magnitude. We used nitrogen-vacancy centers in a diamond rotating at periods comparable to the spin coherence time, and characterize the dependence of magnetic sensitivity on measurement time and rotation speed. Our method upconverts only the dc field of interest and preserves the quantum coherence of the sensor. These results definitively improve the sensitivity of a quantum magnetometer to dc fields, an important and useful addition to the quantum sensing toolbox.