Dc Quantum Magnetometry below the Ramsey Limit

We demonstrate quantum sensing of dc magnetic fields that exceeds the sensitivity of conventional T2? - -limited dc magnetometry by more than an order of magnitude. We use 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 up-converts 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, 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 sensitivity improvement.