  We demonstrate quantum sensing of dc magnetic fields that exceeds the
sensitivity of conventional $T_2^\ast$-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
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, an important and useful addition to the quantum
sensing toolbox.
