Order of Magnitude Improvement in NV Ensemble T2* via Control and Cancellation of Spin Bath Induced Dephasing

We deploy two complementary techniques, spin bath control and double quantum magnetometry, to attain up to a 14-fold improvement in $T_2^*$ for Nitrogen-Vacancy ensembles in diamond. Depending on the Nitrogen concentration of the sample, three regimes can be differentiated: for low concentrations ($N \ll 1$ ppm), the dephasing is strain-dominated and can be suppressed by working in the NV's double quantum basis $\{-1, +1\}$. In an intermediate regime ($N \simeq 1$ ppm), the combination of double quantum and spin bath control achieves a 14-fold increase in $T_2^*$. At greater Nitrogen concentrations ($N \geq 10$ ppm), dipolar interactions with electronic Nitrogen spins dominate the broadening which are mitigated by spin bath driving. Our results elucidate sources of dephasing over a range of spin concentrations and outline a direct path forward to improving coherence times for sensing and quantum science.