Toward a Laser-Free Diamond Magnetometer for Microwave Fields

The nitrogen-vacancy (NV) center in diamond is a unique magnetometer. Its atomic size enables integrations of a tremendous amount ($n_{\mathrm{NV}}$) of NV centers in a bulk diamond with a sensitivity scaling as $1/\sqrt{n_{\mathrm{NV}}}$. However, such a bulk sensor requires a high-power laser to polarize and read out the NV centers. The increasing thermal damage and additional noises associated with high-power lasers hinder the growth of $n_{\mathrm{NV}}$, and thus limit the sensitivity at picotesla level. Here, it shows a relaxometry-based microwave magnetometer that the power density is determined by the relaxation time $T_1$. By cooling the diamond sensor to prolong the $T_1$ ($\approx$s), the required power density further reduces to $0.077{\mathrm{Wcm}}^{-2}$, $\approx {10}^{-6}$ of the saturation value. This work paves the way for the utilization of large-size diamond to promote the sensitivity of diamond magnetometer to femtotesla level and beyond.