Sensitive ac and dc magnetometry with nitrogen-vacancy-center ensembles in diamond

Quantum sensing with solid-state spins offers the promise of high spatial resolution, bandwidth, and dynamic range at sensitivities comparable to more mature quantum sensing technologies, such as atomic vapor cells and superconducting devices. However, despite comparable theoretical sensitivity limits, the performance of bulk solid-state quantum sensors has so far lagged behind these more mature alternatives. A recent review [Barry et al., Rev. Mod. Phys. 92, 015004 (2020)] suggests several paths to improve performance of magnetometers employing nitrogen-vacancy defects in diamond, the most-studied solid-state quantum sensing platform. Implementing several suggested techniques, we demonstrate the most sensitive nitrogen-vacancy-based bulk magnetometer reported to date. Our approach combines tailored diamond growth to achieve low strain and long intrinsic dephasing times, the use of double-quantum Ramsey and Hahn-echo magnetometry sequences for broadband and narrowband magnetometry, respectively, and P1 driving to further extend dephasing time. Notably, the device does not include a flux concentrator, preserving the fixed response of the nitrogen vacancies to magnetic field. The magnetometer realizes a minimum sensitivity of $460\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{1/2}$ in a broadband dc-sensitive mode and $210\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{1/2}$ in a narrowband ac mode. We describe the experimental setup in detail and highlight potential paths for future improvement.