Avoiding power broadening in optically detected magnetic resonance of single NV defects for enhanced dc magnetic field sensitivity

We report a systematic study of the magnetic field sensitivity of a magnetic sensor consisting of a single nitrogen-vacancy (NV) defect in diamond, by using continuous optically detected electron spin resonance (ESR) spectroscopy. We first investigate the behavior of the ESR contrast and linewidth as a function of the microwave and optical pumping power. The experimental results are in good agreement with a simplified model of the NV defect spin dynamics, leading to an optimized sensitivity around $2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}$T$/\sqrt{\mathrm{Hz}}$ for a single NV defect in a high-purity diamond crystal grown by chemical vapor deposition. We then demonstrate an enhancement of the magnetic sensitivity by one order of magnitude by using a simple pulsed-ESR scheme. This technique is based on repetitive excitation of the NV defect with a resonant microwave $\ensuremath{\pi}$ pulse followed by an optimized readout laser pulse, allowing to fully eliminate power broadening of the ESR linewidth. The achieved sensitivity is similar to that obtained by using Ramsey-type sequences, which is the optimal magnetic field sensitivity for the detection of a dc magnetic field.