We demonstrate an all-optical approach of nanoscale magnetic resonance (MR) spectroscopy whereby quantum relaxation $\left(T_1\right)$ of a single probe spin in diamond is monitored during a precise static magnetic field sweep to construct a spectrum of the surrounding spin environment. The method is inherently noninvasive as it involves no driving fields, and instead relies on the natural resonance between the quantum probe and target spins. As a proof of concept, we measure the $T_1-\mathrm{MR}$ spectra across a wide band [megahertz (MHz) to gigahertz (GHz)] of a small ensemble of ${}^{14}\mathrm{N}$ impurities surrounding a single probe spin, providing information on both electron spin transitions (in the GHz range) and nuclear spin transitions (in the MHz range) of the ${}^{14}\mathrm{N}$ spin targets. Analysis of the $T_1-\mathrm{MR}$ spectrum reveals that the electron spin transitions are probed via dipole interactions with the probe, while the relatively weak nuclear spin resonances are dramatically enhanced by hyperfine coupling in an electron-mediated process. With a projected sensitivity to external single-proton spins, this work establishes $T_1-\mathrm{MR}$ as a powerful noninvasive wide-band technique for nanoscale MR spectroscopy.

• Received 1 April 2016

DOI:https://doi.org/10.1103/PhysRevB.94.155402