Photoswitchable Radicals as Reporter Spins for Quantum Sensing with Spin Defects in Diamond

The rapid decay of target signal strength with distance from the sensor presents a key challenge in nanoscale magnetic sensing with the nitrogen-vacancy (NV) center in diamond, limiting the scope of accessible information as well as the sensitivity and spatial resolution with which that information can be recovered. Here, we introduce a strategy to overcome these limitations by leveraging radical anions formed from rhodamine-derived organic dyes localized to the diamond surface. These radicals, generated through photoreduction, are optically identifiable and persist on time scales exceeding an hour. We experimentally demonstrate their coherent manipulation and detection using single, shallow NV centers for readout. We observe heterogeneity in the local magnetic environments of the photoactivated spins from site to site, likely due to variations in inter-radical dipolar couplings across our measurements. Looking forward, our approach enables correlative nanoscale magnetic/optical imaging and opens new pathways for single-molecule magnetic resonance spectroscopy and quantum many-body simulations in strongly interacting dipolar-coupled spin ensembles.