Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity.

The negatively charged boron vacancy (V) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, V preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of V defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of V with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled V defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single V defects.