  Two-dimensional hexagonal boron nitride (hBN) has been known to host a
variety of quantum emitters with properties suitable for a broad range of
quantum photonic applications. Among them, the negatively charged boron vacancy
(VB-) defect with optically addressable spin states has emerged recently due to
its potential use in quantum sensing. Compared to spin defects in bulk
crystals, VB- preserves its spin coherence properties when placed at
nanometer-scale distances from the hBN surface, enabling nanometer-scale
quantum sensing. On the other hand, the low quantum efficiency of VB- has
hindered its use in practical applications. Several studies have reported
improving the overall quantum efficiency of VB- defects using plasmonic
effects; however, the overall enhancements of up to 17 times reported to date
are relatively modest. In this study, we explore and demonstrate much higher
emission enhancements of VB- with ultralow-loss nano-patch antenna (NPA)
structures. An overall intensity enhancement of up to 250 times is observed for
NPA-coupled VB- defects. Since the laser spot exceeds the area of the NPA,
where the enhancement occurs, the actual enhancement provided by the NPA is
calculated to be ~1685 times, representing a significant increase over the
previously reported results. Importantly, the optically detected magnetic
resonance (ODMR) contrast is preserved at such exceptionally strong
enhancement. Our results not only establish NPA-coupled VB- defects as
high-resolution magnetic field sensors operating at weak laser powers, but also
provide a promising approach to obtaining single VB- defects.
