Synergistic ion beam engineering of high-performance spin–photon interfaces in hexagonal boron nitride

Deterministically coupling spin defects to optical microcavities is critical for scalable quantum sensing yet remains a fabrication bottleneck. Here, we demonstrate the deterministic integration of negatively charged boron vacancy (VB−) spin defects in hexagonal boron nitride (h-BN) with circular Bragg grating microcavities using a dual-beam focused ion beam approach. While standard heavy-ion lithography often degrades emitter quality through lattice amorphization, we demonstrate that helium ion (He+) irradiation enables the surgical creation of VB− centers with preserved long-range crystalline order. By combining Ga+ ions milling for cavity definition with in situ He+ ion writing, we achieve sub-15 nm spatial alignment between the emitter and the cavity mode volume, eliminating registration errors inherent to overlay lithography. The fabricated device exhibits highly directional emission with a Purcell factor of 1.57, evidenced by a reduction in fluorescence lifetime. These photonic enhancements translate to high-fidelity capabilities in optically detected magnetic resonance spin readout, establishing this monolithic fabrication strategy as a robust pathway for scalable, high-sensitivity quantum sensing arrays.