Coupling Nanostructured Plasmon–Strain Microwave Waveguide to Spin Defects in Hexagonal Boron Nitride for High‐Sensitivity Quantum Sensors

Despite seamless integration of hexagonal boron nitride (hBN) with on-chip devices, the intrinsically low optical quantum yield of spin-active boron vacancy ( V B - VB^ - ) defects remains a significant limitation to the sensitivity of hBN-based quantum sensors. Here, we demonstrate an hBN quantum sensor with enhanced quantum yield and high DC magnetic field sensitivity (ηDC), achieved by coupling V B - VB^ - defects in hBN with a nanostructured plasmon-strain microwave waveguide architecture.This platform is realized by fabricating arrays of alumina-coated gold nanopillars, or plasmonic nanoresonators (PNRs), onto the constricted region of a microwave-efficient, single-port gold coplanar waveguide. The alumina coating acts as a dielectric barrier that suppresses photoluminescence (PL) quenching, while gold nanopillars enhance local electromagnetic fields and induce strain-driven perturbations of the defect energy levels, causing accelerated photo-emission. This synergistic effect results in a ∼tenfold enhancement in PL and improves optically detected magnetic resonance to -17% for on-PNR regions, exceeding comparable prior works by over an order of magnitude. Consequently, we achieve an ηDC of 9.4 µT/√Hz, approaching the highest reported values for V B - VB^ - defects. This research establishes a strategy for designing and fabricating highly sensitive quantum sensors that operate at room temperature without requiring extensive optimization of laser or microwave fields.