  Optically addressable spin defects in hexagonal boron nitride (hBN) have
become a promising platform for quantum sensing. While sensitivity of these
defects are limited by their interactions with the spin environment in hBN,
inefficient microwave delivery can further reduce their sensitivity. Hare, we
design and fabricate a microwave double arc resonator for efficient
transferring of the microwave field at 3.8 GHz. The spin transitions in the
ground state of VB- are coupled to the frequency of the microwave cavity which
results in enhanced optically detected magnetic resonance (ODMR) contrast. In
addition, the linewidth of the ODMR signal further reduces, achieving a
magnetic field sensitivity as low as 42.4 microtesla per square root of hertz.
Our robust and scalable device engineering is promising for future employment
of spin defects in hBN for quantum sensing.
