Spin-State-Selective Excitation in Spin Defects of Hexagonal Boron Nitride.

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional platform for quantum sensing due to its optically addressable spin defects, such as the negatively charged boron vacancy (V). Spectral overlap of spin transitions due to large hyperfine interactions has limited its magnetic sensitivity. Here, we demonstrate spin-selective excitation of V spin defects in hBN driven by a circularly polarized microwave. Using a cross-shaped microwave resonance waveguide, we superimpose two orthogonally linearly polarized microwaves shifted in phase from an FPGA to generate circularly polarized microwaves. This enables selective spin |0⟩ → |-1⟩ or |0⟩ → |1⟩ excitation of V defects, as confirmed by optically detected magnetic resonance experimentally and supported computationally. We also investigate the influence of the magnetic field on spin-state selectivity. Our technique enhances the hBN platform for quantum sensing through better spin state control and magnetic sensitivity at low and zero fields.