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_B^-). Spectral overlap of spin transitions due to large hyperfine interactions has limited its magnetic sensitivity. Here, we demonstrate spin-selective excitation of V_B^- 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_B^- 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.