Spin-coherence characterization of boron vacancy defects in hexagonal boron nitride with broadband microwave pulses

Negatively charged boron vacancy (VB−) defects in hexagonal boron nitride (hBN) are promising for nanoscale-proximity quantum sensing. To evaluate their performance, it is important to characterize the spin-coherence times T2* and T2. In this study, we realized subgigahertz Rabi oscillations of VB− using an isotopically enriched h10B15N thin film directly stamped onto a narrow gold wire. Using these strong microwave pulses, we performed Ramsey interference and Hahn-echo measurements. The Ramsey interference signal showed Gaussian-like decay, yielding T2*=13.8 ± 0.5 ns. The Hahn-echo measurement gave T2=108.7 ± 5.5 ns and a stretch factor of αHE=1.25 ± 0.11. These results experimentally clarify the spin-coherence properties of VB− and provide an effective method for evaluating the coherence of spin defects in van der Waals thin films with broad resonance linewidths.