The negatively charged boron vacancy ($V_{\text{B}}^{-}$) in two-dimensional (2D) hexagonal boron nitride (hBN) has emerged as a promising candidate for quantum sensing. The coherence time of $V_{\text{B}}^{-}$ spins which coherent quantum sensing resides in is limited by spin-phonon interactions, while the underlying physical mechanism of the interactions is still not fully understood. Here, we probe the relaxation rates on $V_{\text{B}}^{-}$ centers’ $|0⟩\leftrightarrow |\pm ⟩$ (single-quantum, SQ) and $|+⟩\leftrightarrow |-⟩$ (double-quantum, DQ) over the temperature range from 198 to 393 K. The results show that both relaxation rates increase with increasing temperature, and the increase of the DQ relaxation rate is much stronger than that of the SQ relaxation rate. At high temperature (above 393 K), our theoretical estimations indicate that the DQ relaxation rate exceeds three times the SQ relaxation rate, implying that the contribution of DQ relaxation surpasses that of SQ relaxation in the decoherence channel of spin-phonon interactions. Moreover, the experimental results are consistent with calculations based on the theoretical model of second-order spin-phonon interactions, which can be further simplified to a form that interacts with three effective phonon modes (23.48, 77.39, and 165.75 meV). Our findings contribute to a further understanding and provide guidance for practical sensing applications.

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• Coherent control
• Nuclear & electron resonance
• Optical pumping
• Spin relaxation