Nanodiamonds (NDs) hosting nitrogen-vacancy (NV) centers are promising for applications of quantum sensing. Long spin relaxation times ($T_1$ and $T_2$) are critical for high sensitivity in quantum applications. It has been shown that fluctuations of magnetic fields due to surface spins strongly influences $T_1$ and $T_2$ in NDs. However, their relaxation mechanisms have yet to be fully understood. In this paper, we investigate the relation between surface spins and $T_1$ and $T_2$ of single-substitutional nitrogen impurity (P1) centers in NDs. The P1 centers located typically in the vicinity of NV centers are a great model system to study the spin relaxation processes of the NV centers. By employing high-frequency electron paramagnetic resonance (EPR) spectroscopy, we verify that air annealing removes surface spins efficiently and significantly reduces their contribution to $T_1$.

中文翻译：

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减少纳米金刚石中表面自旋诱导的电子自旋弛豫

承载氮空位 (NV) 中心的纳米金刚石 (ND) 有望用于量子传感应用。长自旋弛豫时间（$T_1$ 和 $T_2$）对于量子应用中的高灵敏度至关重要。已经表明，由于表面自旋引起的磁场波动强烈影响 ND 中的 $T_1$ 和 $T_2$。然而，它们的松弛机制尚未完全了解。在本文中，我们研究了表面自旋与 ND 中单取代氮杂质 (P1) 中心的 $T_1$ 和 $T_2$ 之间的关系。P1 中心通常位于 NV 中心附近，是研究 NV 中心自旋弛豫过程的绝佳模型系统。通过采用高频电子顺磁共振 (EPR) 光谱，