Longitudinal relaxation is the process by which an excited spin ensemble decays into its thermal equilibrium with the environment. In solid-state spin systems, relaxation into the phonon bath usually dominates over the coupling to the electromagnetic vacuum^{1,2,3,4,5,6,7,8,9}. In the quantum limit, the spin lifetime is determined by phononic vacuum fluctuations¹⁰. However, this limit was not observed in previous studies due to thermal phonon contributions^11,12,13 or phonon-bottleneck processes^{10, 14,15}. Here we use a dispersive detection scheme^16,17 based on cavity quantum electrodynamics^18,19,20,21 to observe this quantum limit of spin relaxation of the negatively charged nitrogen vacancy (NV⁻) centre²² in diamond. Diamond possesses high thermal conductivity even at low temperatures²³, which eliminates phonon-bottleneck processes. We observe exceptionally long longitudinal relaxation times T₁ of up to 8 h. To understand the fundamental mechanism of spin–phonon coupling in this system we develop a theoretical model and calculate the relaxation time ab initio. The calculations confirm that the low phononic density of states at the NV⁻ transition frequency enables the spin polarization to survive over macroscopic timescales.

纵向弛豫是一个过程，激发的自旋系综衰减到与环境的热平衡。在固态自旋系统中，进入声子浴的弛豫通常在耦合至电磁真空^{1,2,3,4,5,6,7,8,9的过程中}占主导地位。在量子极限中，自旋寿命由声子真空波动¹⁰确定。然而，这种限制并没有在以前的研究中观察到由于热声子的贡献^11,12,13或声子瓶颈处理^10，14,15。这里我们使用基于腔量子电动力学^18,19,20,21的色散检测方案^16,17观察负电荷的氮空位的自旋弛豫（NV的这种量子极限^- ）中心²²在金刚石。金刚石即使在低温下也具有很高的热导率²³，从而消除了声子-瓶颈过程。我们观察到异常长的纵向弛豫时间T ₁长达8 h。为了理解该系统中自旋-声子耦合的基本机理，我们建立了一个理论模型并计算了从头算起的弛豫时间。计算确认状态中的所述NV的低声子密度^-过渡频率使自旋极化超过生存宏观时间尺度。