Solid-state electron spin lifetime limited by phononic vacuum modes

The nitrogen-vacancy (NV) center in diamond is important for applications in quantum technologies since it possesses long lifetimes (T1) and coherence times (T2). However, applications requiere knowledge of spin environment interaction.
The process by which a spin ensemble transfers energy to the surrounding is given by T1 relaxation - driven by spin-phonon interaction. The so far available methods did not allow to study T1 at temperatures where quantum effects become relevant. Knowledge of this relaxation phenomenon is important since T1 provides the ultimate limit of coherence.
We study the T1 relaxation of NV spins at low temperatures with a cavity quantum electrodynamics experiment in a 3D lumped element cavity. We measure T1 below the single phonon limit where quantum fluctuations provide the ultimate limit for T1. The low phononic density of states at the NV transition frequency enables the spin polarization to survive over 8h. Our theoretical model describes a direct spin phonon coupling mechanism and we calculate the relaxation rate ab initio based on density functional theory.