  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. In the quantum limit the spin lifetime is
determined by phononic vacuum fluctuations. However, this limit was not
observed in previous studies due to thermal phonon contributions or
phonon-bottleneck processes. Here we use a dispersive detection scheme based on
cavity quantum electrodynamics (cQED) to observe this quantum limit of spin
relaxation of the negatively charged nitrogen vacancy ($\mathrm{NV}^-$) centre
in diamond. Diamond possesses high thermal conductivity even at low
temperatures, which eliminates phonon-bottleneck processes. We observe
exceptionally long longitudinal relaxation times $T_1$ of up to 8h. 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
$\mathrm{NV}^-$ transition frequency enables the spin polarization to survive
over macroscopic timescales.
