We report coherent coupling between two macroscopically separated nitrogen-vacancy electron spin ensembles in a cavity quantum electrodynamics system. The coherent interaction between the distant ensembles is directly detected in the cavity transmission spectrum by observing bright and dark collective multiensemble states and an increase of the coupling strength to the cavity mode. Additionally, in the dispersive limit we show transverse ensemble-ensemble coupling via virtual photons.
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 to observe this quantum limit of spin relaxation of the negatively charged nitrogen vacancy (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 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.
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