Controlling spin relaxation with a cavity

Spontaneous emission of radiation is one of the fundamental relaxation mechanisms for a quantum system. For spins, however, it is negligible compared to non-radiative relaxation~processes due to their weak coupling to the electromagnetic field. In 1946, Purcell~realized [1] that spontaneous emission is strongly enhanced when the quantum system is placed in a resonant cavity~- an effect now used to control the lifetime of systems with an electrical dipole [2]. Here, by coupling donor spins in silicon to a high quality factor superconducting microwave cavity of~small mode volume, we reach the regime where spontaneous emission constitutes the dominant spin~relaxation channel [3]. The relaxation rate is increased by three orders of magnitude when the spins are tuned to the cavity~resonance, showing it can be engineered and controlled on-demand. Our results provide a~novel way to initialize any spin into its ground state, with applications in magnetic resonance and quantum~information processing. They also show for the first time an alteration of spin dynamics by quantum fluctuations, a step towards the coherent magnetic coupling of a~spin to microwave photons. [1] E. M. Purcell, Phys.~Rev. 1946, 69, 681. \newline [2] P. Goy et al., PRL. 50, 1983. [3] A. Bienfait et al., arxiv~:1508.06148