Controlling spin relaxation with a cavity

By coupling donor spins in silicon to a superconducting microwave cavity and tuning the spins to the cavity resonance, the rate of spin relaxation is increased by three orders of magnitude compared to that of detuned spins; in such a regime, spontaneous emission of radiation is the dominant mechanism of spin relaxation. New spin on the Purcell effect The Purcell effect, in which the slow rate of spontaneous emission from a quantum system is accelerated in a resonant cavity, is central to quantum optics. Here, Patrice Bertet and colleagues demonstrate an analogue of the Purcell effect in a system of spins in solids. The spontaneous emission in this system affects spin relaxation, and the authors show how to modulate spin relaxation through three orders of magnitude. This could give researchers a means of controlling and tuning spin relaxation. Spins in solids, in this case donor spins in silicon, are promising platforms for quantum information processing, and this technique could have ramifications for new spin qubit architectures. Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized 1 that the rate of spontaneous emission can be greatly enhanced by placing the quantum system in a resonant cavity. This effect has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave 2 or optical 3 , 4 cavities, and is essential for the realization of high-efficiency single-photon sources 5 . Here we report the application of this idea to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity with a high quality factor and a small mode volume, we reach the regime in which spontaneous emission constitutes the dominant mechanism of spin relaxation. The relaxation rate is increased by three orders of magnitude as the spins are tuned to the cavity resonance, demonstrating that energy relaxation can be controlled on demand. Our results provide a general way to initialize spin systems into their ground state and therefore have applications in magnetic resonance and quantum information processing 6 . They also demonstrate that the coupling between the magnetic dipole of a spin and