Soutenance par visio-conférence (suivre le lien).
Spins in solids interact only weakly with their electromagnetic environment and in usual situations they reach thermal equilibrium by exchanging energy with their host lattice. However, recent experiments have demonstrated that radiative emission can become the fastest energy relaxation channel for the electron spins if the sample is inserted in a resonant microwave cavity of small mode volume and low loss rate, as predicted by Purcell. In this regime spins are then expected to thermalize to the cavity mode regardless of the lattice temperature. This thesis presents the demonstration of this idea, by showing that spins can be radiatively cooled below the sample temperature by coupling the cavity to a cold thermal radiation source. The experiment is realized with an ensemble of electron spins consisting of bismuth donors in silicon coupled to a micron-size superconducting resonator and the spin temperature is inferred by measuring the spin polarization with pulsed electron spin resonance techniques. A more than twofold increase of polarization is observed when the resonator input is connected to a cold resistive load, proving that spins are radiatively cooled with respect to their host lattice. The demonstrated technique represents a new and universal method to enhance electron spin polarization beyond thermal equilibrium, with potential applications in electron spin resonance spectroscopy.