Electron spin hyperpolarization via radiative cooling

Electron spin resonance (ESR) spectroscopy is widely employed for the detection and characterization of paramagnetic species [1]. A high degree of spin polarization is essential to maximize the signal. Here, we are interested in increasing the polarization beyond thermal equilibrium. Techniques such as optical pumping are available only for systems with special electronic structures. In the present work we give a proof of principle of a new universal hyperpolarization scheme based on the coupling of the electron spins to a colder electromagnetic bath via Purcell-enhanced radiative relaxation. A superconducting micro-resonator is used in order to reach the regime in which radiative relaxation constitutes the dominant mechanism of spin thermalization [2]. The spin system under study is an ensemble of bismuth donors implanted into a host silicon crystal. The sample is installed at the 800 mK stage of a dilution cryostat while the resonator is coupled via a switch either to a 10 mK or to a 800 mK thermal source. When the switch is connected to the colder black body, the electronic spins are cooled via radiative relaxation while the silicon crystal remains at 800 mK.
[1] A. Schweiger and G. Jeschke, Oxford University Press (2001)
[2] A. Bienfait et al., Nature 531, 74-77 (2016)