Albanese, Bartolo
Date: Friday, September 20, 2019
Time: 10:30
Place: ETH Zurich, Hönggerberg, HPF G 6
Host: Yiwen Chu
Cooling a spin ensemble with a cavity
Bartolo Albanese
CEA-Saclay, France
Electron spin resonance (ESR) spectroscopy is widely employed for the detection and characterization of paramagnetic species and their magnetic and chemical environment [1]. In ESR the spins precessing around an applied static magnetic field are first excited by microwaves and subsequently emit a signal into an inductively coupled resonant cavity. A high degree of polarization is essential to maximize the signal. Hyperpolarization techniques based on optical pumping have been developed in the past decades but the requirements on the optical properties limit their range of application. Here we present a new universal hyperpolarization scheme based on radiative cooling, where a spin ensemble in the mode volume of a cavity is thermalized to a black body radiation colder than the sample. For spins in free space spontaneous emission of photons is orders of magnitude slower than any other relaxation process. However, by coupling the spins to a cavity of small mode volume and low loss rate it is possible to reach the regime in which radiative relaxation is the fastest thermalization process [2], as predicted by Purcell [3].
The spin system under study is an ensemble of bismuth donors implanted into a host silicon crystal and inductively coupled to a high quality factor superconducting niobium resonator. The sample is installed at the 900 mK stage of a dilution cryostat while the resonator is coupled via a switch either to a 20 mK or to a 900 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 900 mK, leading to an increase of ESR signal by more than a factor 2.
[1] A. Schweiger and G. Jeschke, Oxford University Press, 225, (2001).
[2] A. Bienfait, Nature, 531, 74-77, (2016).
[3] E. Purcell, Phys. Rev., 69, 681, (1946).