Microwave-enhanced electron spin polarization at millikelvin temperatures

Electron paramagnetic resonance methods are widely used in many areas of research, however sometimes the sensitivity of standard tools is not enough to register the signal. To increase signal intensity, few polarization methods, among them are expensive low temperature/high field experiments, or reserved to special samples light-induced polarization and chemically-induced polarization are used. In the current project, we would like to present a modification of recently demonstrated radiative cooling of spins by Albanese et al [1]. In our project we propose to introduce efficient heat sink to reduce the spin temperature below the equilibrium field temperature. Briefly, in our planned experiments we will fabricate multimode planar superconducting resonator in a way that low-frequency resonator in RF domain will be in Purcell regime with electron spins below. High-frequency MW resonator will be effectively coupled to cold environment of dilution fridge. Photon conversion process between the RF-MW fields, which will reduce the field temperature of the RF mode, is present due to non-linear kinetic inductance of the superconducting material and will be modulated by microwave pumps. In turn, Purcell relaxation of the spins to the RF field will cool them well below fridge temperature.