Polarizing electron spins with a superconducting flux qubit

TL;DR

This paper proposes a method to actively polarize electron spins using a superconducting flux qubit by matching Rabi and Larmor frequencies, overcoming thermal limitations at low magnetic fields.

Contribution

It introduces a spin-lock technique with dephasing to enhance electron spin polarization via a superconducting flux qubit, enabling high polarization in realistic conditions.

Findings

High electron spin polarization achievable with the proposed scheme.

Adding dephasing improves polarization by preventing dark state trapping.

The method is effective under realistic experimental parameters.

Abstract

Electron spin resonance (ESR) is a useful tool to investigate properties of materials in magnetic fields where high spin polarization of target electron spins is required in order to obtain high sensitivity. However, the smaller magnetic fields becomes, the more difficult high polarization is passively obtained by thermalization. Here, we propose to employ a superconducting flux qubit (FQ) to polarize electron spins actively. We have to overcome a large energy difference between the FQ and electron spins for efficient energy transfer among them. For this purpose, we adopt a spin-lock technique on the FQ where the Rabi frequency associated with the spin-locking can match the resonance (Larmor) one of the electron spins. We find that adding dephasing on the spins is beneficial to obtain high polarization of them, because otherwise the electron spins are trapped in dark states that cannot be coupled with the FQ. We show that our scheme can achieve high polarization of electron spins in realistic experimental conditions.