  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.
