It should be noticed that the transfer of polarization to the nuclear spins is almost instantaneous upon switching on the microwave field: a stationary nuclear polarization is achieved coherently in a time of the order 𝑇 "# * . It is due to mixing of the electron and nuclear spin states induced by the microwave field, when it is tuned near the resonance frequency of the electron spins. Therefore, we refer to the effect as resonant mixing (RM). It is also a differential effect because the resulting nuclear polarization is the difference between the contributions of ZQ and the DQ matrices. Furthermore, not all electron polarization is transferred to the nuclear spin.