The response of a SERF atomic magnetometer to a repetitive short-pulsed pump was investigated. Quantum sub-resonances at a repetition rate of $1/n$ of the Larmor frequency of the magnetic field inside the shield are experimentally observed and theoretically explained. This is a type of synchronization phenomenon. Investigations in single alkali atoms cells as well as mixed alkali atoms of K and Rb are presented. In the later, one species is pumped while the probe is on the other specie polarized by spin exchange. The effect of spin destruction, spin exchange and collisions are studied in order to account for the width of the resonances. Quantum calculations of a three levels $Lambda$ model for this phenomenon exhibit a dip at the resonance frequency in the absorption spectrum for both cases of pulsed and CW pump modes and an evidence for EIT.
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