Optical Magnetometer: Quantum Resonances at pumping repetition rate of 1/n of the Larmor frequency

Quantum sub-resonances at 1/n are observed experimentally in a SERF type pulsed optical magnetometer. This is a type of synchronization phenomenon. Sequential short pump laser pulses of 70-200 $\mu$sec, circularly polarized are employed for optical pumping of the alkali vapor in 5 layers shielded magnetometer. The repetition rate of these pulses is 1/n of the Larmor frequency at specified magnetic field produced within the shield. Signal resonances are obtained at these 1/n frequencies. Mixed alkali atoms of K and Rb cells are studied, where one specie is pumped while the probe is on the other species that is 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 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. Such a decrease in the absorption intensity which is the result of quantum interference may cause false sensitivity in the optical magnetometer and in the determination of tiny magnetic fields. Methods in which the magnetic detection employ only frequency changes do not suffer from this effect.