Theory of quantum beat and polarization interference in four-wave mixing

We analyse theoretically the quantum beat and the polarisation interference phenomena observed as oscillations in pulsed four-wave mixing. We show that resolution of the signal with respect to the delay between the incoming pulses as well as the detection frequency provides valuable information on the nature of the oscillations. Most importantly, it is very simple to distinguish between polarisation interference from two independent optical transitions and quantum beats from a three-level system. In case of polarisation interference, the phase of the oscillations shifts π when the detected frequency passes any of the resonances. In case of quantum beat, the phase has little dependence on the detected frequency near the resonance