Interference between competing quantum-mechanical pathways for four-wave mixing

The interference between two different quantum-mechanical pathways for four-wave mixing in sodium vapor has been observed. The four-wave mixing was enhanced by two-photon resonance with the 5s state, for one pathway, and the 4d state, for the other. The phase of the interference could be varied continuously by varying laser frequencies, and constructive and destructive interference effects were studied. A model based on third-order perturbation theory gives a good description of the features observed.