Interference in the resonance fluorescence of two incoherently coupled transitions

The fluorescence light emitted by a four-level system in J=1/2 to J=1/2 configuration driven by a monochromatic laser field and in an external magnetic field is studied. We show that the spectrum of resonance fluorescence emitted on the pi transitions shows a signature of spontaneously generated interference effects. The degree of interference in the fluorescence spectrum can be controlled by means of the external magnetic field, provided that the Lande g factors of the excited and the ground state doublet are different. For a suitably chosen magnetic field strength, the relative weight of the Rayleigh line can be completely suppressed, even for low intensities of the coherent driving field. The incoherent fluorescence spectrum emitted on the pi transitions exhibits a very narrow peak whose width and weight depend on the magnetic field strength. We demonstrate that the spectrum of resonance fluorescence emitted on the sigma transitions shows an indirect signature of interference. A measurement of the relative peak heights in the spectrum from the sigma transitions allows us to determine the branching ratio of the spontaneous decay of each excited state into the sigma channel.