Polarization-correlated photon pairs in the fluorescence from a bichromatically driven four-level atom

We examine the correlation between photons in the resonance fluorescence of a bichromatically driven four-level atom. The four-level atom consists of two twofold degenerate states and, as a consequence, spontaneous emission events become sensitive to polarization. Depending on the interaction strength with the bichromatic field as well as its polarization direction, nonlinear two-photon emission takes place which, in turn, leads to extremely strong correlations between certain polarized photons. On the other hand, correlations between photons differently polarized from the distinctive ones are suppressed. Thus, the light source establishes a promising candidate for the generation of Bell-type correlated photon pairs. We explain the unusual large correlations with a nonlinear four-photon process which crucially depends on the strength of the driving as well as its polarization. (Author)