Intensity-intensity correlations as a probe of quantum interferences /sup n/der conditions of none in the intensity

Summary form only given. We study the intensity-intensity correlation function and demonstrate that for two four-level atoms quantum interference in the second order correlations of the emitted fluorescence light can be observed for the case of joint detection with two detectors. We show that the depth of modulation in such coincidences can be 100% for both /spl sigma/- and /spl pi/-polarised light, independent of the strength of the driving field. In contrast, the visibility of the interference pattern of the far field intensity profile, which can be observed in the case of /spl pi/-polarised emitted radiation, strongly depends on the power of the laser driving the atoms. We can interpret these results in terms of interferences in the two photon decay channels of the system and show that such interferences are absent in case of single photon decay. The observed interference in intensity-intensity correlations can also be explained in terms of the entanglement induced by the detection of the first photon.