Strong Correlation And Anticorrelation Of Resonance Fluorescence Via Coherent Population Trapping

Atom coherence and quantum interference have led to many interesting optical phenomena such as coherent population trapping (CPT), electromagnetic-induced transparency, laser without inversion and control of spontaneous emission. Recently, much attention has been paid to the effect of quantum coherence and interference on photon correlation. We describe the photon correlation from the system by normalized two-time intensity correlation function. Sometimes, intensity correlation takes its maximal values ( much larger than 2), or it remains below unity for a long time ( several periods or all time). For convenience of discrimination, we refer the former as 'strong correlation' and to the later as 'anticorrelation' .Here, we present the phase control of photon correlation in a driven four-level system in the double lambda configuration. We concern the two fluorescent fields radiated from the excited states, and take the approximate detunings between the driving fields and atom. Simply changing the collective phase of the fields can lead to the switch from strong correlation to anticorrelation. The coherent control of intensity correlation is based upon the phase-dependence coherent population trapping. When the system operates near CPT, the excited states tend to radiate and almost no atoms stay at these states, then we get strong correlation. When the system is far away from CPT, the atoms tends to stay at the excited states and not to radiate, then we get anticorrelation.