Generation of two-photon correlation in a four-level system

We study the quantum correlation of a coherently foul-level atomic system coupled by two coherent driven fields. Used the wave function technique, an explicit expression of the photon-photon correlation function is derived. The influence of the driven field as well as the detuning on the second correlation function are investigated. We find that the second correlation function can be controlled by the driven field or the detuning of two coupling lasers. So we can control the magnitude of the joint detection probability by the driving fields. More importantly, we show that this correlated photon states can serve as atomic memory.