Generating correlated (2+1) -photon pairs in an active-Raman-gain medium

A scheme of generating controllable (2+1) photons in a double- Λ atomic system based on active Raman gain is presented in this paper. Such (2+1) photons can be a potential candidate to generate a correlated photon pair as one photon of "2" photons acts as a trigger. Our proposal is an alternative approach to generate photon pairs where the frequencies are tunable for subsequent atomic system experiments compared to spontaneous parametric down-conversion case. Compared to other schemes of generating correlated photon pairs, our scheme exhibits several features due to the exploit of the stimulated Raman process and injection-seeding mechanism.