Generating correlated (2+1)-photon in an active Raman gain medium
  A scheme of generating controllable (2+1) photons in a double-$% \Lambda$ 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 the photon `1' acts as a trigger. Compared to other schemes of generating correlated photon pairs, our scheme exhibits a number of advantages due to the exploit of the stimulated Raman process. 
Chun-Hua Yuan, Cui-Ping Lu, Weiping Zhang, and L. Deng