  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.

Quantum Physics