  The generation of the narrowband strong-correlated biphotons via spontaneous
four-wave mixing can be effectively controlled and enhanced by an additional
driving field which drives a transition with its upper level being a Rydberg
state. We study the properties of the noise of the generated biphotons and show
that in the region of weak pumping and low atomic density, a high degree of the
photon correlation is maintained with the photon-pair generation rate
siginificantly enhanced.
