Photon-pair generation on resonance via a dark state

We present a scheme to generate photon-pair states through resonant spontaneous four-wave mixing (SFWM) in a five-level double-bowtie (Da) configuration subject to two pumping fields and one coupling field when a quasidark state is created between two of three ground levels. The quasidark state is used here to largely suppress linear resonant absorption (gain) of the generated photon pairs while supporting strong SFWM nonlinearities and nonclassical cross-correlation. Numerical results for cold atomic samples show that the generation efficiency may be comparable to or even larger than that obtained in the off-resonance four-level single-bowtie configuration with only one pumping field. This scheme is robust to fluctuations of atomic populations and enables one to easily control the ratio of two rates with which distinct SFWM processes generate photon-pair states.