Generation of nonclassical narrowband photon pairs from a cold rubidium cloud

We report the generation of nonclassical narrowband photon pairs via the spontaneous four-wave mixing process in a cold Rubidium (Rb-87) cloud prepared in a magneto-optical trap. The generated photon pairs exhibit strong violation of the Cauchy-Schwarz inequality by a factor of 625 +/- 45, confirming nonclassical correlation between the photons. The biphoton wave packets, measured for several different experimental conditions, are in good agreement with the theoretical results.