Nonclassical photon pairs generated from a room-temperature atomic ensemble

Abstract
We report experimental generation of nonclassically correlated photon pairs from collective emission in a room-temperature atomic vapor cell. The nonclassical feature of the emission is demonstrated by observing a violation of the Cauchy-Schwarz inequality. Each pair of correlated photons is separated by a controllable time delay up to 2μs. This experiment demonstrates an important step towards the realization of the Duan-Lukin-Cirac-Zoller scheme for scalable long-distance quantum communication.