Efficient retrieval of a single excitation stored in an atomic ensemble
  We report significant improvements in the retrieval efficiency of a single excitation stored in an atomic ensemble and in the subsequent generation of strongly correlated pairs of photons. A 50% probability to transform the stored excitation into one photon in a well-defined spatio-temporal mode at the output of the ensemble is demonstrated. These improvements are illustrated by the generation of high-quality heralded single photons with a suppression of the two-photon component below 1% of the value for a coherent state. A broad characterization of our system is performed for different parameters in order to provide input for the future design of realistic quantum networks. 
Julien Laurat, Hugues de Riedmatten, Daniel Felinto, Chin-wen Chou,   Erik Schomburg, H. Jeff Kimble