  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.
