  We experimentally demonstrate that a non-classical state prepared in an
atomic memory can be efficiently transferred to a single mode of
free-propagating light. By retrieving on demand a single excitation from a cold
atomic gas, we realize an efficient source of single photons prepared in a
pure, fully controlled quantum state. We characterize this source using two
detection methods, one based on photon-counting analysis, and the second using
homodyne tomography to reconstruct the density matrix and Wigner function of
the state. The latter technique allows us to completely determine the mode of
the retrieved photon in its fine phase and amplitude details, and demonstrate
its nonclassical field statistics by observing a negative Wigner function. We
measure a photon retrieval efficiency up to 82% and an atomic memory coherence
time of 900 ns. This setup is very well suited to study interactions between
atomic excitations, and to use them in order to create and manipulate more
sophisticated quantum states of light with a high degree of experimental
control.
