QUROPE Quantum Information Processing and Communication in Europe

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 non-
classical field statistics by observing a negative Wigner function. We measure a photon retrieval
efficiency up to 80% 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.