Figure 1: Schematic view of the experimental setup.

(a) A cold cloud of ⁸⁷Rb atoms confined in a magneto-optical trap (MOT) serves as quantum memory for light. Inelastic scattering of a classical write pulse results in the emission of a Stokes photon, which heralds the presence of a collective spin excitation. A subsequent read pulse maps the atomic state onto an anti-Stokes photon. The retrieved light is sent to a PPLN waveguide, together with a strong pump at 1,569 nm. The 780 nm photons are converted to 1,552 nm by DFG. After conversion, residual pump light is blocked by means of a bandpass filter (BF). The converted light is then coupled in a single mode optical fibre. A fibre Bragg grating filters out the broadband noise generated by the pump. The non-classical nature of the storage, retrieval and conversion processes is probed by measuring the cross-correlation between Stokes and (un)converted anti-Stokes photons. (b) Level scheme used for the DLCZ memory. The write pulse off-resonantly couples the ground state |g› to the excited level |e›. A Stokes photon is emitted with probability p, thus creating a single collective excitation to the storage state |s›. During the retrieval process, a pulse resonant with the |s›→|e› transition (read beam) collectively transfers back the atom to the initial state |g› while emitting an anti-Stokes single photon in a well-defined spatio-temporal mode.