  We report a coherence-preserving photon frequency down-conversion experiment
based on difference-frequency generation in a periodically poled Lithium
niobate waveguide, at the single-photon level. The coherence of the process has
been demonstrated by measuring the phase coherence of pseudo single-photon
time-bin qubits after frequency conversion with an interference visibility of >
96 %. This interface could be of interest for quantum repeater based hybrid
networks.
