  By using parametric down-conversion process with a strong signal field
injection, we demonstrate coherent frequency down-conversion from a pump photon
to an idler photon. Contrary to a common misunderstanding, we show that the
process can be free of quantum noise. With an interference experiment, we
demonstrate that the coherence is preserved in the conversion process. This may
lead to a high fidelity quantum state transfer from high frequency photon to
low frequency photon and connects a missing link in a quantum network. With
this scheme of coherent frequency down-conversion of photons, we propose a
method of single-photon wavelength division multiplexing.
