  We propose a method that enables efficient frequency conversion of quantum
information based on recently demonstrated strong parametric coupling between
two single-photon pulses propagating in a slow-light atomic medium at different
group velocities. We show that an incoming single-photon state is efficiently
converted into another optical mode in a lossless and shape-conserving manner.
The persistence of initial quantum coherence and entanglement within frequency
conversion is also demonstrated. We first illustrate this result for the case
of small frequency difference of converted photons, and then discuss the
modified scheme for conversion of photon wavelengths in different spectral
ranges. Finally we analyze the generation of a narrow-band single-photon
frequency-entangled state.

Quantum Physics