  We propose a method that enables efficient conversion of quantum information
frequency between different regions of spectrum of light based on recently
demonstrated strong parametric coupling between two narrow-band single-photon
pulses propagating in a slow-light atomic medium [1]. We show that an input
qubit at telecom wavelength is transformed into another at visible domain in a
lossless and shape-conserving manner while keeping the initial quantum
coherence and entanglement. These transformations can be realized with a
quantum efficiency close to its maximum value.
