Observation of quantum frequency conversion.

Quanfum frequency conversion, a process with which an input beam of light can be converted info an output beam of a differen frequency while preserving the quantum state, is experimentally demonstrated for the first time. Nonclassical intensity correlation (≃3 dB) between two beams at 1064 nm is used as the input quantum property. When the frequency of one of the beams is converted from 1064 to 532 nm, nonclassical intensity correlations (≃1.5 dB) appear between the up-converfed beam and the remaining beam