  State-preserving frequency conversion in the optical domain is a necessary
component in many configurations of quantum information processing and
communication. Thus far, nonlinear crystals are used for this purpose. Here, we
report on a new approach based on coherent anti-Stokes Raman scattering (CARS)
in a dense molecular hydrogen gas. This four-wave mixing process sidesteps the
limitations imposed by crystal properties, it is intrinsically broadband and
does not generate an undesired background. We demonstrate this method by
converting photons from 434 nm to 370 nm and show that their polarization is
preserved.
