  Raman effect in gases can generate an extremely long-living wave of coherence
that can lead to the establishment of an almost perfect periodic variation of
the medium refractive index. We show theoretically and numerically that the
equations, regulate the pulse propagation in hollow-core photonic crystal
fibers filled by Raman-active gas, are exactly identical to a classical problem
in quantum condensed matter physics -- but with the role of space and time
reversed -- namely an electron in a periodic potential subject to a constant
electric field. We are therefore able to infer the existence of Wannier-Stark
ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally
associated with condensed matter physics only, now realized with purely optical
means in the temporal domain.
