WannierStark Ladders and Bloch Oscillations in Superlattices

Conduction electrons in real crystalline solids behave very much like electrons in free space, moving in straight lines between collisions when subject to an electric field. But in an ideal (although cold) world, free from scattering by impurities, imperfections and thermal vibrations of the lattice, how would conduction electrons behave? That question, answered in principle long ago in light of the then newly developed quantum mechanics, was purely academic until recently Optical experiments in semiconductor superlattices in electric fields hove shown the existence of a long debated quantum mechanical phenomenon, bringing us closer to demonstrating an extremely fast emitter of radiation.