Bloch oscillations and an accelerator for cold atoms

Ultracold atoms in a periodic optical potential subjected to a constant external force provide an experimental realization of Bloch's theory of quantum transport, originally developed for electrons in solids. We observe the momentum distribution of Bloch states and Bloch oscillations in the lowest two bands of the potential. In addition to a solid-state analysis, we give a quantum optics interpretation of this effect in terms of photon exchanges between the atoms and the laser waves. The coherent transfer of a large number of photon momenta (approximate to 100) by means of adiababtic rapid passages leads to an efficient and dissipation-free acceleration of atoms. This technique produces atomic beams with a very small longitudinal velocity spread, which may find applications in atom optics and precision measurements.