BLOCH OSCILLATIONS AND AN ACCELERATOR FOR COLD ATOMS

Ultracold atoms in a periodic optical potential subjected to a constan t external force provide an experimental realization of Bloch's theory of quantum transport, originally developed for electrons in solids. W e observe the momentum distribution of Bloch states and Bloch oscillat ions 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. T he coherent transfer of a large number of photon momenta (approximate to 100) by means of adiababtic rapid passages leads to an efficient an d dissipation-free acceleration of atoms. This technique produces atom ic beams with a very small longitudinal velocity spread, which may fin d applications in atom optics and precision measurements.