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.