  The coherent manipulation of wave packets is an important tool in many areas
of physics. We demonstrate the experimental realization of quasi-free wave
packets of ultra-cold atoms bound by an external harmonic trap. The wave
packets are produced by modulating the intensity of an optical lattice
containing a Bose-Einstein condensate. The evolution of these wave packets is
monitored in-situ and their reflection on a band gap is observed. In direct
analogy with pump-probe spectroscopy, a probe pulse allows for the resonant
de-excitation of the wave packet into localized lattice states at a long,
controllable distance of more than 100 lattice sites from the main component.
This coherent control mechanism for ultra-cold atoms thus enables controlled
quantum state preparation, opening exciting perspectives for quantum metrology
and simulation.
