1. Forschungszentrum Jülich, Institute of Quantum Control (PGI-8), D-52425 Jülich, Germany
2. University of Cologne, Institute of Theoretical Physics, , D-50937 Köln, Germany
3. University of Innsbruck, Institute for Theoretical Physics, A-6020 Innsbruck, Austria
4. Freie Universität Berlin, Dahlem Center for Complex Quantum Systems, D-14195 Berlin, Germany.