1. Institute for Theoretical Physics, University of Cologne, D-50937 Cologne, Germany
2. Peter Grünberg Institute-Quantum Control (PGI-8), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
3. Dipartimento di Fisica e Astronomia “G. Galilei,” Università degli Studi di Padova, I-35131 Padua, Italy
4. Faculty of Natural Sciences and Technology, Physics, Universität des Saarlandes, Campus E2.6, D-66123 Saarbrücken, Germany
5. Department of Physics, University of Kaiserslautern, Erwin Schrödinger Strasse, 67663 Kaiserslautern, Germany
6. Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Padova, I-35131 Padua, Italy