1. Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, I-90123 Palermo, Italy
2. Lundbeck Foundation Theoretical Center for Quantum System Research and Department of Physics and Astronomy, University of Aarhus, DK 8000 Aarhus C, Denmark
3. Dipartimento di Fisica, Università di Trento, and CRS INFM-BEC, I-38050 Povo, Italy
4. Institut für Quanten-Informationsverarbeitung, University of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany