[1] Dipartemento di Matematica e Informatica, Università degli Studi di Salerno, Fisciano (SA), Italy
[2] The School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
[3] Departament de Física, Universitat Autònoma de Barcelona Bellaterra, E-08193, Catalonia, Spain
[4] Max-Planck-Institut fur Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748, Garching, Germany