1. Department of Engineering and Information Technology, Brescia University, Brescia, Italy
2. European Laboratory for Nonlinear Spectroscopy (LENS) and Istituto Nazionale di Ottica (INO), Firenze, Italy
3. Scuola Normale Superiore and CNISM, Pisa, Italy
4. INO-CNR BEC Center and Dipartimento di Fisica, Trento University, 38123 Povo, Italy