1. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain
2. Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany
3. ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain
4. Univ. Bordeaux, CNRS, LOMA, UMR 5798, F-33400 Talence, France