1. Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany.
2. Department of Physics and Astronomy, Aarhus University, DK 8000 Aarhus C, Denmark.
3. Department of Physics, University of California, Berkeley, CA 94720, USA.
4. Fakultät für Physik, Ludwig-Maximilians-Universität München, 80799 München, Germany.