1. Institut für Theoretische Physik, Nichtlineare Optik und Quantenelektronik, Technische Universität Berlin, Hardenbergstr. 36, EW 7-1, 10623 Berlin, Germany.
2. JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309–0440, USA.
3. Department of Physics, University of Colorado, Boulder, Boulder, CO 80309–0390, USA.
4. Department of Physics, University of Michigan, Ann Arbor, MI 48105–1040, USA.
5. Department of Physics and Astronomy, University of Denver, Denver, CO 80208–6900, USA.