1. JILA, University of Colorado Boulder and the National Institute of Standards and Technology, Boulder, Colorado 80309, United States
2. Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, United States
3. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
4. Université de Lyon, CNRS, Université Claude Bernard Lyon 1, Institut Lumière Matière, 69622 Villeurbanne, France
5. Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris 05, France
6. Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States