1. Univ Lyon, Univ Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Villeurbanne, France
2. Department of Physics, Ludwig-Maximilians-Universität Munich, Am Coulombwall 1, 85748 Garching, Germany
3. Max Planck Institute of Quantum Optics, Hans-Kopfermann-Straße 1, 85748 Garching, Germany