1. Fakultät für Physik, Ludwig-Maximilians-Universität München, Schellingstrasse 4, 80799 Munich, Germany.
2. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany.
3. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Germany.
4. Graduate School Materials Science in Mainz, Gottlieb-Daimler-Strasse 47, 67663 Kaiserslautern, Germany.
5. Department of Physics, Harvard University, Cambridge, MA 02138, USA.
6. Institute for Quantum Information and Matter, Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA.
7. Department of Physics, Stanford University, Stanford, CA 94305, USA.
8. Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK.