1. Department of Physics and JILA, University of Colorado and NIST, Boulder, CO 80309, USA.
2. Research Center OPTIMAS, University of Kaiserslautern, Erwin-Schroedinger Strasse 46, 67663 Kaiserslautern, Germany.
3. Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, CO 80305, USA.
4. Department of Physics and Astronomy, University Uppsala, S-75120 Uppsala, Sweden.
5. Georg-August-Universität Göttingen, I. Physikalisches Institut, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany.
6. School of Science and Technology, Örebro University, SE-701 82 Örebro, Sweden.
7. Department of Engineering Sciences and Mathematics, Luleå University, SE-E186 Luleå, Sweden.