1. Institut für Physik, Johannes Gutenberg Universität-Mainz, 55128 Mainz, Germany
2. Helmholtz-Institut Mainz, GSI Helmholtzzentrum für Schwerionenforschung, 55128 Mainz, Germany
3. ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
4. Institute for Bioengineering of Catalonia, 08028 Barcelona, Spain
5. State Key Laboratory of Brain and Cognitive Science, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China
6. Department of Physics, University of California, Berkeley, California 94720-7300, United States