1. Physik-Institut, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
2. Department of Biology, University of Fribourg, Chemin du Musée 10, 1700 Fribourg, Switzerland
3. Department of Molecular Life Sciences, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland