1. Max Planck Institute for Multidisciplinary Sciences, D-37077 Göttingen, Germany.
2. 4th Physical Institute – Solids and Nanostructures, University of Göttingen, D-37077 Göttingen, Germany.
3. Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
4. Center for Quantum Science and Engineering, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland.