1. Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
2. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Göteborg, Sweden
3. Institute for Theory of Statistical Physics, RWTH Aachen, 52056 Aachen, Germany
4. Peter Grünberg Institut and JARA, Forschungszentrum Jülich, 52425 Jülich, Germany