1. Department of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden
2. Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
3. Department of Physics, University of Chicago, Chicago, Illinois 60637, USA
4. 3rd Institute of Physics, University of Stuttgart and Institute for Quantum Science and Technology IQST, 70569 Stuttgart, Germany
5. Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany
6. Center for Molecular Engineering and Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA