1. Laboratory for Solid State Physics, ETH Zurich, 8093 Zurich, Switzerland
2. Department of Quantum Nanoscience, TU Delft, 2600 GA Delft, The Netherlands
3. Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland
4. Swiss Light Source, Paul Scherrer Institut, PSI, 5232 Villigen, Switzerland