1. Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, Copenhagen DK-2100, Denmark.
2. Institute of Physics, University of Muenster, Muenster 48149, Germany.
3. CeNTech—Center for Nanotechnology, Muenster 48149, Germany.
4. SoN—Center for Soft Nanoscience, Muenster 48149, Germany.
5. Wolfson Institute for Biomedical Research, University College London, London, UK.
6. Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstrasse 150, Bochum D-44780, Germany.
7. Heidelberg University, Im Neuenheimer Feld 227, Heidelberg 69120, Germany.