1. HH Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, United Kingdom
2. School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
3. Centre for Nanoscience and Quantum Information, Tyndall Avenue, Bristol BS8 1FD, United Kingdom
4. Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7-9, 55128 Mainz, Germany