1. Walther-Meißner Institute, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
2. Physics Department, Technical University of Munich, 85748 Garching, Germany
3. Nanosystems Initiative Munich, 80799 Munich, Germany
4. Munich Center for Quantum Science and Technology (MCQST), 80799 Munich, Germany
5. Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany
6. Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA