1. Nanoinstitut München Department of Physics Ludwig‐Maximilians‐Universität Munich Königinstraße 10 80539 München Germany
2. Chair of Nano and Quantum Sensors Department of Electrical and Computer Engineering Technical University of Munich Theresienstraße 90 80333 München Germany
3. Walter Schottky Institut and Department of Physics Technical University of Munich Am Coulombwall 4 85748 Garching Germany
4. Department of Physics Imperial College London Londond SW7 2AZ UK