1. Danish Fundamental Metrology (DFM), Matematiktorvet 307, DK-2800 Kgs. Lyngby, Denmark, and Mikroelektronik Centret (MIC), Technical University of Denmark (DTU), Building 345 east, DK-2800 Kgs. Lyngby, Denmark, and Department of Physics and the Quantum Protein Center, Technical University of Denmark (DTU), Building 309, DK-2800 Kgs. Lyngby, Denmark