1. Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland
2. Quantum Center, ETH Zürich, CH-8093 Zürich, Switzerland
3. Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan