1. Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne CH-1015, Switzerland
2. Vienna Center for Quantum Science and Technology (VCQ), University of Vienna, 1090 Vienna, Austria
3. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, U.K.
4. Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States