1. HRL Laboratories, 3011 Malibu Canyon Rd, Malibu, California 90265, USA
2. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
3. Department of Physics and Astronomy, Purdue University, 525 Northwestern Ave., West Lafayette, Indiana 47907, USA
4. Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
5. Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark