1. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
2. QuTech, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands, EU
3. Institute of Solid State Physics, Friedrich Schiller University of Jena, Max-Wien-Platz 1, 07743 Jena, Germany
4. Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan