1. NanoLund and Division of Solid State Physics, Lund University, Box 118, S-22100 Lund, Sweden
2. Department of Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden
3. Beijing Key Laboratory of Quantum Devices, Key Laboratory for the Physics and Chemistry of Nanodevices, and Department of Electronics, Peking University, Beijing 100871, China
4. Beijing Academy of Quantum Information Sciences, West Bld. #3, No.10 Xibeiwang East Rd., Haidian District, Beijing 100193, China