1. Department of Physics; University of Science and Technology Beijing; Beijing 100083 P. R. China
2. School of Physics and Astronomy; University of Nottingham; Nottingham NG7 2RD UK
3. State Key Laboratory of Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 P. R. China
4. College of Materials Science and Opto-Electronic Technology; University of Chinese Academy of Science; Beijing 100049 P. R. China
5. Center for Excellence in Topological Quantum Computation; University of Chinese Academy of Science; Beijing 100049 P. R. China