1. College of Materials Science and Opto-Electronic Technology &, Center of Materials Science and Optoelectronics Engineering &, CAS Center for Excellence in Topological Quantum Computation & CAS Key Laboratory of Vacuum Physics University of Chinese Academy of Sciences Beijing 100049 P. R. China
2. Department of Physics, Chemistry and Biology (IFM) Linköping University 58183 Linköping Sweden
3. Department of Physics The Chinese University of Hong Kong New Territories Hong Kong 999077 P. R. China