1. College of Materials Science and Opto-Electronic Technology Center of Materials Science and Optoelectronics Engineering CAS Center for Excellence in Topological Quantum Computation and CAS Key Laboratory of Vacuum Physics University of Chinese Academy of Sciences Beijing 100049 P.R. China
2. National Laboratory of Solid State Microstructures School of Physics and Collaborative Innovation Center for Advanced Microstructures Nanjing University Nanjing 210093 P.R. China