1. Institute of Physics and University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P.R. China
2. CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, P.R. China
3. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, P.R. China
4. Department of Physics and Astronomy and Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, United States