1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences , Beijing 100094 , P. R. China
2. Songshan Lake Materials Laboratory , Dongguan 523808 , Guangdong , P. R. China
3. State Key Laboratory of Low-Dimensional Quantum Physics , Tsinghua University , Beijing 100084 , P. R. China
4. University of Chinese Academy of Sciences , Beijing 100049 , P. R. China
5. Frontier Science Center for Quantum Information , Beijing 100084 , P. R. China