1. Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China.
2. Department of Physics, Zhejiang University, Hangzhou 310027, Zhejiang, China.
3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
4. State Key Laboratory for Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China.
5. Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.
6. Department of Physics, Stanford University, Stanford, CA 94305, USA.
7. Center for Advanced Study, Tsinghua University, Beijing 100084, China.