1. State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, P. R. China
2. Department of Physics and Beijing Key Laboratory of Opto-electronic Functional Materials and Micro-nano Devices, Renmin University of China, Beijing 100872, P. R. China
3. International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, P. R. China
4. Frontier Science Center for Quantum Information, Beijing 100084, P. R. China