1. Department of Physics, Tsinghua University, Beijing 100084, China
2. Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China
3. State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China
4. Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, P. R. China
5. Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
6. Frontier Science Center for Quantum Information, Beijing 100084, China
7. Key Laboratory of Quantum State Construction and Manipulation (Ministry of Education) and Laboratory for Neutron Scattering, Renmin University of China, Beijing 100872, P. R. China