1. Department of Computer Science and Technology, Tsinghua University, Beijing 100084, P. R. China
2. National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, P. R. China
3. School of Software, Tsinghua University, Beijing 100084, P. R. China
4. Department of Physics, Tsinghua University, Beijing 100084, P. R. China
5. State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, P. R. China