1. Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100080, China
2. CCAST (World Laboratory), P.O. Box 8730, Beijing 100080, China
3. Department of Modern Applied Physics, Center of Atomic and Molecular Sciences, Tsinghua University, Beijing 100084, China
4. Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-99, Beijing 100080, China