1. Hefei National Laboratory for Physical Sciences at the Microscale; University of Science and Technology of China; Hefei Anhui 230026 P.R. China
2. Synergetic Innovation Center of Quantum Information & Quantum Physics; University of Science and Technology of China; Hefei Anhui 230026 P.R. China
3. Beijing National Laboratory for Condensed Matter Physics; Institute of Physics; Chinese Academy of Sciences; Beijing 100190 P.R. China
4. Department of Theoretical Chemistry and Biology; School of Biotechnology; Royal Institute of Technology; S-10691 Stockholm Sweden