1. CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China.
2. Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
3. National Institute for Materials Science, Namiki 1-1, Ibaraki 305-0044, Japan.
4. CEMS, RIKEN, Wako-shi, Saitama 351-0198, Japan.
5. Department of Physics, University of Michigan, Ann Arbor, MI 48109–1040, USA.