1 State Key Laboratory of Precision Spectroscopy, Quantum Institute for Light and Atoms, Department of Physics, East China Normal University, Shanghai 200062, China
2 School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei 230009, China
3 Department of Physics, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202, USA
4 School of Physics and Astronomy, and Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240, China
5 e-mail: chyuan@phy.ecnu.edu.cn
6 e-mail: lqchen@phy.ecnu.edu.cn