1. Key Laboratory of Artificial Structures and Quantum Control, and Shanghai Center for Complex Physics, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
2. Department of Physics, University at Buffalo, the State University of New York, Buffalo, New York 14260, USA
3. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
4. Department of Physics, University of Nebraska-Omaha, Omaha, Nebraska 68182, USA