1. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, P. R. China
2. Centre for Atom Optics and Ultrafast Spectroscopy, Swinburne University of Technology, Melbourne 3122, Australia
3. Department of Physics and Astronomy, and Rice Quantum Institute, Rice University, Houston, TX 77251, USA
4. Center for Cold Atom Physics, Chinese Academy of Science, Wuhan 430071, China