1. National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
2. Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd St., Philadelphia, Pennsylvania 19104, USA
3. School of Earth and Space Exploration, Arizona State University, 781 S Terrace Rd., Tempe, Arizona 85281, USA
4. Department of Physics, Stanford University, Stanford, California 94305, USA
5. Quantum Optoelectronics Laboratory, Southwest Jiaotong University, Chengdu, China