[1] Natl Inst Stand & Technol, Quantum Phys Div, Boulder, CO 80309 USA
[2] E China Normal Univ, Quantum Opt Lab, Shanghai 200062, Peoples R China
[3] Univ Colorado, Joint Inst Lab Astrophys, Boulder, CO 80309 USA
[4] Duke Univ, Dept Phys, Durham, NC 27708 USA
[5] Calif Polytech Univ, Quantum Opt Grp, Pasadena, CA 91125 USA