1. College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, United States
2. Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, United States
3. Viterbi School of Engineering, University of Southern California, Los Angeles, California 90089, United States
4. Max Planck Institute of Quantum Optics, Hans-Kopfermann-Straße 1, Garching 85748, Germany