1. JILA, National Institute of Standards and Technology and University of Colorado, Boulder, CO 80309, USA.
2. Department of Physics, University of Colorado, Boulder, CO 80309, USA.
3. Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck, Austria.
4. Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria.
5. Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA.
6. Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland, College Park, MD 20742, USA.