1. JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, United States
2. Department of Physics, University of Colorado, Boulder, Colorado 80309, United States
3. Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
4. Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, Colorado 80309, United States