1. JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, United States
2. Quantum
Electronics and Photonics Division, National Institute of Standards and Technology, Boulder, Colorado 80305, United States
3. Department
of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States
4. Department of Molecular, Cellular, and Developmental
Biology, University of Colorado, Boulder, Colorado 80309, United States