1. Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA
2. Quantum Measurement Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
3. Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634, USA
4. Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
5. Department of Physics, University of Colorado, Boulder, Colorado 80309, USA