1. Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, USA
2. Associate of the National Institute of Standards and Technology, Boulder, Colorado 80305, USA
3. Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
4. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
5. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
6. Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA