1. Department of Physics and Astronomy, George Mason University, Fairfax, Virginia 22030, United States
2. Quantum Science and Engineering Center, George Mason University, Fairfax, Virginia 22030, United States
3. Department of Physics, University at Buffalo, Buffalo, New York 14260, United States
4. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States