1. Joint Quantum Institute, NIST/University of Maryland, College Park, Maryland 20742, United States
2. Quantum Technology Center, NIST/University of Maryland, College Park, Maryland 20742, United States
3. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, U.K.
4. Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, United States