1. Sensor Science Division, National Institute of Standards and Technology 1 , Gaithersburg, Maryland 20899, USA
2. Joint Quantum Institute 2 , College Park, Maryland 20742, USA
3. Physics Department, University of Maryland 3 , College Park, Maryland, 20742, USA
4. Quantum Measurement Division, National Institute of Standards and Technology 4 , Gaithersburg, Maryland 20899, USA