1. National Institute of Standards and Technology, Gaithersburg, MD, USA
2. Joint Center for Quantum Information and Computer Science, University of Maryland, College Park, MD, USA
3. Quantum Information Processing Group, Raytheon BBN Technologies, Cambridge, MA, USA
4. Centre for Quantum Information & Quantum Control and Department of Physics, University of Toronto, Toronto, ON, Canada
5. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, CA, USA