1. Department of Physics and Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
2. Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106, USA
3. Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA
4. Station Q, Microsoft Research, Santa Barbara, California 93106-6105, USA