1. ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain
2. Quantum Information Processing Group, Raytheon BBN Technologies, Cambridge, Massachusetts 02138, United States
3. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States
4. Department of Physics and Astronomy, The University of Manchester, Oxford Road, M13 9PL, Manchester, United Kingdom