1. ICFO, Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain
2. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. Quantum Information Processing Group, Raytheon BBN Technologies, Cambridge, Massachusetts 02138, United States
4. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States