1. Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands
2. Department of Mathematics, University of Michigan, Ann Arbor, Michigan 48109, USA
3. Australian Research Council Centre of Excellence for Quantum Computation and Communications, School of Physics, University of New South Wales, Sydney 2052, Australia
4. Institute for Systems based on Optoelectronics and Microtechnology (ISOM), Universidad Politecnica de Madrid, Avenida Complutense 30, 28040 Madrid, Spain