1. University College of London, Department of Physics and Astronomy, London WC1E 6BT, United Kingdom
2. Centre for Quantum Technologies, National University of Singapore, Singapore
3. QuTech, Delft University of Technology, Lorentzweg 1, 2611 CJ Delft, The Netherlands
4. Institute für Theoretische Physik, Universität Ulm, D-89069 Ulm, Germany