1. Centre for Quantum Photonics, H. H. Wills Physics Laboratory, and Department of Electrical and Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK.
2. Department of Physics, Imperial College London, London SW7 2AZ, UK.
3. Nokia Technologies, Broers Building, 21 J. J. Thomson Avenue, Cambridge CB3 0FA, UK.
4. NTT Basic Research Laboratories, Nippon Telegraph and Telephone (NTT) Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan.
5. NTT Device Technology Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan.