[1] Univ London Imperial Coll Sci Technol & Med, Blackett Lab, Quantum Opt & Laser Sci Grp, London SW7 2BW, England
[2] Univ Oxford, Clarendon Lab, Oxford OX1 3PU, England
[3] Natl Univ Singapore, Ctr Quantum Technol, Singapore 117543, Singapore
[4] UCL, Dept Phys & Astron, London WC1E 6BT, England
[5] Univ Politecn Cartagena, E-30202 Cartagena, Spain
[6] Univ Autonoma Barcelona, Dept Fis, E-08193 Bellaterra, Spain
[7] ICFO Inst Ciencies Foton, Barcelona, Spain