[1] Univ Oxford, Dept Phys, Clarendon Lab, Oxford OX1 3PU, England
[2] Natl Univ Singapore, Ctr Quantum Technol, Singapore 117543, Singapore
[3] Monash Univ, Sch Phys & Astron, Clayton, Vic 3800, Australia
[4] Abdus Salam Int Ctr Theoret Phys ICTP, Trieste, Italy