1. Centre for Quantum Dynamics and Centre for Quantum Computation and Communication Technology, Griffith University, Brisbane, Queensland 4111, Australia.
2. Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.
3. Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering, University of Bristol, BS8 1FD, UK.
4. National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA.