1. School of Physics, University of Melbourne VIC 3010 (Australia)
2. Present address: Department of Experimental Physics, University of Torino, Torino 10126, Italy
3. Clarendon Laboratory, University of Oxford Oxford, OX1 3PU (UK)
4. Department of Materials, University of Oxford Oxford, OX1 3PH (UK)
5. Quantum Communications Victoria, School of Physics University of Melbourne VIC 3010 (Australia)