1. Centre of Excellence for Quantum Computer Technology, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney NSW 2052, Australia
2. Department of Applied Physics/COMP, Helsinki University of Technology, P.O. Box 5100, FI-02015 TKK, Finland
3. Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland
4. Centre of Excellence for Quantum Computer Technology, School of Physics, University of Melbourne, Melbourne VIC 3010, Australia