1. Center for Spintronics and Quantum Computation, University of California, Santa Barbara, Santa Barbara, CA 93106, USA.
2. Centre for Quantum Computation and Communication Technology, School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney 2052, Australia.
3. School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
4. Department of Physics, Princeton University, Princeton, NJ 08544, USA.