1. Australia Research Council Centre of Excellence for Quantum Computation and Communication Technology and School of Physics; University of New South Wales; Sydney NSW 2052 Australia
2. Department of Electronic Engineering and Material Science and Technology Research Center; The Chinese University of Hong Kong; Shatin N.T. Hong Kong SAR
3. Laboratory of Thin Film Materials, School of Materials Science and Engineering; Beijing University of Technology; Beijing 100022 People's Republic of China