1. Quantum Spin Information Project, ICORP-JST, Atsugi, Kanagawa 243-0198, Japan
2. Institute for Nano Quantum Information Electronics, University of Tokyo, Meguro, Tokyo 153-8505, Japan
3. Low Temperature Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
4. NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan
5. Department of Applied Physics, School of Engineering, University of Tokyo, Bunkyo, Tokyo 133-8656, Japan
6. Institute for Microstructural Sciences M50, National Research Council, Ottawa, Ontario K1A 0R6, Canada