1. Quantum Spin Information Project, International Cooperative Research Project, Japan Science and Technology Agency, Morinosato Wakamiya 3-1, Atsugi-shi, Kanagawa, 243-0198, Japan.
2. Center for Nanoscale Systems, Harvard University, Cambridge, MA 02138, USA.
3. Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.