1. Department of Applied Physics, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
2. ICORP Spin Information Project, Japan Science and Technology Agency, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
3. Institute for Quantum Computing, University of Waterloo, 200 University Ave. W., Waterloo, N2L 3G1, Canada