1. Graduate School of Information Sciences, Tohoku University, Sendai 980-8579, Japan
2. Graduate School of Informatics, Kyoto University, Kyoto 606-8501, Japan
3. Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152-8551, Japan
4. Sigma-i Co., Ltd., Tokyo 108-0075, Japan