1. School of Computing, Tokyo Institute of Technology, Yokohama 226-8503, Japan
2. Graduate School of Information Science, Hiroshima City University, Hiroshima 731-3194, Japan
3. Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan
4. Physics and Informatics Laboratories, NTT Research Inc., Sunnyvale, CA 94085, United States of America
5. E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, United States of America