1. NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi 243-0198, Japan.
2. Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan.
3. Solution Oriented Research for Science and Technology (SORST), Japan Science and Technology Agency, 4-1-8 Honmachi, Kawaguchi 331-0012, Japan.
4. Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan.