1. NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan
2. CREST, JST, 4-1-8 Honmachi, Kawaguchi, 331-0012, Japan
3. Institute of Physics, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8571, Japan