1. Department of Physics, Kinki University, Higashiosaka, Osaka 577-8502, Japan
2. Research Center for Quantum Computing, Interdisciplinary Graduate School of Science and Engineering, Kinki University, Higashiosaka, Osaka 577-8502, Japan
3. Department of Physics, Okayama University, Okayama 700-8530, Japan
4. Condensed Matter Theory Laboratory, RIKEN, Wako, Saitama 351-0198, Japan