1. Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
2. Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan
3. Department of Physics, and Research and Education Center for Natural Sciences, Keio University, Yokohama 223-8511, Japan
4. ERATO Macroscopic Quantum Control Project, JST, Tokyo 113-8656, Japan