1. Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522
2. Macroscopic Quantum Coherence Laboratory, FRS, RIKEN, Saitama 351-0198
3. Tsukuba Research Center for Interdisciplinary Materials Science and Institute of Physics, University of Tsukuba, Ibaraki 305-8571
4. UC Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.
5. Department of Physics, Sophia University, Tokyo 102-8544