1. Quantum-Phase
Electronics Center
and Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
2. Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan, and PRESTO,
Japan Science and Technology Agency, Chiyodaku, Tokyo 102-0076, Japan
3. Center for Emergent Matter Science
(CEMS), Riken, Hirosawa 2-1, Wako 351-0198,
Japan