1. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
3. RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan