1. Department of Electrical Engineering, The University of Tokyo, Tokyo 113-8656, Japan
2. Nanoscale Quantum Photonics Laboratory, RIKEN, Saitama 351-0198, Japan
3. Quantum Optoelectronics Research Team, RIKEN Center for Advanced Photonics, Saitama 351-0198, Japan
4. Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan