1. School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China
2. School of Physics, Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Capital Normal University, Beijing 100048, P. R. China
3. Department of Physics, National Taiwan University, Taipei 106, Taiwan, ROC
4. Institute of Physics, Faculty of Electrical Engineering and Information Technology, University of the Bundeswehr Munich, Neubiberg 85577, Germany
5. Quantum Information Center, Chung Yuan Christian University, Taoyuan 32023, Taiwan, ROC