1. School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China
2. Western Seoul Center, Korea Basic Science Institute, Seoul 03579, Republic of Korea
3. Department of Materials Science and Engineering, Hongik University, Sejong 30016, Republic of Korea
4. Department of Electronics Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 807, Taiwan, ROC
5. Institute of Physics, EIT 2, Faculty of Electrical Engineering and Information Technology, University of the Bundeswehr Munich, Neubiberg 85577, Germany
6. Quantum Information Center, Chung Yuan Christian University, Taoyuan 32023, Taiwan, ROC
7. Department of Physics, National Taiwan University, Taipei 106, Taiwan, ROC