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. Chuncheon Center, Korea Basic Science Institute, Chuncheon 24341, Republic of Korea
4. Institute of Physics, EIT 2, Faculty of Electrical Engineering and Information Technology, University of the Bundeswehr Munich, Neubiberg 85577, Germany
5. Centre for Nanoscience and Nanotechnology, Panjab University, Chandigarh 160014, India
6. Department of Electronics Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 807, Taiwan, ROC
7. Quantum information center, Chung Yuan Christian University, Taoyuan 32023, Taiwan, ROC
8. Department of Physics, National Taiwan University, Taipei 106, Taiwan, ROC