1. Department of Physics and Center for Quantum Technology, National Tsing Hua University, Hsinchu, Taiwan
2. Department of Physics, University of Tokyo, Tokyo, Japan
3. National Synchrotron Radiation Research Center, Hsinchu, Taiwan
4. Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
5. Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan