1. Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
2. Quantum Universe Center, Korea Institute for Advanced Study, Seoul 02455, Republic of Korea
3. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
4. Quantum Information Science Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
5. Department of Physics, Hanyang University, Seoul 04763, Republic of Korea
6. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021Karlsruhe, Germany
7. Max Planck School of Photonics, 07745 Jena, Germany
8. Department of Physics, Stellenbosch University, Stellenbosch 7602, South Africa