1. Department of Physics, University of Gothenburg, 41296 Gothenburg, Sweden
2. Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
3. Chalmers University of Technology, 41296 Gothenburg, Sweden
4. Center for Joint Quantum Studies and Department of Physics, Tianjin University, Tianjin 300072, China