1. Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, A-6020 Innsbruck, Austria
2. Department of Physics, Stockholm University, 106 91 Stockholm, Sweden
3. Center for Joint Quantum Studies and Department of Physics, Tianjin University, Tianjin 300072, China
4. Department of Physics, University of New Hampshire, Durham, New Hampshire 03824, USA