1. Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
2. Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, 30167 Hannover, Germany
3. MOE Key Laboratory of Fundamental Physical Quantities Measurement, School of Physics, Huazhong University of Science and Technology, 430074 Wuhan, People’s Republic of China