High-Resolution Frequency Measurement of the Ground-State Hyperfine Splitting of 113Cd+ Ions

Time-keeping clock is one of the most important parts in COMPASS global navigation satellite system (GNSS) of China and the synchronization of these time-keeping clocks is the basis of GNSS. Recently, we have engaged in developing a transportable atomic clock based on Cd-113(+) ions, which is potentially applied in the comparison of atomic clocks in different locations, including the time-keeping clocks at the ground stations of the COMPASS system. To operate a high precision microwave atomic clock, one has to measure its clock transition frequency precisely at zero external magnetic field. In this paper, the progress of the precision measurement of the ground-state hyperfine splitting of laser-cooled Cd-113(+) ions is reported. In the previous experiment, the hyperfine splitting was measured to be 15,199,862,854.96(12) Hz, using Ramsey's separated oscillation fields technique. Recently, by upgrading the ion trap apparatus, the control time sequence, and the magnetic field stabilization, we obtained a preliminary improved result of 15,199,862,855.013 Hz. This value is nearly one order of magnitude more accurate than the results obtained before.