High Precision Atomic Clock Based On Cadmium Ions

Time-keeping clock is one of the most important parts in BeiDou Navigation Satellite System（BDS） of China and the synchronization of these time-keeping clocks is the basis of BDS. For thepotential application in BDS, we have engaged in developing the¹¹³Cd⁺ions atomic clock, which istransportable and can have high stability performance.The15.2GHz ground-state hyperfine splitting of¹¹³Cd⁺ions is used as the clock transition. Alarge ion cloud is confined in the quadrupole Paul trap, hence the first-order Doppler effect is deeplysuppressed. In order to reduce the second-order Doppler effect, the trapped ions are cooled down vialaser cooling. Furthermore, since the energy level of¹¹³Cd⁺ion is simple, only one laser is needed inthe experiment to accomplish laser cooling, optical pump, and optical detection. Using Ramsey’sseparated oscillation fields technique, we obtained a narrow Ramsey fringe of¹¹³Cd⁺ions. The firstexperimental system （JMI-1） is for verification. In JMI-1, we obtained the Ramsey fringe of theclock transition successfully, and on that basis, the ground-state hyperfine splittings of¹¹¹Cd⁺and¹¹³Cd⁺ions are measured to be14530507349.9（1.1） Hz and15199862854.96（12） Hz,respectively. The result of¹¹¹Cd⁺ion fills in the gap of high-precision measurement of the transition.And the value of¹¹³Cd⁺ion agrees with previous publications.In order to develop a high-performance clock, we have to obtain a narrower and more preciseRamsey fringe. This thesis reports the upgraded system （JMI-2）. In JMI-2, we re-designed the iontrap, magnetic field, optical part and timing control sequence. In the experiments, approximately104ions were laser cooled to16±3mK. And a2-second free processing time during the Ramseyinterrogation was demonstrated. The clock transition of¹¹³Cd⁺ions at zero external magnetic fieldwas measured to be15199862855.0125（87） Hz, with an frequency uncertainty of5.7×10^-13. Theprecision is improved by one order of magnitude compared to the previous result measured in JMI-1system. Moreover, we realized close-loop locking of the atomic clock based on¹¹³Cd⁺ions for thefirst time. The stabilities at1second and1000seconds are approximately4×10¹3and5×10¹⁴,respectively.Furthermore, this thesis gives the outlook about the next experiment system （JMI-3）. JMI-3willbe upgraded in several ways according to the problems that limit the performance of JMI-1andJMI-2and designed to be more compact. The stability at1000seconds of JMI-3is expected to reachthe10-15level and could be comparable to the hydrogen maser. JMI-3can be potentially applicablein time-keeping clock comparison between the ground stations of the BDS.