地面原子时标是我国正在研制的北斗导航系统中最重要的部分之一，地面原子时标核心守时钟之间的高精度同步是整个导航系统的基础。由于镉离子原子钟在高稳定度、小型化设计方面有着极大的潜力和独特的优势，因此针对这一需求，我们展开了镉离子原子钟的研究。镉离子原子钟以113Cd+离子基态超精细能级分裂0-0跃迁作为钟跃迁能级，跃迁频率约为15.2GHz。实验中，利用四极线型Paul离子阱囚禁离子以抑制一阶多普勒效应对原子钟的影响，并利用激光冷却的方法进一步降低二阶多普勒效应带来的影响。由于镉离子能级结构简单，实验中只需利用一台激光器即可实现离子的激光冷却、光泵浦和光探测。为得到线宽极窄的中心谱线，提高频标的准确度，实验中采用Ramsey分离振荡场技术对镉离子能级进行探测。在第一代验证性实验系统（JMI-1）上，成功地探测到镉离子的Ramsey谱线信号，并精确地测量了111Cd+和113Cd+离子基态超精细能级分裂，二者的频率值分别为14 530 507 349.9(1.1)Hz和15 199 862 854.96(12)Hz。111Cd+离子的测量结果填补了该跃迁线高精度测量的空白，113Cd+离子测量结果在历史测量数据误差范围内，精度略有提高。为了得到谱线更窄、精度更高的钟跃迁谱线，以实现镉离子原子钟高稳定度的设计指标，实验中针对包括离子阱、磁场、光路系统、控制系统等装置进行了改进。在搭建完成的改进型实验系统（JMI-2）上，实现了对约104个离子的有效冷却，温度为16±3mK，Ramsey自由演化时间提高至2秒。在此系统上进一步测量了113Cd+离子零磁场钟跃迁频率，同时对测量过程中的二阶多普勒频移、二阶塞曼频移和黑体辐射频移等系统频移进行了评估，最终测得的频率值为15 199 862 855.0125(87)Hz，不确定度为5.7×10-13，测量结果在JMI-1测量误差范围内，精度提高了近一个数量级，该结果是目前国际上报道的精度最高的测量结果。在JMI-2实验系统上，我们还首次实现了镉离子频标的闭环锁定，与中国计量院提供的标准时频信号对比测量后，实验系统的秒级稳定度为4×10-13，千秒稳定度达到5×10 14。这是目前国际上第一个实现闭环锁定的激光冷却镉离子微波原子钟。在上述工作基础上，本文对正在开展的小型化镉离子频标实验系统（JMI-3）的研究进行了展望。JMI-3实验系统针对之前限制实验系统性能的主要因素实施改进，并考虑了小型化的设计。预期JMI-3系统千秒稳定度可进入10-15量级，与氢钟指标接近，并实现可搬运，将有望应用于北斗地面守时钟之间的高精度比对。
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 the potential application in BDS, we have engaged in developing the 113Cd+ ions atomic clock, which is transportable and can have high stability performance.The 15.2 GHz ground-state hyperfine splitting of 113Cd+ ions is used as the clock transition. A large ion cloud is confined in the quadrupole Paul trap, hence the first-order Doppler effect is deeply suppressed. In order to reduce the second-order Doppler effect, the trapped ions are cooled down via laser cooling. Furthermore, since the energy level of 113Cd+ ion is simple, only one laser is needed in the experiment to accomplish laser cooling, optical pump, and optical detection. Using Ramsey’s separated oscillation fields technique, we obtained a narrow Ramsey fringe of 113Cd+ ions. The first experimental system (JMI-1) is for verification. In JMI-1, we obtained the Ramsey fringe of the clock transition successfully, and on that basis, the ground-state hyperfine splittings of 111Cd+ and 113Cd+ ions are measured to be 14 530 507 349.9(1.1)Hz and 15 199 862 854.96(12)Hz, respectively. The result of 111Cd+ ion fills in the gap of high-precision measurement of the transition. And the value of 113Cd+ ion agrees with previous publications.In order to develop a high-performance clock, we have to obtain a narrower and more precise Ramsey fringe. This thesis reports the upgraded system (JMI-2). In JMI-2, we re-designed the ion trap, magnetic field, optical part and timing control sequence. In the experiments, approximately 104 ions were laser cooled to 16±3 mK. And a 2-second free processing time during the Ramsey interrogation was demonstrated. The clock transition of 113Cd+ ions at zero external magnetic field was measured to be 15 199 862 855.0125(87) Hz, with an frequency uncertainty of 5.7×10-13. The precision is improved by one order of magnitude compared to the previous result measured in JMI-1 system. Moreover, we realized close-loop locking of the atomic clock based on 113Cd+ ions for the first time. The stabilities at 1 second and 1000 seconds are approximately 4×10 13 and 5×10 14, respectively.Furthermore, this thesis gives the outlook about the next experiment system (JMI-3). JMI-3 will be upgraded in several ways according to the problems that limit the performance of JMI-1 and JMI-2 and designed to be more compact. The stability at 1000 seconds of JMI-3 is expected to reach the 10-15 level and could be comparable to the hydrogen maser. JMI-3 can be potentially applicable in time-keeping clock comparison between the ground stations of the BDS.
