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Laser-cooled trapped ions can be considered as a nearly isolated system.Hence,in this system the ions are unperturbed and have a long decoherence time.In recent years,quantum information processing experiments taking advantage of this characteristic of the trapped ions have made great progress.[1-4] Due to the long interaction time between the radiation field and the atomic ions,high-resolution spectroscopic measurement of the trapped ions can also be performed.Practically,this offers several promising candidates for atomic clocks with both ultra-high stability and accuracy.[5,6] Compared to ions cooled by collision with buffer gas,the frequency standards based on laser-cooled ions can greatly reduce the secondorder Doppler frequency shift (SODFS) by minimizing micromotion and thermal secular motion.