Optical atomic clocks have many applications in the definition of the time and the length, measurement of fundamental constants and their potential variations with time, testing of relativity, very long baseline interferometry, geodesy, and global navigation satellite systems. An optical clock consists of three parts: ultra-stable laser, frequency comb and trapped ions or atoms. The accuracy of optical atomic clocks can be affected by Doppler shift, Zeeman shift, Stark shift, collision shift, blackbody radiation (BBR) shift, etc. The BBR shift is very difficult to be characterized and controlled. Among all optical atomic clocks under current development, Al
+ optical clock has the smallest BBR shift at a level of
4.0×10−19, therefore the Al
+ is a promising ion for high-accuracy optical frequency standard. In this article we give a review on the three parts of optical frequency standards, the basic working principle of Al
+ ion optical frequency standard based on quantum logic technique, and research progress of an Al
+ optical clock under development at Huazhong University of Science and Technology.
The accuracy of state-of-the-art optical atomic clocks has surpassed that of microwave clocks by more than two orders of magnitude. This revolution in accuracy has prompted international dialogue concerning the redefinition of the SI second away from the current cesium standard. Optical clocks employing a single trapped ion are promising candidates for realizing this redefinition as they offer exceptional fractional frequency uncertainty. However, there is still room for improvement: the leading contributions to the uncertainty of most single-ion clocks, such as the 88Sr+ clock operated by the National Research Council of Canada (NRC), are the blackbody radiation shifts and collisional frequency shifts. The uncancelled blackbody radiation shift dominates both the total shift and uncertainty of the existing room temperature strontium ion clocks. To address these sources of inaccuracy, I contributed to developing a strontium ion trap for operation at 4 K. Operating at cryogenic temperatures suppresses the blackbody radiation shift by eight orders of magnitude and collisional shifts by at least two orders of magnitude, enabling substantial improvements to the performance of single-ion clocks. The proposed cryogenic clock is capable of achieving fractional frequency uncertainty below 10−19. This thesis describes the design, construction, and evaluation of the cryogenic ion trap towards realizing a more accurate atomic clock.
An overview of the key methods used to reduce the uncertainty of the systematic shifts of the strontium ion optical frequency standard is presented. An uncertainty of 10 mHz at a clock frequency of 445 THz is obtained, limited by the uncertainty of the blackbody radiation shift. The uncertainty estimates of other shifts such as micromotion and electric quadrupole shifts are on the order of 1 mHz or less. The effectiveness of the methods used to reduce or cancel the shifts is illustrated with experiments comparing two ion traps. One has strong micromotion levels while the other, the reference trap, provides good control over the shifts. Despite nominal shifts of 70 Hz in the trap with large micromotion levels, the agreement with the reference trap is at the sub-Hz level.