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Chinese Physics Letters

Cooling and Crystallization of Trapped 113 Cd + Ions for Atomic Clock

Shiguang Wang, Jianwei Zhang, K. Miao, Zhengbo Wang, Lijun Wang

Published Jan 1, 2013• 8 citations
Publisher

Abstract

We report recent experimental progress toward a trapped 113Cd+ ion microwave frequency standard. With laser cooling, a large number of trapped cadmium ions are cooled to crystallization and the temperature of the ion crystal is measured to be 16±3 mK. The second order Doppler frequency shift, a main limiting factor of the atomic clock's performance, is estimated to be less than 1 × 10−16.

DOI
10.1088/0256-307x/30/1/013703

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Applied Physics Letters•Aug 21, 2023•8 citations
We report recent experimental progress toward a trapped 113Cd+ ion microwave frequency standard. With laser cooling, a large number of trapped cadmium ions are cooled to crystallization and the temperature of the ion crystal is measured to be 16±3 mK. The second order Doppler frequency shift, a main limiting factor of the atomic clock's performance, is estimated to be less than 1 × 10−16.
This paper reports direct temperature determination of sympathetically cooled 113Cd+ ions with laser-cooled 24Mg+ in a linear Paul trap. The sympathetically cooled ion species distribute in the outer shell of the large ensembles, which contain up to 3.3 × 105 ions. With optimized parameters, the minimum temperature of the sympathetically cooled 113Cd+ ions was measured to be on the order of 10 mK. These results are promising for the performance of microwave atomic clocks. The second-order Doppler frequency shift is two orders of magnitude lower (from 1.88 × 10−14 to 6.26 × 10−16) and the Dick effect is suppressed.
Abstract As a candidate for the next generation microwave clock, microwave atomic clocks based on trapped ions are currently studied worldwide. Since 2010, our team at Tsinghua University has been committed to developing a high-performance 113Cd+ microwave ion clock. In 2021, the laser-cooled cadmium-ion microwave clock constructed in our laboratory demonstrated remarkable performance, with a short-term instability of 4.2 × 10 − 13 / τ and a relative frequency uncertainty of 1.8 × 10−14, improving nearly three orders of magnitude compared to the result from JPL. The 0-0 ground-state hyperfine splitting frequency of 113Cd+ was determined to be 15199862855.02799(27) Hz, currently the most accurate measurement result for this transition frequency. Apart from that, we introduced the sympathetic cooling technology into the experiment of cadmium-ion microwave clock. Using 40Ca+ as the coolant ion, we obtained Ramsey patterns with a free evolution time of up to 100 seconds, ultimately improving the short-term instability of the system to 3.48 × 10 − 13 / τ . Moreover, a microwave frequency standard based on laser-cooled 171Yb+ ions has also been developed in our laboratory since 2021. The short-term frequency instability was measured to be 8.5 × 10 − 13 / τ . The ground-state hyperfine splitting frequency of 171Yb+ was determined to be 12642812118.4674(8) Hz with a fractional frequency uncertainty of 6.33 × 10−14.
Microwave frequency standards based on ions are promising as the next generation atomic clocks. This paper reports the recent progresses of the microwave frequency standard based on laser-cooled113Cd+ions developed at Tsinghua University. Based on the current experimental setup, the ground-state hyperfine splitting of113Cd+is measured with higher precision and a better short-term frequency stability is obtained than before. To improve the performance of the Cd+clock further, a new experimental setup with dual-traps is designed, and bi-ions operation is proposed.
A microwave frequency standard based on113Cd+ions at Tsinghua University has been carried out for twelve years. To overcome the limitations of the Dick effect and the second-order Doppler frequency shift (SODFS), we propose to use heavy174Yb+as the coolant ions to sympathetically cool the large113Cd+cloud, which is expected to further improve the performance of the cadmium-ion microwave frequency standard. This paper reports our recent experimental and simulation progress towards the cadmium-ion microwave frequency standard.
We reported the realization of a 174Yb+–113Cd+ bi-species Coulomb crystal comprising 174Yb+, a heavier ion, as coolant and verified the potential of applying a coolant ion with a greater mass than the clock ion to sympathetically cooled microwave frequency standard. The two species of massive ions stably trapped in a Paul trap make up this large two-component crystal. The 113Cd+ ions are trapped in the center, which reduces considerably RF heating and excess micro-motion to which the 113Cd+ ions are subjected. Under this scheme, the uncertainty due to the second-order Doppler effect is reduced to 5×10−16, which represents an order of magnitude improvement over sympathetic cooled 40Ca+–113Cd+ crystal. The second-order Zeeman frequency shift, which contributes the largest term of the systematic frequency shifts to the microwave ion frequency standard, is estimated to be as low as 7.133×10−13 corresponding to the low value of the operating magnetic field. The relevant AC Stark shift uncertainty is estimated to be 4×10−19. These results demonstrate that using a heavier coolant ion is far superior and confirm the feasibility of a sympathetic-cooled cadmium-ion microwave clock system employing a 174Yb+–113Cd+ bicrystal.