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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Progress on Ion Microwave Frequency Standards at Tsinghua University

H. R. Qin, Y.Z. Zheng, L. J. Wang, N. C. Xin, S. N. Miao, Y. T. Chen, Wenxiong Shi, J. Z. Han, Jianwei Zhang

Published May 15, 2023• 0 citations
Publisher

Abstract

SummaryThe ion microwave frequency standard is a candidate for the next generation of practical microwave frequency standards, which has a very wide application potential. Tsinghua University has continuously been working on the research and development of ion microwave frequency standards. A demonstration experiment of two laser-cooled cadmium ion microwave clocks with zero dead time shows a short-term stability of τ-0.95averaging. Besides, a microwave frequency standard based on cadmium ions sympathetically cooled by ytterbium ions has been realized, and the clock signal has been obtained. In addition, two new laser-cooled ytterbium ion microwave clocks are also being built.

DOI
10.1109/eftf/ifcs57587.2023.10272154

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SummaryThe ion microwave frequency standard is a candidate for the next generation of practical microwave frequency standards, which has a very wide application potential. Tsinghua University has continuously been working on the research and development of ion microwave frequency standards. A demonstration experiment of two laser-cooled cadmium ion microwave clocks with zero dead time shows a short-term stability of τ-0.95averaging. Besides, a microwave frequency standard based on cadmium ions sympathetically cooled by ytterbium ions has been realized, and the clock signal has been obtained. In addition, two new laser-cooled ytterbium ion microwave clocks are also being built.
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.
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-performance113Cd+^{113}\text{Cd}^{+}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 of4.2×10−13/τ4.2\times 10^{-13}/\sqrt{\tau}4.2×10−13/τ​and a relative frequency uncertainty of1.8×10−141.8\times 10^{-14}1.8×10−14, improving nearly three orders of magnitude compared to the result from JPL. The ground-state hyperfine splitting of113Cd+^{113}\text{Cd}^{+}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. Using40Ca+^{40}\text{Ca}^{+}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 to3.48×10−13/τ3.48\times 10^{-13}/\sqrt{\tau}3.48×10−13/τ​.
The ion microwave frequency standard is a candidate for the next generation of microwave frequency standard with the potential for very wide applications. The Dick effect and second-order Doppler frequency shift (SODFS) limit the performance of ion microwave frequency standards. The introduction of sympathetic cooling technology can suppress the Dick effect and SODFS and improve the stability and accuracy of the frequency standard. However, the sympathetically cooled ion microwave frequency standard has seldom been studied before. This paper reports a high-performance sympathetically cooled ion microwave frequency standard in a Paul trap. Using laser-cooled 40Ca+{}^{40}{\mathrm{Ca}}^{+}40Ca+ as coolant ions, 113Cd+{}^{113}{\mathrm{Cd}}^{+}113Cd+ ion crystal is cooled to below 100 mK and has a coherence lifetime of over 40 s. The short-term frequency stability reached $3.48\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}/{\ensuremath{\tau}}^{1/2}$, which is comparable to that of the mercury ion frequency standard. Its uncertainty is $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$, which is better than that of directly laser-cooled cadmium ion frequency standard.