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European Frequency and Time Forum

Precision Determination of the Ground-State Hyperfine Splitting in a 113Cd+ Microwave Clock

S. N. Miao, Y. H. Zheng, Yiting Chen, Wen‐Xin Shi, Jianwei Zhang, Lijun Wang

Published Jun 25, 2024• 1 citation
Publisher

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$^{113}\text{Cd}^{+}$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$4.2\times 10^{-13}/\sqrt{\tau}$and a relative frequency uncertainty of$1.8\times 10^{-14}$, improving nearly three orders of magnitude compared to the result from JPL. The ground-state hyperfine splitting of$^{113}\text{Cd}^{+}$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$^{40}\text{Ca}^{+}$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$3.48\times 10^{-13}/\sqrt{\tau}$.

DOI
10.1109/eftf61992.2024.10722764

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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.
The precision of Cadmium ion microwave clock based on linear quadrupole trap is mainly determined by the measurement accuracy of hyperfine splitting of the ground state of113Cd+. To reduce the second order Doppler frequency shift(SODFS), which is one of the main frequency shifts, we take advantages of sympathetic cooling by24Mg+ions to keep the Cadmium ion clouds at relatively low temperature during the interrogation of clock transition. By comparing the experiment results with molecular-dynamics(MD) simulation, we discuss the efficiency of sympathetic cooling and its influence on our clock's performance.
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.