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

Research On Sympathetic Cooling 113 Cd + - 174 Yb + System By Molecular Dynamics Simulation

S. N. Miao, Jianwei Zhang, N. C. Xin, L. M. Guo, H. X. Hu, Wenxiong Shi, H. R. Qin, J. Z. Han, L. J. Wang

Published Jul 7, 2021• 2 citations
Publisher

Abstract

A microwave ion clock based on113Cd+ions at Tsinghua University has been carried out for eleven years. To improve the performance of the ion clock with lower second-order Doppler frequency shift and suppressed Dick effect, a high-performance scheme for sympathetically cooling113Cd+ions based on molecular dynamics simulation is reported.
DOI
10.1109/eftf/ifcs52194.2021.9604306

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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.
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.
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.
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.
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.