Scientists have developed room-temperature calcium ion photoclocks with uncertainty of the order of E-18

Recently, the research team of Gao Kelin and Guan Hua of the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences developed a room-temperature calcium ion clock with an uncertainty of 4.8×10-18, which laid a solid foundation for the next step to achieve a portable calcium ion light clock of the order of 10-18. The findings were published in Physical Review Applied.

Achieving high-precision portable light clocks is a key and necessary condition for realizing light clock applications. In 2017, the International Bureau of Weights and Measures proposed a roadmap for redefining seconds with reference to light clocks, one of which is that the frequency ratio between different light clocks is better than 5×10-18, and transporting high-precision light clocks to various laboratories for frequency comparison is one of the important methods. In the application of relativistic geodesy, the uncertainty of the light clock of 1×10-18 corresponds to the elevation difference of about 1 cm, and the comparison of light clocks with an uncertainty of 10-18 is expected to achieve centimeter-level or sub-centimeter-level elevation difference measurement, which provides a new scheme for elevation measurement. At the same time, the portable light clock is used in the construction of a new generation of integrated PNT system, which can significantly improve the comprehensive performance index of the system.

The research team previously developed a portable calcium ion clock with an uncertainty of 1.3×10-17 and transported it to the Beijing Changping Campus of the Chinese Academy of Metrology, realizing the absolute frequency measurement of calcium ion light clock on the order of 10-16. At the 22nd CCTF meeting held on March 19, 2021, the absolute frequency value of the calcium ion light clock measured by the team was adopted for the fourth time. On April 13, 2022, the International Bureau of Weights and Measures officially adopted calcium ion optical frequency transition as a new reference for the definition of secondary seconds.

The main factor limiting the uncertainty of calcium ion light clock into 10-18 is the BBR shift uncertainty of blackbody radiation. The frequency shift of blackbody radiation is related to the chosen light clock system (the difference in polarizability of the atomic frequency transition) and is proportional to the 4th power of the ambient temperature, which is very sensitive to temperature – the ambient temperature at which the ions are located and the fluctuations in temperature. To suppress the temperature effect, two methods can be used experimentally – directly reducing the ambient temperature of the ion and reducing the temperature change of the environment where the ion is located. These two solutions are suitable for laboratory-type photoclocks and transportable photoclocks with higher requirements for robustness, respectively. The team previously reduced the blackbody radiation shift and uncertainty of calcium ion photoclocks by using a liquid nitrogen cryogenic system to reduce the ambient temperature of ions from room temperature (about 300 K) to liquid nitrogen temperature (about 80 K). After hard work, the blackbody radiation frequency shift uncertainty of liquid nitrogen low-temperature calcium ion photoclock was finally reduced to 2.7×10-18.

In this study, a new room temperature calcium ion photoclock was built to reduce the temperature change of the environment in which the ions are located in order to realize the transportability, and the physical system of the photoclock was actively controlled by the water cooling system, which reduced the ambient temperature uncertainty to ±0.3 oC during the operation of the photoclock. At the same time, finite element analysis was used to calculate the effective stereoscopic angle of each component of the ion trap to calcium ions, and the average temperature of each component of the ion trap was monitored and evaluated by combining the temperature probe and infrared camera in the vacuum, and finally the blackbody radiation frequency shift uncertainty of the room temperature calcium ion clock was reduced to 4.6×10-18. At the same time, through the careful evaluation of the remaining systematic error terms of the calcium ion photoclock, the total system uncertainty of the calcium ion photoclock is 4.8×10-18. On this basis, the total uncertainty obtained by comparing the frequency of the newly constructed room temperature calcium ion photoclock with the existing low-temperature calcium ion photoclock in the laboratory was 7.5×10-18 (statistical uncertainty was 4.9×10-18, and the system uncertainty was 5.7×10-18). This result verifies the reliability of the blackbody radiation frequency shift assessment.

With the continuous improvement of the uncertainty index of calcium ion photoclock, combined with the relatively simple characteristics of calcium ion photoclock, it can be developed into miniaturized, quasi-continuous and high-reliability high-precision portable photoclock, and will be widely used in precision measurement physics, time reference, relativistic geodesy, navigation and positioning, etc.

The research work is supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Chinese Academy of Sciences. (Source: Institute of Precision Measurement Science and Technology Innovation, Chinese Academy of Sciences)

Active temperature-controlled room temperature calcium ion light clock. Left: room temperature calcium ion light clock; Right: Active temperature control based on water cooling system.

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