Special issue

1 Progress in atomic clocks and the redefinition of the "second"

CHEN Weiliang , LIU Kun , DAI Shaoyang , ZHENG Fasong , ZUO Yani , FANG Fang

2025, 45(2):5-25. DOI: 10.11823/j.issn.1674-5795.2025.02.01

[Abstract](1510) [HTML](195) [PDF 3.76 M](2823)

Abstract:
This paper introduces the current development status of high-performance cold atomic clocks serving as frequency standards，elaborates on the fundamental principles, performance specifications, applications in metrology and other fields, and development trends of fountain clocks and optical clocks. The analysis focuses on the impact of atomic clock technology advancements on the redefinition of the "second", explores the evolutionary pathways and current status of the redefinition of the "second". It is pointed out that the performance of atomic clocks can be improved by increasing the coherent interaction time, reducing the atomic temperature, and optimizing the uncertainty evaluation strategy. It is proposed that the uncertainty level of atomic clocks can be further improved and their application scope expanded by implementing integrated space-time measurements, optimizing the evaluation methods for gravitational redshift, and establishing high-level intercontinental remote comparison links.

2 Research progress on two-way quantum time synchronization

DONG Ruifang , XIANG Xiao , QUAN Run'ai , HONG Huibo , SHI Bingke , LIU Tao , ZHANG Shougang

2025, 45(4):1-11. DOI: 10.11823/j.issn.1674-5795.2025.04.01

[Abstract](1177) [HTML](162) [PDF 12.33 M](399)

Abstract:
Quantum time synchronization is an interdisciplinary frontier technology that integrates quantum technology with time-frequency technology. By leveraging the intrinsic nonlocal time correlation of frequency-entangled biphoton sources, two-way quantum time synchronization not only improves the precision of existing time synchronization by 1～2 orders of magnitude but also possesses inherent security advantages. This provides a new generation of transformative technical solutions for significantly enhancing time service precision and ensuring time service security. This paper focuses on the research progress achieved by the National Time Service Center of the Chinese Academy of Sciences in the field of two-way quantum time synchronization: a model for evaluating the accuracy of two-way quantum time synchronization has been established; the first international demonstration of 10-femtosecond-level ultra-high-precision quantum time synchronization was reported; successful demonstrations of sub-picosecond-level time transfer were achieved on a 2 km free-space + 7 km field fiber hybrid link, hundred-kilometer field fiber link, and a 250 km ultra-long-distance fiber link, fully validating the high-precision synchronization ability of this technology under high-loss and strong-noise environmental conditions; meanwhile, the security advantages of the quantum time transfer system have been experimentally verified. These research achievements not only mark significant progress in the field of long-distance fiber-based quantum secure time transfer in China but also provide a highly compatible time synchronization solution for the future construction of large-scale quantum networks.

3 Dual-mixing time-delay detection of weak frequency signals enabled by local-oscillator optical enhancement

ZHANG Yunrui , GAO Hao , LUO Bin , YU song

2026, 46(1):55-62. DOI: 10.11823/j.issn.1674-5795.2026.01.04

[Abstract](202) [HTML](97) [PDF 6.76 M](315)

Abstract:
In the process of ultra long span fiber frequency transmission, factors such as power attenuation and additional noise of active devices lead to weak frequency signal at the receiving end, which limits the signal detection resolution and sensitivity. To solve this problem, the research team proposed a dual-mixing time-delay detection of weak frequency signals enabled by local-oscillator optical enhancement. The weak carrier signal power was enhanced by a coherent laser, and the dual-mixing time-delay detection structure was used to improve the receiving sensitivity. At the same time, the high signal-to-noise ratio signal extraction was realized by combining with the balanced detection technology. The experimental results show that compared with the traditional intensity modulation / direct detection method, the dual-mixing time-delay detection of weak frequency signals enabled by local-oscillator optical enhancement can effectively improve the sensitivity of the receiver by about 10 dB, the RF power is increased by 25 dB under the same input power condition, and this method achieves excellent frequency stability, with Allen deviation of 2 × 10^-13@1 s and 2.1 × 10^-15@10 000 s, and stability of 3 × 10^-17@1 s and 3 × 10^-19@10 000 s under the condition of difference frequency of 10 kHz, which verifies the feasibility and significant advantages of the proposed method in high-precision optical fiber frequency transmission.

4 Research progress of active optical clock

张佳 , 史田田 , 缪健翔 , 陈景标

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.01

[Abstract](2029) [HTML](289) [PDF 1.49 M](1720)

Abstract:
Since 2005, active optical clock (AOC) has undergone nearly 20 years of development. The AOC utilizes an atomic ensemble as the gain medium, and its stimulated radiation can be used as the clock laser signal directly. Because the AOC works in the bad-cavity region, it has two significant advantages of cavity-pulling suppression and narrow linewidth, which can effectively overcome the cavity length thermal noise problem of the passive optical clock. Due to its superior performance, the AOC has received wide attention from international counterparts. According to the different implementation methods, this paper classifies AOCs into atomic beam type, laser cooling and optical-lattice-trap type, atomic beam and optical lattice "hybrid" type, Faraday atomic filter type, ion-trap type, and thermal atomic cell type. For different types of AOCs, this paper presents the experimental and theoretical research progress in detail and analyzes their advantages and disadvantages. Finally, the application of AOCs in the field of precision measurement is analyzed, and the future development direction of AOCs is prospected, so as to provide reference for promoting the wide application of AOCs.

5 Chip-scale coherent population trapping atomic clock

陈杰华

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.05

[Abstract](2113) [HTML](355) [PDF 1.31 M](2143)

Abstract:
The chip-scale atomic clock technology based on coherent population trapping (CPT) atoms is reviewed, including the basic principle of CPT atomic clock, Ramsey-CPT atomic clock technology, schemes of CPT atomic clock suitable for miniaturization, and the development and current status of the chip-scale CPT atomic clock. The key technologies， such as laser frequency modulation, Ramsey technology, left and right circularly polarized light pumping (push-pull), laser and microwave frequency stabilization, and micro optical-electro-mechanical system (MOEMS), are analyzed and discussed. Finally, it is concluded that the CPT atomic clock is developing in the direction of low power consumption, chip-based and high clock precision.

6 Progress of domestic high performance optically pumped compact cesium clocks

贺轩 , 袁志超 , 陈佳源 , 陈徐宗 , 王青 , 齐向晖

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.09

[Abstract](1923) [HTML](387) [PDF 2.33 M](1451)

Abstract:
The advantage of optically pumped compact cesium clocks is that they have a higher utilization of atoms compared to traditional magnetic state-selection cesium clocks. The group in Peking University has made breakthroughs in the frequency stability of optically pumped compact cesium clocks. The key factors for achieving high performance of optically pumped compact cesium clocks are the cesium beam tube, laser frequency stabilization, and circuits. The frequency stability of the optimized optically pumped compact cesium clock exceeds by more than twice that of the 5071A high-performance cesium beam tubes, with a typical value of 3 × 10^-12 / τ^1/2. Eight optically pumped compact cesium clocks have been developed in the past three years. And the commercial high performance optically pumped compact cesium clocks have been preliminarily realized

7 Mercury ion microwave clock and its research progress

颜碧波 , 陈义和 , 柳浩 , 佘磊

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.10

[Abstract](2182) [HTML](382) [PDF 3.33 M](2131)

Abstract:
Mercury ion microwave clock is an ideal candidate for the next generation spaceborne atomic clock and ground-based timekeeping atomic clock. This paper briefly introduces the operation procedure and research status of mercury ion microwave clock, especially the latest research progress made by our group in the key technologies, including the ion trapping, buffer gas cooling, microwave synthesizer and mercury discharge lamp. On this basis, a miniaturized prototype of mercury ion clock has been built using these technologies and demonstrated frequency stability of 2.3 × 10^-15/10⁵ s. An extended linear ion trap mercury ion clock is also under development. The ions can be trapped and shuttled back and forth efficiently in the extended ion trap. Frequency stability of 3.45 × 10^-13/τ^1/2(τ = 10～10 000 s) has been measured in preliminary close-loop operating. All these works lay an important foundation for the further application of mercury ion microwave clock technology.

8 Experimental study on clock-offset measurement of sub 100 femtosecond resolution over 100 km fiber link with dual-comb system

吴云峰 , 孟飞 , 阿布都维力·阿布力克木 , 宋晏蓉 , 张志刚

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.13

[Abstract](1020) [HTML](303) [PDF 2.83 M](1124)

Abstract:
In order to solve the problem that the clock-offset measurement over a long fiber link is limited to picosecond level, we have completed the experimental study on clock-offset measurement over a 100 km fiber link based on dual-comb linear optical sampling method. The net dispersion of long-distance fiber link and the line width of comb teeth are optimized by using dispersion compensation and comb compression technology. The central time of interferograms can be accurately obtained, and the clock-offset is measured with higher resolution. The time delays over fiber link measured with time interval counter and dual-comb linear optical sampling method are compared, and the clock-offset over a 100 km fiber link is measured. The experimental results demonstrate that the clock-offset measured based on the linear optical sampling method is less than 100 fs. High resolution clock-offset measurement is the key technology of long distances time synchronization. High precision time synchronization technology has important applications in accurate navigation, high speed communication and timing system.

9 Research progress and prospect of transportable optical clocks

潘多 , 刘天宇 , 陈景标

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.04

[Abstract](2030) [HTML](346) [PDF 2.94 M](2064)

Abstract:
Transportable optical clocks are divided into three categories: high-precision transportable optical clocks, portable optical clocks, and balanced optical clocks. The working principles and research progress of these three types of transportable optical clocks are introduced, and their performance advantages and development limitations are analyzed from the perspectives of stability, uncertainty, and system integration. On this basis, the application prospect of transportable optical clock in mobile time service, geodesy, space exploration, micro positioning and other scenarios is prospected. It is proposed that the mobile performance of the high-precision transportable optical clock system should be improved by reducing the environmental sensitivity of the core laser components and improving the robustness of the laser components and vacuum system, the long-term stability of portable optical clocks should be improved by combining vacuum technology, artificial intelligence and other means, and the accuracy of the balanced portable optical clock should be further improved through the iteration of experimental schemes.

10 Research progress of microwave frequency standards based on trapped ions

秦浩然 , 张建伟 , 王力军

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.03

[Abstract](1179) [HTML](340) [PDF 1.54 M](1740)

Abstract:
Microwave frequency standards have been widely used and played an indispensable role in many fields such as satellite navigation, precision measurement, electric power and communication. In recent years, many scientific research institutes in the world have been carrying out research on new-type microwave frequency standards. Among them, the microwave frequency standards based on trapped ions have the advantages of high performance and miniaturization, and have become a new generation of microwave frequency standards of great concern. This paper reviews the research status of ion trap microwave frequency standards, introducing the working principle of Penning trap and Paul trap, and the research motivation, application fields, physical solutions and technical specifications of various ion trap microwave frequency standards, including frequency standards based on ¹⁹⁹Hg⁺, ¹¹³Cd+, ¹⁷¹Yb⁺ and so on. Finally, the application prospects of ion trap microwave clocks in timekeeping, deep space exploration and other fields are introduced.

11 Ca⁺ optical frequency standard with systematic uncertainty and stability at the 10^-18 level

黄垚 , 管桦 , 高克林

2023(3). DOI: 10.11823/j.issn.1674-5795.2023.03.11

[Abstract](977) [HTML](326) [PDF 3.22 M](1156)

Abstract:
This paper mainly reviews the work on the physical system design of the liquid-nitrogen-cooled ⁴⁰Ca⁺ optical clock, the evaluation of its uncertainty and the optimization of the stability for the ⁴⁰Ca⁺ optical clocks in the Innovation Academy for Precision Measurement Science and Technology of the Chinese Academy of Sciences. The liquid-nitrogen-cooled system creates a liquid-nitrogen temperature environment (～ 80 K) for the ⁴⁰Ca⁺, which greatly reduces the blackbody radiation (BBR) frequency shift of the ⁴⁰Ca⁺ optical clock, and improve its systematic uncertainty to 3.0?×?10^-18, becoming the fifth type of atomic/ion optical clocks with uncertainty reaching to the 10^-18 level. In order to optimize the stability of the ⁴⁰Ca⁺ optical clock, the clock laser frequency was referenced to the Ramsey fringe, and the reference fringe determination algorithm and the automatic peak-finding algorithm were introduced, making the ⁴⁰Ca⁺ optical clocks run stably for a long time and reach a stability of 6.3?×?10^-18 with an averaging time of 524 000 s.

12 Development of time synchronization performance measurement and calibration device for optical fiber network

LIN Pingwei , QI Miaomiao , SONG Zhenfei

2023(2). DOI: 10.11823/j.issn.1674-5795.2023.02.10

[Abstract](886) [HTML](314) [PDF 3.52 M](978)

Abstract:
In order to solve the problem of low accuracy of long-distance transmission in fiber optic time transfer system, the basic principles and technical characteristics of four mainstream high-precision fiber optic time transfer methods, namely round-trip, dual-fiber bidirectional time synchronization, bidirectional wavelength division multiplexing and bidirectional time division multiplexing, were analyzed. Based on this, a single-channel time-frequency transmission device was developed. The device adopts the electrical phase compensation method to realize frequency transfer synchronization, and uses the round-trip method combined with bidirectional time division multiplexing to achieve time transfer synchronization. It uses 1 PPS and 100 MHz to jointly mark the time signal, with 1 PPS as the coarse time marker and 100 MHz as the fine time marker, to achieve high precision time synchronization. Experiments show that the uncertainty of the single-channel time-frequency transmission device is about 13 ps, which can meet the requirements of the existing fiber optic time-frequency synchronization network metrology calibration. The device has broad application prospects in the fields such as cascaded time transfer synchronization and fiber optic time-frequency synchronization network metrology calibration. It provides an important technical support for the construction of high stability, high reliability and high precision time service system.

13 Experimental study on time constant calibration technology of turbine flowmeter

ZHANG Yongsheng , DANG Jing , YYU Xiaoli

2023(6). DOI: 10.11823/j.issn.1674-5795.2023.06.11

[Abstract](711) [HTML](305) [PDF 3.22 M](882)

Abstract:
In order to calibrate the time constant of the turbine flowmeter, a liquid step flow test device was established. The device uses a screw pump to transport fluid to form a platform flow, and an internal meshing gear pump to transport fluid to form a step flow component. The step flow component is superimposed on the platform flow by controlling the opening and closing of the high-speed solenoid valve, and the step flow is generated ultimately. The generation time of the step flow was evaluated by using a pitot tube, and the result showed that the rise time of the step flow was less than 9 ms. The time constant calibration tests of turbine flowmeter were carried out under different step conditions, and the results showed that the time constant of the turbine flowmeter was not invariable, and was greatly affected by the steady flow before the step flow, with a trend of decreasing with the increase of the steady flow before the step change, and the time constant was less affected by the step flow amplitude. In the small flow range, the dynamic flow coefficient obtained from actual measurement deviated significantly from the theoretical model derivation, while in the large flow range, the two were more consistent. Finally, the uncertainty of the calibration result of the time constant was evaluated, and the maximum expanded uncertainty was about 10 ms (k = 2). The research results are helpful for evaluating the dynamic characteristics of flowmeters and are expected to improve the accuracy of unsteady flow measurement.