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.

Abstract:Dual?comb spectroscopy (DCS) is an advanced precision spectral measurement tool with high resolution, high frequency accuracy, rapid measurement speed, and broad bandwidth. Thus, it has been widely used in many fields such as spectral lidar, greenhouse gas monitoring, and combustion diagnosis. As new principles, new schemes and new technologies of dual?comb spectroscopy are emerging, it is necessary to sort out and summarize their current development status. In this paper, the principles and technical performance indexes of dual?comb spectroscopy are introduced, and the experimental schemes and advantages of four typical DCS measurement systems are analyzed and compared, including optical frequency reference, electro?optical modulation, single?cavity dual?comb, and optical?optical modulation. Meanwhile, the development status of dual?comb spectroscopy in the expansion of the operating band is analyzed. Finally, the development trend and application prospects of DCS systems are summarized and prospected, which can provide a reference for further improvement of dual?comb spectroscopy in full?band spectral measurements and multi?scene applications.

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.

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.

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.

Abstract:The basic principles and development history of quantum correlated imaging are briefly reviewed, and the research progress of quantum correlated imaging in microscopic imaging is introduced in detail from the perspective of quantum light sources and classical light sources. Quantum correlated imaging based on classical light sources is easy to implement and low in cost, making it more promising in microscopic imaging.

Abstract:In this paper, basic physics of quantum correlation imaging is introduced, with discussion on its properties of high sensitivity, robustness, efficiency of information acquisition, as well as the capability of single?pixel and lensless imaging. Towards its applications in Lidar, issues related to imaging of moving objects and influences from the atmosphere are discussed. By improving the sampling rate, enhancing the means of tracking and optimizing the strategy of imaging reconstruction, the application performance of quantum correlation imaging can be upgraded. The development direction of quantum correlation imaging lidar in fields such as scouting and early warning are prospected. Further improvements of the discovery probability, tracking accuracy, discrimination accuracy and effective working distance in the future by studying imaging technology under extremely low photon flux, optimizing design of illumination patterns, establishing cooperation among multiple systems, and developing algorithms including artificial intelligence and information fusion are expected.

Abstract:Single photon detection and laser heterodyne detection are important tools for detecting weak echo light, and the extraction of multidimensional information from weak echo light is currently an important area of laser sensing. However, in practice, background noise and decoherence of the echo light can seriously affect the perception of multidimensional information from single photon detection and heterodyne detection techniques. These problems in the detection of weak echo light are difficult to solve effectively by using conventional solutions. The quantum heterodyne precision measurement method is a new measurement method based on single?photon detection combined with heterodyne detection, which can overcome the shortcomings of single?photon detection sensitivity limited by background noise. In addition, the quantum heterodyne has extremely low requirement for local oscillation intensity, which can effectively reduce the requirement for local oscillation intensity in large array heterodyne detection. This paper further summarises and analyses the research developments in quantum heterodyne precision measurement methods. The review and analysis of the existing research results will help to understand and grasp the current research status and problems of quantum heterodyne precision measurement method, and lay the foundation for the future development of quantum heterodyne precision measurement methods.

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

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.

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.

Abstract:In the ytterbium ion optical clock experiments, the ions are cooled and manipulated by lasers, and the laser frequency shifts will affect the atomic clock system. Aiming at the laser frequency shifts, based on the digital PID control method, a new multi?channel frequency?digital signal conversion method for frequency stabilization is designed to lock the multi?channel and multi?wavelength laser frequency to the reference frequency of the wavelength meter. The laser frequency data before and after locking are acquired for a certain amount of time and compared. The result shows that the laser frequency drift is stabilized from 800 MHz to ± 0.8 MHz, the laser frequency short?term instability decreases from 9.29 × 10^-10@1 s to 2.79 × 10^-10@1 s, and the long?term instability reaches 3.85 × 10^-12@1 000 s. The system is simple and easy to implement, and has the advantages of miniaturization and strong adaptability.

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.

Abstract:In recent years, low?noise femtosecond fiber lasers are widely demanded in high?precision and high?speed research such as ultra?stable photogenerated microwave and long?distance time?frequency transfer. To realize a laser seed with low?timing?jitter noise, an 100 MHz all?polarization?maintaining mode?locked Er:fiber laser was demonstrated on "optical cube". The"optical cube" configuration can effectively suppress the mechanical noise and the drift of the repetition rate. Based on the balanced optical cross?correlator technique, this is the first time to perform high?precision time jitter measurement of the pulse train directly from an all polarization?maintaining mode?locked Er:fiber laser. The integrated root?mean?square timing jitter from 10 kHz to 1 MHz offset frequency is only 98.36 as.

Abstract:Frequency transmission based on optical fibers plays an important role in the construction and application of time?frequency synchronization systems. In order to quantify the effect of delay fluctuations and dispersive residuals introduced by temperature and wavelength asymmetries on the stability of the stable radio frequency transmission system, a simple frequency transmission system and a 3 000 km fiber link are constructed using optical simulation software for simulation and analysis of 10 GHz RF signal transmission. Through analysis, it is found that the temperature asymmetry introduced in the link has a small effect on system frequency stability, while the wavelength asymmetry has a larger effect. The stability of the system deteriorates by more than 10^-13/ s as the difference between the positive and negative wavelength increases. This study has a practical guiding effect on the construction and optimization of the long distance optical fiber frequency transmission system in the actual environment.

Abstract:The current research on self?injection?locking frequency comb theory is incomplete in exploring the influence of different parameters on self?starting effect. The results of the operating?point locking theory used to explain the locking mechanism and numerical simulation are quite different and cannot explain some experimental phenomena well. Based on the Lugiato?Lefever Equation (LLE) taking into consideration of the backscattering effect, a self?starting Kerr microcomb model is constructed. The influence of key parameters in the model on the self?started locking state of the frequency comb are discussed by using the numerical simulation based on split?step Fourier method and combining the physical connotations of the parameters. Then from the basic formula of the model, the operating?point locking theory is modified to overcome the inadequacies of the self?injection locking micro?comb theory. It is demonstrated that the modified theory can improve the accuracy of the prediction of the locking state in the self?starting numerical simulation, as well as the compatibility between the theoretical prediction and the numerical simulation or experimental results.