Since its inception in 1949, atomic clocks have undergone more than 70 years of development, refining modern timekeeping, establishing the quantum definition of the second, and providing the foundation for precision measurement, fundamental constants, global navigation satellite systems (GNSS), and national time standards. The principle of the atomic clock lies in the implementation of oscillation frequency of corresponding to the quantum transition refers to a specific atomic energy level, thereby enabling ultra-precise time measurement. The evolution from microwave to optical quantum frequency standards has significantly enhanced accuracy, leading to groundbreaking advancements in precision measurement, fundamental physics, and quantum metrology. While single atomic ensemble comprises multiple accessible quantum transition energy levels, traditional atomic clocks utilize only one single transition at a time, thereby constraining their full potential. Optimizing the effective use of quantum resource to enhance precision measurement and expand atomic clock capabilities, remains a challenging yet promising field for research.

Professor Jingbiao Chen and Associate Researcher Duo Pan from Peking University, in collaboration with Researcher Deshui Yu from the National Time Service Center, Chinese Academy of Sciences, achieved the first realization of dual-wavelength optical frequency standards within a single atomic ensemble by leveraging modulation transfer between multi-wavelength quantum transitions. This breakthrough enables precise frequency locking of dual optical clocks within the same atomic ensemble. The research team developed a theoretical framework and experimentally implemented the dual-optical-transition modulation transfer spectroscopy (DOT-MTS) scheme, demonstrating the feasibility of operating two high-stability optical clocks simultaneously within a single atomic ensemble. Relevant research results were recently published in Photonics Research, Volume 13, Issue 3, 2025. [Jie Miao, Jingming Chen, Deshui Yu, Qiaohui Yang, Duo Pan, Jingbiao Chen, "Single-atomic-ensemble dual-wavelength optical frequency standard," Photonics Res. 13, 721 (2025)]

This work extends conventional single-quantum-transition modulation transfer spectroscopy (MTS) to dual-optical-transition modulation transfer spectroscopy (DOT-MTS), As shown in Fig. 1, the modulation signal from the 780 nm laser (Rb D2 line) is concurrently transferred to both the 780 nm (Rb D2 line) and 795 nm (Rb D1 line) quantum transitions, realizing 780–795 nm DOT-MTS. This advancement enables the precise and simultaneous implementation of optical frequency standards at both 780 nm and 795 nm. By achieving high-precision dual optical clocks locking within a single atomic ensemble, this work establishes a dual-wavelength optical frequency standard. This breakthrough opens a new avenue for more efficient utilization of atomic systems in precision measurement and quantum technologies.

Fig. 1. 780–795 nm DOT-MTS. (a) The modulation transfer process in 780–795 nm DOT-MTS ; (b) The optical setup of the 780–795 nm DOT-MTS ; (c) The energy levels of 780–795 nm DOT-MTS.

Looking ahead, future research will focus on extending the DOT-MTS technique to a multi-frequency scheme, incorporating V-type, Λ-type, and ladder-type configurations across various atomic structures. The next step involves investigating the iodine molecule, whose rich spectral features present significant potential for the development of multi-wavelength standards to further enhance precision measurements. These advancements will not only extend applications beyond optical clocks and length metrology but also establish a new frontier in multi-wavelength optical clocks within a single quantum atomic ensemble. Furthermore, this progress will significantly advance laser spectroscopy, with profound implications for quantum metrology.