Optical clocks, as the next-generation time and frequency standards, achieve ultra-low systematic uncertainty and frequency instability by precisely referencing the local oscillator frequency to the optical atomic transition frequency. Since the successful development of the first all-optical ¹⁹⁹Hg⁺ optical clock in the early 21st century, optical atomic clocks have made remarkable progress over the past two decades. Currently, state-of-the-art optical clocks have achieved systematic uncertainties and frequency stabilities at the 10^–19 level, surpassing traditional microwave atomic clocks by more than two orders of magnitude. This breakthrough has opened up new research areas in fundamental physics and precision measurement.

This paper begins by reviewing landmark developments in ion optical clocks and optical lattice clocks. Corresponding tables are provided to summarize the best performance metrics achieved by all known research groups, along with the specific optical clock types developed by each group. The main focus of the paper is a review of precision measurement applications based on optical clocks, covering four key areas. First, the method and typical setup for steering international atomic time (TAI) using optical clocks are introduced. The principles underlying optical frequency measurement data submission are summarized, followed by an overview of progress in TAI steering with optical clocks. Second, the principles for constraining variations in fundamental physical constants through optical clock comparisons are briefly outlined. Recent results regarding the fine-structure constant and the proton-to-electron mass ratio are presented to demonstrate the ability of optical clocks to probe such variations. Third, tests of Einstein’s equivalence principle are discussed, including principles and recent advances in examining local position invariance and local Lorentz invariance using optical clocks. Local position invariance is tested by measuring gravitational frequency shifts between clocks at different geopotential heights or within distinct regions of a vertical optical lattice. Local Lorentz invariance is tested by comparing optical clocks with different quantization axes; recent advances have raised the upper limit on Lorentz-violation coefficients for electron-photon systems to the order of 10^–21. Finally, chronometric leveling based on optical clock comparisons is presented. A comparison with traditional geodetic methods is provided, highlighting the advantages of the chronometric approach. The paper also details recent experimental progress in chronometric leveling.

In the outlook section, the paper analyzes potential research directions for further enhancing the performance of optical clocks. It also explores the possible advancements in precision measurement applications, such as constraining the variation rates of fundamental physical constants, as the performance of optical clocks continues to improve.