E-22 uncertainty optical frequency divider

The time/frequency unit is the most accurate among the seven basic units, so many measurement studies that pursue ultra-high accuracy and sensitivity will be transformed into frequency measurements to achieve higher measurement accuracy and sensitivity. For example, by measuring the relative changes in the ratio of different atomic transition frequencies, ultralight dark matter can be detected or constants can be studied to determine whether they change over time. By measuring different locations To verify the correctness of theories such as local position invariance and gravitational redshift, the frequency changes of light clocks at different times are used.

The essence of time/frequency measurement is to measure the frequency ratio between the measured object and the frequency standard. Therefore, the accuracy and sensitivity of frequency measurement depend on the performance of the frequency standard and the frequency ratio measurement device. In recent years, the development of optical clocks based on the electronic level transitions of atoms in the optical band has been rapid: currently, the optical clock with the lowest uncertainty has entered the 10-19 level, and the long-term frequency instability has also entered the 10-19 level. Researchers have begun to explore effective ways to gradually achieve the performance of optical clocks in the 10-21 level. In terms of optical frequency ratio measurement, the most accurate result at present was achieved by the State Key Laboratory of Precision Spectrum of East China Normal University in 2016: the influence of optical frequency noise and microwave frequency standard performance on optical frequency division or frequency ratio measurement was further eliminated by using the titanium sapphire femtosecond optical comb with frequency precision phase-locked to ultra stable narrow linewidth laser, combined with optical comb transmission oscillator technology and optical frequency auto reference microwave frequency standard technology, As a result, the additional noise introduced by the frequency ratio measurement is between 6-19 (1-second average time) and 4-21 (104 second average time), and the uncertainty of the frequency ratio measurement is between 1.4-10-21, which is much smaller than the frequency instability and uncertainty of the current optical clock. Therefore, it can meet the application requirements of the current optical clock.

In order to meet the application of the 10-21 uncertainty optical clock in the future and realize the frequency measurement with the accuracy of 10-21, the State Key Laboratory of Precision Spectroscopy of East China Normal University has improved the stability of the mechanical structure and the effective optical path of the system, and has adopted the titanspar femtosecond optical comb whose frequency is locked in the hydrogen clock, so as to realize the long-term stable operation of the system and overcome the impact of the periodic change of the environment on the frequency ratio measurement, And also using optical comb transmission oscillator technology and optical frequency self reference microwave frequency standard technology to reduce the influence of optical comb frequency noise and microwave frequency standard frequency noise, it was verified that the noise introduced by the optical divider in the optical frequency ratio measurement process can reach 4 × 10-18 (1 second average time) and 6 × 10-22 (105 second average time), and the uncertainty of optical frequency ratio measurement can reach 3 × 10-22, maintaining a world leading position in this research direction. In this device, they achieved high-precision and low-noise optical frequency division using a stable 10-13 second high noise comb, providing ideas for using chip combs to achieve high-precision optical frequency division in the future.

This research achievement was first published in APL Photonics 8, 100802 (2023) by the State Key Laboratory of Precision Spectroscopy Science and Technology of East China Normal University. On the basis of this article, they also developed a high-precision portable optical frequency ratio measurement device for studying the frequency ratio measurement of different optical clocks.

Figure 1: Schematic diagram of optical frequency divider