An atomic clock research team from the National Time Service Center of the Chinese Academy of Sciences has proposed and implemented a compact optical clock based on quantum interference enhanced absorption spectroscopy, which is expected to play an important role in micro-positioning, navigation, timing (μPNT) and other systems.

Inspired by the successful story of coherent population trapping (CPT) based chip-scale microwave atomic clock and the booming of optical microcombs, chip-scale optical clock was also proposed and demonstrated with better frequency stability and accuracy, which is mainly based on two-photo transition of Rubidium atom ensemble.

However, the typically required high cell temperatures (~100 ℃) and laser powers (~10 mW) in such configuration are not compliant with the advent of a fully miniaturized and low-power optical clock.

To address these limitations, the researchers developed an innovative approach that utilizes enhanced-absorption sub-Doppler resonances on the D₁ line of rubidium atoms. 

By employing monochromatic light and carefully tuned polarization settings for counterpropagating pump and probe beams, the researchers observed enhanced absorption due to the constructive or destructive interference between two dark states prepared by the pump and probe beams, respectively. The observed absorption-enhanced Doppler-free resonance with high ratio of signal amplitude to linewidth is favorite to implement a high-performance optical clocks.

What's more, the spectroscopic lines are obtained for modest laser powers (around 100 µW) and cell temperatures (around 40 ℃), all these features are of significant interest for demonstrating a compact optical reference.

The researchers presented a theoretical model that highlights the significant contribution of Zeeman dark states in this spectroscopic scheme. And the theoretically calculated spectroscopic signals agree well with the experimental observations.

To measure the frequency stability of this optical clock, two identical diode lasers were frequency-stabilized onto enhanced-absorption sub-Doppler resonances. The influence of key parameters on the sub-Doppler resonance features is thoroughly investigated. Using this simple-architecture setup, the researches demonstrate the locked laser beat-note with a fractional frequency stability of 1.8 E−12 at 1s and below E−11 at 10,000 s, which is improved by more than two orders of magnitude compare with the free-running case. 

These results demonstrate the potential of this scheme for the implementation of a compact or even chip-scale optical frequency reference, which might find applications in instrumentation, navigation, and metrology.

This work is a cooperation with Prof. Rodolphe Boudot from the Franche-Comté Électronique Mécanique Thermique et Optique - Sciences et Technologies (FEMTO-ST) Institute in France, and the results were published in PHYSICAL REVIEW APPLIED.

Fig. 1. Theoretically calculated (left) and experimentally observed (right) spectroscopic signals in proposed method.(Image by Peter Yun)

Fig. 2. Compact optical clocks implemented with enhanced-absorption of sub-Doppler resonances, experimental setup (left) and measured laser frequency stability (right).(Image by Peter Yun)