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Researchers from the National Time Service Center of the Chinese Academy of Sciences have developed a new compact optical clock that utilizes quantum interference enhanced absorption spectroscopy. This advancement has significant implications for micro-positioning, navigation, and timing (μPNT) systems.
The team drew inspiration from the established success of coherent population trapping (CPT)-based chip-scale microwave atomic clocks, along with the rapid advancements in optical microcombs. Their new chip-scale optical clock exhibits improved frequency stability and accuracy, primarily leveraging the two-photon transition of rubidium atom ensembles.
Despite these advancements, earlier configurations required elevated cell temperatures (approximately 100 ℃) and significant laser power (around 10 mW), which posed challenges to miniaturization and power efficiency.
To overcome these challenges, the researchers introduced an innovative technique that employs enhanced-absorption sub-Doppler resonances on the D1 line of rubidium atoms. By using monochromatic light and precisely calibrated polarization settings for the pump and probe beams, they were able to achieve enhanced absorption through constructive and destructive interference. This process successfully produced a highly favorable absorption-enhanced Doppler-free resonance, characterized by a strong signal amplitude relative to its linewidth, making it well-suited for high-performance optical clocks.
Moreover, the researchers were able to achieve these spectroscopic results while operating at modest laser powers of approximately 100 µW and cell temperatures around 40℃, which are crucial factors for demonstrating a compact optical reference.
A theoretical model was presented by the team, illustrating the important role of Zeeman dark states in their spectroscopic method. The theoretical predictions closely matched the experimental results observed in their studies.
To assess the frequency stability of this innovative optical clock, the researchers frequency-stabilized two identical diode lasers onto enhanced-absorption sub-Doppler resonances. They meticulously explored how various parameters influenced the sub-Doppler resonance characteristics. This straightforward setup enabled the team to demonstrate a locked laser beat-note with impressive fractional frequency stability of 1.8 x 10−12 at 1 second and below 10−11 at 10,000 seconds, significantly surpassing the performance of free-running configurations.
These findings underscore the potential of this approach to create a compact or chip-scale optical frequency reference, opening up opportunities in fields such as instrumentation, navigation, and metrology.
This research project was conducted in collaboration with Prof. Rodolphe Boudot from the Franche-Comté Électronique Mécanique Thermique et Optique—Sciences et Technologies (FEMTO-ST) Institute in France. The results of this investigation are published in Physical Review Applied.
More information: Peter Yun et al, Enhanced absorption in Doppler-free spectroscopy of the Rb atom D1 line with monochromatic light: Application to laser-frequency stabilization, Physical Review Applied (2025). DOI: 10.1103/PhysRevApplied.23.034063
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phys.org