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Review of Scientific Instruments

Coherent population trapping resonance scheme of simultaneously noise suppression and multiple constructive interferences

Suping Qu, Yi Yin, Xianyang Lu

Published Jun 1, 2026• 0 citations
Publisher

Abstract

Traditional coherent population trapping (CPT) atomic clocks use single circularly polarized coherent multichromatic light to interact with atoms for preparing CPT dark states. While this configuration is simple, it suffers from the trap states and involves only the ±1st-order sidebands in the CPT resonance, excluding the carrier and other sidebands. This results in a lower CPT signal amplitude and higher laser noise, limiting the frequency stability of CPT atomic clocks. To address this issue, we developed and experimentally validated a scheme utilizing left- and right-circularly polarized light based on multiple coherent superpositions. This scheme significantly enhances the amplitude of the CPT resonance. Furthermore, by incorporating a Michelson interferometer, the polarization direction of the ±1st-order sidebands in the coherent multichromatic light is adjusted from parallel to perpendicular, enabling the filtering of one sideband to be detected while eliminating other unwanted sidebands and effectively suppressing laser noise. The results show that, under identical conditions, the short-term frequency stability of this method is theoretically improved by a factor of seven compared to the traditional circularly polarized light scheme, providing an effective pathway for the development of high-performance CPT atomic clocks.

DOI
10.1063/5.0324040

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Traditional coherent population trapping (CPT) atomic clocks use single circularly polarized coherent multichromatic light to interact with atoms for preparing CPT dark states. While this configuration is simple, it suffers from the trap states and involves only the ±1st-order sidebands in the CPT resonance, excluding the carrier and other sidebands. This results in a lower CPT signal amplitude and higher laser noise, limiting the frequency stability of CPT atomic clocks. To address this issue, we developed and experimentally validated a scheme utilizing left- and right-circularly polarized light based on multiple coherent superpositions. This scheme significantly enhances the amplitude of the CPT resonance. Furthermore, by incorporating a Michelson interferometer, the polarization direction of the ±1st-order sidebands in the coherent multichromatic light is adjusted from parallel to perpendicular, enabling the filtering of one sideband to be detected while eliminating other unwanted sidebands and effectively suppressing laser noise. The results show that, under identical conditions, the short-term frequency stability of this method is theoretically improved by a factor of seven compared to the traditional circularly polarized light scheme, providing an effective pathway for the development of high-performance CPT atomic clocks.
This scheme is based on a Michelson interferometer in which the relative optical path difference between the two arms is adjusted to control the relative phase between the copropagating left- and right-handed ( σ + - σ − ) circularly polarized light. This configuration converts the dominant ±1 st sidebands from parallel to orthogonal linear polarizations, while the 0 th sideband becomes circularly polarized light. The ±1 st sidebands, forming a double-Λ configuration, interact with 87 Rb atoms to generate an interference-enhanced coherent population trapping (CPT) resonance. A partial reflector placed behind the vapor cell enables multiple interactions between the σ + - σ − circularly polarized light and the atoms. After the polarizer, only the +1 st sideband resonance is detected; the −1 st sideband is completely suppressed, and the 0 th sideband is partially attenuated, thereby reducing the light background. Under the same conditions, the multi-pass configuration improves the CPT contrast fourfold and reduces noise by roughly half an order of magnitude compared with the single-pass case. With only a few additional optical components, this scheme improves both contrast and noise, positioning it as a promising candidate for high-performance CPT atomic clocks.
We present a digital differential detection scheme using a single photodiode to suppress the diode laser frequency noise and improve the short-term stability in atomic clocks based on coherent population trapping (CPT) resonance. One laser light is divided into two beams via modulation of an acousto-optic modulator. One beam induces CPT resonance with circularly polarized light, and the other beam induces only absorption with linearly polarized light. A differential algorithm counteracts common-mode noise from the two beams. A stability of 6.86×10−12 at 100 s and 2.26×10−12 at 1000 s has been demonstrated. The short-term stability is improved by a factor of 2.4 at 100 s. The scheme promises to eliminate laser-related noise and explore potential applications in chip-scale atomic clocks.
We present a new polarization-selective detection scheme that enables simultaneous dual-frequency laser stabilization and implementation of high-contrast coherent population trapping (CPT) resonance. The proposed configuration splits elliptically polarized light into two optical paths after atomic interaction. The component reflected from the polarizing beam splitter, containing Doppler-broadened spectral features, is employed for precise laser frequency locking. The reflected signal exhibits a frequency discrimination slope nine times steeper than that of the transmitted polarizing beam splitter component. Simultaneously, the transmitted component isolates the CPT resonance signals from the positive and negative first-order sidebands for microwave frequency stabilization. Experiment demonstrates excellent agreement with theoretical simulations derived from the Liouville equation. Compared with conventional circular polarization methods, this approach achieves a fourfold enhancement in short-term frequency stability. These advancements make the technique particularly promising for developing compact CPT atomic clocks with improved metrological performance.
We have developed and experimentally studied a coherent population trapping (CPT) state preparation scheme for atomic clock application with co-propagating left and right circularly polarized lasers. With realization of constructive interference and spin-polarized dark state free in CPT state preparation, we have obtained CPT resonance signal 3 times larger than that of the conventional scheme used in atomic clock. Polarization fluctuations and CPT signal sensitivity to laser power behaviors are both improved with the scheme. Our study reveals that it is a promising candidate for both normal-size and chip-scale CPT atomic clocks.
Coherent population trapping (CPT) atomic clocks are prevailingly realized by exciting CPT resonance with circularly polarized bichromatic coherent light (σ+-σ+ scheme), while atom utilization efficiency is relatively low in the scheme. For improving efficiency, we experimentally study the scheme of realizing CPT resonance through the interaction between atoms and parallel linearly polarized bichromatic coherent light field (lin//lin scheme). In the same experimental conditions the experimental results show that the signal-noise ratio of the CPT resonance signal is twice, and the differential slope is 1.65 time higher than those of σ+-σ+ scheme. The experimental results reveal that the scheme is an ideal candidate for low power consuming atomic clock and magnetometer.
A scheme is studied for a coherent population trapping (CPT) atomic clock with counterpropagating left and right circularly polarized lasers, wherein the vertical-cavity surface-emitting laser and the partially reflecting mirror form an external cavity of an established length. With cancellation of spin-polarized dark states and interaction repeated with atoms, a CPT signal is obtained that is eight times larger than that of the conventional scheme. For the improvement of modulation efficiency, CPT resonance still can be excited with a lower microwave power compared with the conventional scheme. The experimental results suggest that it is an alternative for low-power atomic clocks.