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Journal of Physics Conference Series

A study of irregular-shaped Ramsey cavity for cold-atom microwave clocks

Siqi Jing, Fasong Zheng, Wenxia Song, Kun Liu, Weiliang Chen, Fang Fang

Published Apr 1, 2026• 0 citations
Publisher

Abstract

Abstract To improve the Ramsey fringe contrast for a portable free-falling-typed cold-atom microwave clock based on a TE 013 -mode cylindrical microwave resonator, this paper innovatively designs an irregular-shaped Ramsey cavity based on the combination of TE 011 and TE 013 modes cylindrical microwave resonators. To ensure b 1 𝜏 1 = b 2 𝜏 2 during the two Ramsey microwave pulses, this study constructs a 3D electromagnetic model to analyze the relationship between the matching factor η = b 1 𝜏 1 /b 2 𝜏 2 and the cavity structural parameters. Simulation results demonstrate that the irregular-shaped Ramsey cavity can effectively generate four spatially separable microwave standing wave fields, with two of them differing in height. The cavity is designed to achieve a central Ramsey fringe with a full width at half maximum (FWHM) of about 9 Hz. By iteratively refining the cavity geometry, the optimal parameters for the cavity dimensions were obtained, with a TE 013 mode height of 100.56 mm and a TE 011 mode height of 34.09 mm. This configuration achieves an amplitude-transit time matching factor η of unity ( η = 1).

DOI
10.1088/1742-6596/3228/1/012004

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Abstract To improve the Ramsey fringe contrast for a portable free-falling-typed cold-atom microwave clock based on a TE 013 -mode cylindrical microwave resonator, this paper innovatively designs an irregular-shaped Ramsey cavity based on the combination of TE 011 and TE 013 modes cylindrical microwave resonators. To ensure b 1 𝜏 1 = b 2 𝜏 2 during the two Ramsey microwave pulses, this study constructs a 3D electromagnetic model to analyze the relationship between the matching factor η = b 1 𝜏 1 /b 2 𝜏 2 and the cavity structural parameters. Simulation results demonstrate that the irregular-shaped Ramsey cavity can effectively generate four spatially separable microwave standing wave fields, with two of them differing in height. The cavity is designed to achieve a central Ramsey fringe with a full width at half maximum (FWHM) of about 9 Hz. By iteratively refining the cavity geometry, the optimal parameters for the cavity dimensions were obtained, with a TE 013 mode height of 100.56 mm and a TE 011 mode height of 34.09 mm. This configuration achieves an amplitude-transit time matching factor η of unity ( η = 1).
Compact cold-atom frequency standards, characterized by low frequency drift and high accuracy, continue to attract significant interest to address the growing demand for portable, high-performance time-keeping solutions. This paper presents and experimentally demonstrates a proof-of-principle prototype of a compact cold-atom Ramsey clock. The system is based on a multi-standing-wave field microwave cavity operating in the TE₀₁₃ mode and employs a free-falling atom scheme to simplify optical frequency control. The chosen TE₀₁₃ cavity is a cylindrical microwave resonator (CMR) fabricated from oxygen-free copper (OFC), which offers advantages in size and microwave leakage control over the conventional TE₀₁₁ cavity for this free-fall configuration. Its structure, which also serves as the vacuum chamber, is sealed with ceramic windows and coupled via rectangular waveguides. During operation, atoms interact with the first and third standing-wave fields to implement Ramsey interrogation. Experimental results show optimal pulse timing with τ1=16 ms, which aligns closely with theoretical predictions (τ1=16.1 ms). The measured central Ramsey fringe has a full width at half maximum (FWHM) of 8.9 Hz and a contrast of 89.1%, corresponding to a measured relative frequency instability of 6.4×10−13τ-1/2.
We introduce a compact atomic clock based on laser-cooled atoms trapped inside a loop-gap microwave cavity. The cavity occupies a volume eight times smaller than conventional cylindrical cavities while maintaining a high quality factor in the presence of the ten apertures required for optical manipulation of cold atoms. The measured linewidth of the Ramsey spectrum, which is limited by the free fall distance of the atomic cloud in the cavity, was 19.6 Hz. The corresponding frequency instability was σy(τ) = 2.5 × 10−12 τ−1/2, which could be further improved by reducing the local oscillator noise. This work paves a way toward high-performance portable atomic clocks.
Additive manufacturing (AM) of passive microwave components is of high interest for the cost-effective and rapid prototyping or manufacture of devices with complex geometries. Here, we present an experimental study on the properties of recently demonstrated microwave resonator cavities manufactured by AM, in view of their applications to high-performance compact atomic clocks. The microwave cavities employ a loop-gap geometry using six electrodes. The critical electrode structures were manufactured monolithically using two different approaches: Stereolithography (SLA) of a polymer followed by metal coating and Selective Laser Melting (SLM) of aluminum. The tested microwave cavities show the desired TE011-like resonant mode at the Rb clock frequency of ≈6.835 GHz, with a microwave magnetic field highly parallel to the quantization axis across the vapor cell. When operated in an atomic clock setup, the measured atomic Rabi oscillations are comparable to those observed for conventionally manufactured cavities and indicate a good uniformity of the field amplitude across the vapor cell. Employing a time-domain Ramsey scheme on one of the SLA cavities, high-contrast (34%) Ramsey fringes are observed for the Rb clock transition, along with a narrow (166 Hz linewidth) central fringe. The measured clock stability of 2.2 × 10−13 τ−1/2 up to the integration time of 30 s is comparable to the current state-of-the-art stabilities of compact vapor-cell clocks based on conventional microwave cavities and thus demonstrates the feasibility of the approach.
A portable cold87Rb atomic clock is under development at NIM. The clock uses87Rb atoms which are trapped and cooled in a small three-dimensional magneto-optical trap (3D-MOT) chamber. Within the 3D-MOT chamber, the free-falling atoms are prepared in∣F=2,mF=0⟩\vert F=2, m_{F}=0\rangle∣F=2,mF​=0⟩clock state with optical pumping (OP) directly after post cooling by2−2′2-2^{\prime}2−2′OP light and repumping. Following optical state selection, the free-falling atoms enter a microwave cavity where they undergo microwave interrogation. Eventually, the number of atoms in two clock states are detected individually by the fluorescence detection method in the detection chamber. The preliminary results show that, the selected atom number has been improved about 3 times with the optical state selection compared to the microwave state selection method. And a Ramsey fringe has been obtained experimentally, and the contrast of the central Ramsey fringe exceeds 88% with a full width half maximum (FWHM) of less than 9 Hz.
We demonstrate operation of a Ramey-mode Rb vapor-cell atomic clock based on a microwave cavity realized by additive manufacturing (3D-printing). The cavity design is based on a loop-gap approach and its critical electrode structure is realized in one monolithic piece by stereolithography of a polymer which simplifies the assembly of the cavity. The microwave magnetic field in the cavity shows excellent uniformity and high homogeneity across the Rb cell, which results in high-contrast Ramsey fringes observed on the clock transition. A measured clock stability of 2.2×10-13τ-1/2demonstrates the feasibility of the approach. We discuss aging studies performed on 3D-printed test samples that are of relevance for long-term operation of the clock.