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Frontiers in Physics

Frequency shift properties of a calcium beam optical clock

Yu Chen, Yabei Su, Jiyang Ma, Jing Peng, Ming Ma, Xiaobo Xue, Shengkang Zhang

Published May 16, 2025• 0 citations
Publisher

Abstract

The optical frequency transitions with long-lifetime spin triplet states produced by the two valence electrons in the outermost layer of alkaline earth atoms make calcium atom a promising quantum frequency reference. Electron-shelving detection improves the signal-to-noise ratio of the clock transition spectrum, enabling a calcium atomic beam optical frequency standard. This scheme employs the 1S0-1P1 transition at 423 nm to detect the 1S0-3P1 transition at 657 nm, emphasizing the importance of 423 nm laser stability. While the performance of these clocks is well-studied, research on the atomic beam itself is limited. This study investigates frequency shifts in neutral calcium atoms’ 1S0-1P1 transition at 423 nm and their 1S0-3P1 transition at 657 nm. We measure the frequency shift caused by temperature, laser power, and laser alignment, providing insights to optimize the design of the calcium beam’s physical apparatus.

DOI
10.3389/fphy.2025.1597737

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The optical frequency transitions with long-lifetime spin triplet states produced by the two valence electrons in the outermost layer of alkaline earth atoms make calcium atom a promising quantum frequency reference. Electron-shelving detection improves the signal-to-noise ratio of the clock transition spectrum, enabling a calcium atomic beam optical frequency standard. This scheme employs the 1S0-1P1 transition at 423 nm to detect the 1S0-3P1 transition at 657 nm, emphasizing the importance of 423 nm laser stability. While the performance of these clocks is well-studied, research on the atomic beam itself is limited. This study investigates frequency shifts in neutral calcium atoms’ 1S0-1P1 transition at 423 nm and their 1S0-3P1 transition at 657 nm. We measure the frequency shift caused by temperature, laser power, and laser alignment, providing insights to optimize the design of the calcium beam’s physical apparatus.
The narrow linewidth of1S0-3P1657 nm clock transition makes it a promising quantum frequency reference. In this experiment, the fast1S0-1P1423 nm transition is used to detect the narrow1S0-3P1657 nm clock transition. Therefore, the frequency stability of the 423 nm laser determines the performance of the optical frequency standard. Here we investigate the frequency shift properties of the neutral calcium atoms'1S0-1P1transition at 423 nm. We measure the frequency shift caused by temperature, laser power, and laser beam pointing.
The Ca beam optical frequency standard has the potential to be small optical clock like small Cs beam clock in microwave regime. For improving the signal-to-noise ratio of the clock transition spectroscopy, the electron shelving detection, which uses the fast1S0-1P1423nm transition to detect the narrow1S0-3P1657nm clock transition, was applied for Ca beam optical frequency standard. Therefore the frequency stability of 423 nm laser to some extent determines the performance of the optical frequency standard. In this paper, we stabilize the 423 nm laser frequency to Ca beam signal with side-band locking technique and estimate the preliminary stability of 423 nm laser which is 1.4×10-11at 1 s and 2×10-12at 100 s.
Strontium optical lattice clocks at JILA recently demonstrated record-high accuracy and stability. These advances were enabled by an ultrastable laser with fractional frequency stability of 1x10-16 at 1 second. This laser allows us to study systematic shifts of the 1S0 to 3P0 clock transition with unprecedented precision. Density-dependent frequency shifts represent an unavoidable perturbation for clocks based on many atoms. Our studies of atomic interactions in an optical lattice clock system uncover the nature of these interactions and reveal important many-body atomic correlation effects in this open quantum system. By extending our measurements to all ten nuclear-spin sublevels of the clock states, we observe the first direct evidence of SU(N) symmetric interactions in alkaline earth(-like) atoms. Using the techniques we developed in these studies, we also demonstrate a novel technique for measuring the frequency noise spectrum of an ultrastable laser. We discuss designs for the future direction of our experiment which will place 87Sr atoms within a high finesse cavity that is resonant on the 1S0 to 3P1 transition. We will use this system to study collective effects in cavity quantum electrodynamics. Most notably, strong atom-cavity coupling can enhance the spectroscopic sensitivity of our clock by creating spin-squeezed states via quantum non-demolition measurements. As a precursor to future work studying cavity-mediated collective behavior, we use the unique atomic structure of 88Sr to investigate free-space retarded dipolar coupling in an optically thick sample of atoms.
In this paper a small Calcium beam optical frequency standard utilizing fully-sealed Calcium beam vacuum tube owning smaller size compared with vacuum system with flanges is demonstrated. Adopting the electron shelving detection method, we detect the clock transition 657 nm saturation absorbing spectroscopy whose linewidth is 500 kHz. The instability of this small optical frequency standard is 6.8×10-14at 1 second and 1.8×10-15after 1000 seconds of averaging by comparing with another small Calcium beam optical frequency standard whose reported instability is 3.0×10-14at 1 second. The optical part of the whole system is specially designed for robustness and miniaturization.
Optical clocks have become a very important tool of investigating the characteristics of atoms and ions, for the state-of-art instability and uncertainty. While optical clocks with long running time is still a critical issue. In order to improve vacuum system design and minimize atomic beam divergence angle in optical beam clock, the resonance fluorescence signal of the atom interacting with laser is adopted to evaluate atomic beam two-dimensional distribution. The laser frequency is stabilized to atomic transition resonance frequency via absorption spectrum method. Meanwhile, the laser beam fixed on a two-dimensional displacement platform is exploited to depict the atomic beam distribution with the resonance fluorescence intensity variation corresponding to each laser incident position. The experiment is carried on the calcium beam optical clock to obtain the preliminary atomic beam distribution.