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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Atomic Beam Distribution in Calcium Beam Optical Clock

Lei Han, Xiaobo Xue, Hui Zhu, Qianqian Ji, Yu Chen, Shengkang Zhang

Published May 15, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1109/eftf/ifcs57587.2023.10272074

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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.
Optical clocks have become a very important tool of investigating the characteristics of atoms or ions, for the state-of-art instability and uncertainty. Increasing the running time of an optical atomic clock is one critical issue that scientists are still working on. In this paper we report the improvement of the atomic chamber of a calcium beam optical clock. By using a combination of microtubes as the nozzle, we achieved a very high atomic flux with small divergence. The resonant atomic flux is about 1×1012 to 1013 per second and the divergence is smaller than 30mrad. The experimental results show that the system is reasonable in design and meets the technical indicators.
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.
Considering the strong inhomogeneous spatial polarization and intensity distribution of spontaneous decay fluorescence due to the Hanle effect, we propose and demonstrate a universe Hanle detection configuration of electron-shelving method for optical clocks. Experimental results from Ca atomic beam optical frequency standard with electron-shelving method show that a designed Hanle detection geometry with optimized magnetic field direction, detection laser beam propagation and polarization direction, and detector position can improve the fluorescence collection rate by more than one order of magnitude comparing with that of inefficient geometry. With the fixed 423 nm fluorescence, the improved 657 nm optical frequency standard signal intensity is presented. The potential application of the Hanle detection geometry designed for facilitating the fluorescence collection for optical lattice clock with a limited solid angle of the fluorescence collection has been discussed. The Hanle detection geometry is also effective for ion detection in ion optical clock and quantum information experiments. Besides, a cylinder fluorescence collection structure is designed to increase the solid angle of the fluorescence collection in Ca atomic beam optical frequency standard.
Optical frequency standards with the reference to a narrow electronic transition of a laser-cooled collection of neutral atomic particles are becoming essential tools of research in modern precision physics experiments. In the core of a building block of an optical frequency standard is the optical continuous wave laser that has a good spectral purity of the emitted light. Such a stable optical oscillator is highly desirable in high resolution spectroscopy, if it emits in a good quality beam at a short visible wavelength. This Master thesis explores efficient techniques for building such an optical frequency source intended for use in the cooling and trapping of Calcium atoms scheme. The strong dipole transition at the blue wavelength in the atomic Calcium is needed to reduce the kinetic energy of atoms by nearly six orders of magnitude. A further reduction in the thermal energy of the laser cooled atoms is required to locate with ultra-high precision the 400 Hz narrow clock transition of the stable 40Ca isotope. The experimental methods that achieve this and approach sub-microkelvin temperature of the laser cooled bosonic isotopes of alkaline earths are inspected. The blue laser with a uniform intensity distribution in the beam is useful to maintain the trapped number of cold atoms during these experiments. The spectroscopic properties of the relative transitions in Calcium atom are also reviewed following relevant publications in the area. The constructed blue laser can be used as a primary wavelength source in the lasers network for cooling and trapping of Calcium atoms. These experiments will constitute part of the project to build an optical atom clock referenced to 40Ca narrow linewidth transition. The blue laser is constructed by generating second harmonic in a Potassium Niobate crystal, which is temperature controlled to use a type-I noncritical phase-matching of the optical nonlinear process. The power of the intracavity-generated second harmonic depends on the resonance properties of the optical resonator where this nonlinear crystal is placed. The study is aimed at characterising the designed optical resonator and the experimental parameters that describe it. The formula is derived that relates the resonance power enhancement coefficient with finesse and the power coupling contrast of a passive optical cavity. The obtained relationship is verfied during the experiments. The produced efficiency of the intracavity second harmonic generation is approx. 0.0023 mWblue/(mWred)2. The research work also examines the noise characteristics of the infrared diode laser that is used as a pump source for the intracavity generated second harmonic and determines the spectroscopic precision of the produced blue light. The frequency locking experiment is analysed using the unbalanced scheme of the polarisation stabilisation technique. The designed optical buildup cavity became a part of the unbalanced frequency discriminator in such a scheme. The results demonstrate high gain of frequency noise suppression of the stabilised laser. The unbalanced arrangement of the H}ansch-Couillaud technique has been possible due to a very low amplitude noise of semiconductor lasers.
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.