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arXiv:1404.4431 · The Scientific World JOURNAL

Hanle Detection for Optical Clocks

Xiaogang Zhang, Shengnan Zhang, Duo Pan, Peipei Chen, Xiaobo Xue, Wei Zhuang, Jingbiao Chen

Published Jan 1, 2015• 4 citations

Abstract

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.

DOI: 10.1155/2015/614737

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In order to develop small optical clock for various transportable applications with better stability and accuracy than that of commercial small Cs clock and H-maser clock, our group has proposed a small Ca atomic beam clock scheme with electron-shelving detection. Its potential stability was experimentally demonstrated recently. Here, we first demonstrated the dispersive detection of 657 nm transition via the Faraday rotation effect of 423 nm transition based on a thermal Ca atomic beam, this alternative detection method can effectively detect the clock signal by the ground state atoms after the clock laser interrogation. The Faraday anomalous dispersion optical filter of 423 nm transition can reach a maximum transmission of 10%. With a magnetic field of 84 G, the observed 3.3% of the ground state atoms has been excited to the σ±level of metastable3P1(m=±1) state, which are separated 176.6 MHz to m=0 level.

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.

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 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.

Active optical clocks, a new class of optical atomic clocks, work through stimulated emission lasing, a unique mechanism that is distinct from laser absorption spectroscopy of conventional optical clocks. With coherent weak optical feedback from a Fabry-Perot resonator operating in the bad-cavity regime, active optical clocks sustain a lasing oscillation at an atomic transition frequency. They are used directly as quantum optical frequency standards with expected stability better by about two orders of magnitude than that of the best clocks. This review starts with a simplified account of historical developments and the limitations of critical techniques in optical atomic clocks. After an introduction of the basic mechanism underlying active optical clocks, we discuss the most recent experimental results for several configurations and experimental set-ups of active optical clocks. These include those based on a 2-level atomic beam and 3-level laser-cooled trapped-atom system, those based on a 4-level atom scheme, and the Faraday active optical clock. Moreover, following the successful lasing and preliminary results on a Faraday active optical frequency standard with Faraday atomic filter using thermal Cs cell, we describe an experimental scheme of a Faraday active optical clock with a Faraday atomic filter using magic-lattice-trapped Sr atoms at the 689-nm transition, and Faraday optical clocks operating in the good-cavity regime. We conclude with a comprehensive comparison of the different active optical clocks, prospects for active Faraday optical clocks, and potential applications of active optical clocks.