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Journal of Physics D Applied Physics

Density-ratio dependence of spin polarization and spatial homogeneity in hybrid Rb–Cs vapor cells

Ge Jin, Tao shi, Yuxin Chen, Yongjun Zhou, Sheng Zou

Published Apr 21, 2026• 1 citation
Publisher

Abstract

Abstract Hybrid spin-exchange optical pumping (hybrid SEOP) using multiple alkali-metal species is commonly employed to improve the spatial homogeneity of alkali spin polarization in atomic magnetometers and comagnetometers. We investigate the dependence of spin polarization and its spatial homogeneity on the alkali density ratio D r = [ Rb ] / [ Cs ] in Rb–Cs hybrid vapor cells. A series of cells with controlled D r were fabricated and characterized using absolute electron paramagnetic resonance polarimetry at multiple positions along the cell axis. The results show that the density ratio strongly influences both the magnitude and spatial uniformity of alkali polarization through the competition among optical pumping, spin exchange, and spin-destruction processes. An optimal density ratio is observed near D r ≈ 6.59 ± 0.47 , where the polarization reaches its maximum while maintaining high spatial homogeneity (end-to-end ratio ∼89% and RMS homogeneity > 95 % ). Under typical operating conditions, the optimal density ratio lies in the range D r ≈ 4 –10. These results clarify the role of the alkali density ratio in Rb–Cs hybrid SEOP and provide practical guidance for optimizing hybrid vapor cells used in precision atomic magnetometers and comagnetometers.

DOI
10.1088/1361-6463/ae627e

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We measured the spatial homogeneity of spin polarization, which is directly related to the sensitivity and magnetic linewidth of an optically pumped atomic magnetometer with a hybrid cell of K and Rb atoms. This was done by changing the position of the probe beam. Furthermore, for comparison, we also measured the sensitivity and magnetic linewidth of atomic magnetometers with single K and single Rb cells, and found that optically thick atoms were spin polarized homogeneously with the hybrid cell. Optically thin alkali metal vapor can be spin polarized homogeneously, and the spin polarization transfers to an optically thick alkali metal vapor. An atomic magnetometer with the hybrid cell was found to be more effective in realizing simultaneous signal measurements at different locations with the uniform sensitivity and magnetic linewidth.
The hybrid optical pumping atomic magnetometers have not realized its theoretical sensitivity, the optimization is critical for optimal performance. The optimizations proposed in this paper are suitable for hybrid optical pumping atomic magnetometer, which contains two alkali species. To optimize the parameters, the dynamic equations of spin evolution with two alkali species were solved, whose steady-state solution is used to optimize the parameters. The demand of the power of the pump beam is large for hybrid optical pumping. Moreover, the sensitivity of the hybrid optical pumping magnetometer increases with the increase of the power density of the pump beam. The density ratio between the two alkali species is especially important for hybrid optical pumping magnetometer. A simple expression for optimizing the density ratio is proposed in this paper, which can help to determine the mole faction of the alkali atoms in fabricating the hybrid cell before the cell is sealed. The spin-exchange rate between the two alkali species is proportional to the saturated density of the alkali vapor, which is highly dependent on the temperature of the cell. Consequently, the sensitivity of the hybrid optical pumping magnetometer is dependent on the temperature of the cell. We proposed the thermal optimization of the hybrid cell for a hybrid optical pumping magnetometer, which can improve the sensitivity especially when the power of the pump beam is low. With these optimizations, a sensitivity of approximately 5 fT/Hz(1/2) is achieved with gradiometer arrangement.
Abstract The spin polarization and spatial homogeneity of alkali metal atoms are critical characteristics that determine the performance of spin-exchange relaxation-free magnetometers. In buffer gas cells, these characteristics are influenced by optical broadening and alkali atomic motional restriction imposed by the buffer gas. In coated cells, the protective effect of the coating on atomic spins, combined with rapid atomic diffusion, leads to significantly superior spin polarization characteristics compared to those in high-pressure buffer gas cells. However, due to neglecting the influence of atom-coating interactions on atomic motion, the existing spin polarization and spatial distribution models fail to accurately describe the polarization characteristics in coated cells. In this paper, based on the Langevin–Bloch equation, a dynamic model considering interaction effects between alkali metal atoms and the coating as well as between the pump light and the hyperfine levels of alkali metal atoms is proposed to accurately describe the spin polarization and spatial distribution in coated cells. To validate the proposed model, a series of experiments were carried out. The results demonstrate that coated cells can maintain high spin polarization and spatial homogeneity even under conditions of low pump intensity and high temperature, which are in good agreement with the simulations. This work provides a theoretical reference for the design of high-performance and low-power quantum sensors.
Abstract In an optically pumped alkali vapor cell with a high density of atoms, the attenuation of the pump light generates a spatially non-uniform distribution of the electronic spin polarization of alkali atoms, which is detrimental to biomagnetism applications of magnetometers as well as the hyperpolarization of noble gas atoms. Therefore, in this study, we propose a new scheme to generate a nearly uniform, unsaturated spin polarization region based on counter-propagating pump beams and atomic diffusion. A finite element method-based simulation is used to demonstrate the three-dimensional distribution of the spin polarization in a spherical cell. The effects of cell temperature and pump light power on the homogeneity of the spin polarization are studied. The distribution of spin polarization near the center of the cell is experimentally measured and a 1 cm uniform spin polarization region is achieved in the center of the cell. The uniformity of spin polarization in the center region of the cell increased by 50% compared with single beam pumping. The advantage of our proposed scheme is that it can generate an unsaturated uniform region of spin polarization in the center of a cell using a single species of alkali atoms.