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Journal of Physics D Applied Physics

Density-ratio dependence of angular-momentum-transfer efficiencies in hybrid Rb–Cs– 129 Xe vapor cells

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

Published Sep 4, 2026• 0 citations
Publisher

Abstract

Abstract We investigate angular-momentum transfer efficiencies in hybrid Rb–Cs– 129 Xe vapor cells with different alkali compositions. Experiments were performed at a pump power of 40 mW and a cell temperature of 110 ∘ C, where the Rb–Cs spin-exchange rate is comparable to the alkali electron-spin relaxation rate and complete spin-temperature equilibrium is not established. Using Rb-rich cells under Cs pumping and Cs-rich cells under Rb pumping, we measured the steady-state 129 Xe polarization and build-up time and determined the corresponding spin-exchange and photon efficiencies. Within each cell series, both efficiencies exhibit systematic composition-dependent trends arising from the combined effects of optical pumping, incomplete Rb–Cs polarization transfer, alkali relaxation, and alkali–Xe spin exchange. The Rb–Cs polarization-transfer factor converts the nominal alkali density ratio into an effective polarization-weighted ratio, which governs the relative contributions of the Rb–Xe and Cs–Xe spin-exchange channels. These results clarify angular-momentum transfer in hybrid Rb–Cs– 129 Xe systems and provide guidance for optimizing low-power hybrid spin-exchange optical pumping and alkali–noble-gas sensor platforms.

DOI
10.1088/1361-6463/aea2a5

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Abstract We investigate angular-momentum transfer efficiencies in hybrid Rb–Cs– 129 Xe vapor cells with different alkali compositions. Experiments were performed at a pump power of 40 mW and a cell temperature of 110 ∘ C, where the Rb–Cs spin-exchange rate is comparable to the alkali electron-spin relaxation rate and complete spin-temperature equilibrium is not established. Using Rb-rich cells under Cs pumping and Cs-rich cells under Rb pumping, we measured the steady-state 129 Xe polarization and build-up time and determined the corresponding spin-exchange and photon efficiencies. Within each cell series, both efficiencies exhibit systematic composition-dependent trends arising from the combined effects of optical pumping, incomplete Rb–Cs polarization transfer, alkali relaxation, and alkali–Xe spin exchange. The Rb–Cs polarization-transfer factor converts the nominal alkali density ratio into an effective polarization-weighted ratio, which governs the relative contributions of the Rb–Xe and Cs–Xe spin-exchange channels. These results clarify angular-momentum transfer in hybrid Rb–Cs– 129 Xe systems and provide guidance for optimizing low-power hybrid spin-exchange optical pumping and alkali–noble-gas sensor platforms.
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.
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By analyzing the measured spin-relaxation transients of 129Xe^{129}\mathrm{Xe}129Xe nuclear spins and alkali-metal-atomic spins in mixtures of alkali-metal-atom vapors, Xe gas, and larger amounts of N2{\mathrm{N}}_{2}N2​ gas, we have determined the three-body formation rates and the spin-transfer probabilities for alkali- metal--noble-gas van der Waals molecules. Three parameters, in addition to the spin quantum numbers of the alkali-metal and noble-gas nuclei, are needed to predict the spin-transfer rates. These parameters, which we have determined from experimental measurements, are x=\ensuremath{\gamma}N/\ensuremath{\alpha}, the ratio of the spin-rotation interaction \ensuremath{\gamma}N to the spin-exchange interaction \ensuremath{\alpha}; p0{p}_{0}p0​, the third-body pressure for which the molecular breakup rate ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}1}$ is equal to the spin-rotation frequency \ensuremath{\gamma}N/h; and Z, the three-body rate constant for forming van der Waals molecules.
The applicability of commonly used 129Xe nuclear polarization (PXe) buildup models to large optical cell polarizers, which operate at lower gas pressures when compared to small cell polarizers for which spin-exchange optical pumping (SEOP) models were originally developed, has yet to be systematically evaluated. In addition, spatial heterogeneity of Rb vapor density ([Rb]) and its impact on SEOP in low pressure, large optical cell polarizers remains poorly understood. In this theoretical study, it was shown that 1D analytical and numerical PXe buildup models yield diverging PXe and critical flow rate values at low flow rates and high laser absorption, contributing to the discrepancy between theoretical and experimentally measured production rates. Additionally, it was found that [Rb] heterogeneity leads to lower PXe buildup and an undesirable increase in optimal cell temperature, while the optimal laser absorption remains relatively unchanged. The updated simulation framework presented here can be utilized to model other SEOP systems.