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Optics Express

In-situ measurement of the density ratio of K-Rb hybrid vapor cell using spin-exchange collision mixing of the K and Rb light shifts

Kai Wei, Tian Zhao, Xiujie Fang, Yueyang Zhai, Hairong Li, Wei Quan

Published May 22, 2019• 29 citations
Publisher

Abstract

Ne comagnetometer-cells at different temperatures, pump light powers, and mole fractions of K. The method was further verified by the conventional laser-absorption-spectroscopy method. The new approach has the advantage to measure the density ratio of the optically-thick hybrid alkali atoms, while requiring no additional magnetic field necessary for conventional magnetic-field induced Faraday-rotation techniques. It also has the advantage of in-situ measuring the density ratio under exactly the normal operation of the devices, which means that the errors caused by the heating-effect of the strong pump light and the temperature drift during long-term operation can be real-time monitored.

DOI
10.1364/oe.27.016169

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Ne comagnetometer-cells at different temperatures, pump light powers, and mole fractions of K. The method was further verified by the conventional laser-absorption-spectroscopy method. The new approach has the advantage to measure the density ratio of the optically-thick hybrid alkali atoms, while requiring no additional magnetic field necessary for conventional magnetic-field induced Faraday-rotation techniques. It also has the advantage of in-situ measuring the density ratio under exactly the normal operation of the devices, which means that the errors caused by the heating-effect of the strong pump light and the temperature drift during long-term operation can be real-time monitored.
This study presents a novel method for measuring the number density of K in K-Rb hybrid vapor cells using circularly polarized pump light on polarized alkali metal atoms. This proposed method eliminates the need for additional devices such as absorption spectroscopy, Faraday rotation, or resistance temperature detector technology. The modeling process involved considering wall loss, scattering loss, atomic absorption loss, and atomic saturation absorption, with experiments designed to identify the relevant parameters. The proposed method is real-time, highly stable, and a quantum nondemolition measurement that does not disrupt the spin-exchange relaxation-free (SERF) regime. Experimental results demonstrate the effectiveness of the proposed method, as the longitudinal electron spin polarization long-term stability increased by 204% and the transversal electron spin polarization long-term stability increased by 44.8%, as evaluated by the Allan variance.
Abstract Accurate measurement of alkali metal density ratio in hybrid vapor cells is essential for high sensitivity of comagnetometers. We introduce and investigate a new method of measuring the density ratio based on electron spin-relaxation rate. The Bloch equations including the parameter of density ratio are studied, and the density measurement results show that measurement uncertainty is less than 12%. The advantage of this method is measuring the in-situ density ratio, which is more precise and useful for optimizing sensitivity of comagnetometers by adjusting the density ratio in real time.
In a hybrid pumping alkali-metal vapor cell filled with both K and Rb atoms, the K atom spins are optically pumped by the laser and the Rb atom spins are polarized by the K spins through spin exchange. We discovered that the ac Stark shift of the Rb atom spins, caused by the far-off-resonant pumping laser tuned to the K absorption lines, was mixed together with the ac Stark shift of the K atom spins through fast spin exchange. This mixing is studied and we demonstrate a K-Rb-21Ne^{21}\mathrm{Ne}21Ne comagnetometer in which the ac Stark shift of the Rb atom spins is eliminated by compensating for the Rb ac Stark shift with the K ac Stark shift. An ac Stark shift elimination method has been developed.
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.
We investigate the effects of the density ratio of K-Rb hybrid cells on the alkali metal-noble gas comagnetometers. Bloch equations simplified with the density ratio and average-pumping-rate model are presented for numerical simulation, which simplifies equations of complete hybrid spin ensemble and problem of polarization gradient. The spin polarizations of electron and nucleon, total electronic relaxation rates, and the spin-exchange efficiencies are measured with cells of different density ratios. The results are in good agreement with our equivalent model. Based on our theoretical analysis, the K-Rb- 21 Ne comagnetometer achieves maximum output signal by optimizing the combination of density ratio and optical power density. The density ratio is critical to the homogeneity of spin polarization and efficiency of hyperpolarization. The method in this work finds a way to optimize the sensitivity of comagnetometers, which is significant for angular-rotation sensors and new physics research.
The light shift generated by the circularly polarized light leads to an error in the spin-exchange relaxation-free (SERF) comagnetometer, since it turns the fluctuations of the pumping light frequency and intensity into a drift in the SERF comagnetometer. Moreover, the light shift is a source of cross-talk effect between the two sensitive axes. Firstly, the relationship between the vapor cell temperature and the total light shift felt by Rb atoms in the K-Rb hybrid pumping SERF comagnetometer is formulated and simulated. It is found that the total light shift is the result of the collision mixing of K and Rb ac Stark shifts and the increase of cell temperature makes the light shift of K felt by Rb dominate the total light shift. Furthermore, different from the method of adjusting the frequency of laser, a new method that suppresses the total light shift in hybrid pumping by adjusting the cell temperature is proposed and experimentally verified. The total light shift at 165.7°C is -0.00152 nT experimentally, which is 3 orders of magnitude smaller than that at 185.0°C, indicating that it can be suppressed effectively by adjusting the cell temperature. The method does not require any extra hardware or sacrifice the pumping efficiency and can be applied to suppress the error induced by the pumping light and the cross-talk effect between the axes. In addition, the relationship between total relaxation rate and cell temperature is briefly discussed.