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Optics Express

Method to quickly estimate T 1 value by suppressing spin exchange relaxation and magnetic field gradient relaxation in atomic sensors

Xuelei Wang, Jianli Li, Chunyu Qu, Yu Cheng, Junjie Zhang

Published Mar 25, 2024• 6 citations
Publisher

Abstract

The accuracy of atomic sensor is determined by the transverse nuclear spin relaxation time (T 2 ). As the longitudinal nuclear spin relaxation time (T 1 ) is the upper limit of T 2 , it becomes an important metric for evaluating the merits of alkali vapor cells. However, the conventional method for measuring T 1 of pulse delay takes dozens of minutes, which will increase the effect of random errors during the testing process. In order to solve this problem, a method to quickly estimate the value of T 1 is proposed by suppressing the influence of spin exchange relaxation and magnetic field gradient relaxation terms on T 2 . Experimental results indicate that the accuracy is maintained at above 95%, and the testing time is reduced to less than 5%.

DOI
10.1364/oe.522737

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The accuracy of atomic sensor is determined by the transverse nuclear spin relaxation time (T 2 ). As the longitudinal nuclear spin relaxation time (T 1 ) is the upper limit of T 2 , it becomes an important metric for evaluating the merits of alkali vapor cells. However, the conventional method for measuring T 1 of pulse delay takes dozens of minutes, which will increase the effect of random errors during the testing process. In order to solve this problem, a method to quickly estimate the value of T 1 is proposed by suppressing the influence of spin exchange relaxation and magnetic field gradient relaxation terms on T 2 . Experimental results indicate that the accuracy is maintained at above 95%, and the testing time is reduced to less than 5%.
In the atomic cell of the spin exchange optical pumping system, the optical pumping inevitably introduces a magnetic field gradient, which affects the measurement result of the nuclear spin relaxation time a lot. To address this issue, our method achieves the measurement while excluding the influence of this magnetic field gradient. In this method, the pump laser is turned off after the spins of 87Rb and 129Xe have been effectively polarized, which results in the spin polarization of the 87Rb atoms being provided only by the spin exchange with 129Xe. Ultimately, the transverse and longitudinal components of the polarization of 129Xe are detected by the in situ87Rb atom magnetometer to obtain the T1 and T2 of 129Xe under different conditions. Using our method, since the influence of inhomogeneity caused by the pump laser is eliminated, the relaxation characteristic of atomic cells can be measured more quickly, essentially, and conveniently, which helps compare the performance of different atomic cells and find the best cell fabrication process further.
Reducing the transverse relaxation rate of alkali atoms is crucial to improve the sensitivity of the magnetometers. In a perturbative treatment of the effects of the magnetic-field gradients, the transverse relaxation rates of alkali atoms are predicted to be proportional to the square of the longitudinal magnetic-field gradient. The theoretical predictions are confirmed in our experiments. The magnetic-field gradients in the vapor cell are measured by experiments. The magnetic-field gradient in the cell is suppressed drastically through compensating it with the compensation coils. Experimental results show that suppressing the magnetic-field gradient can reduce the transverse relaxation rate of alkali atoms significantly. This study contributes to the development of ultra-high sensitivity alkali atomic magnetometers.
We demonstrate the measurement of the transverse spin relaxation rate of Xe without the influence of the Rb polarization-induced magnetic field gradient. The optical pumping beam and probe beam turn on and off repeatedly during the measurement to reduce the relaxation originating from the Rb polarization-induced magnetic field gradient. During the absence of the optical pumping beam, the nuclear spin of the noble gas atom does not experience the alkali-polarization-induced magnetic field gradient so that the transverse spin relaxation rate ( 1 / T 2 ) decreases. When the duty cycle reaches zero, the measured transverse relaxation time T 2 approaches the longitudinal spin relaxation time T 1 . Our method helps in roughly estimating the longitudinal spin relaxation time with a single measurement of the free induction decay.
We propose a fast and accurate method for 129 X e − R b spin-exchange rate determination based on an atomic magnetometer at low magnetic field and low pressure. The 129 X e − R b spin-exchange rate can be estimated via the product of the maximum transverse spin polarization of 129 X e nuclei and the corresponding Rabi frequency. The velocity-averaged binary spin-exchange cross section and the rate constant for spin-exchange in 129 X e − R b van der Waals complexes are measured to be ⟨ σ ν ⟩ S E = ( 1.26 ± 0.13 ) × 10 − 16 c m 3 / s and κ 4 H e = 1326 ± ( 47 ) s − 1 in 4 H e buffer gas, respectively. With respect to the method based on the longitudinal relaxation rate, the method presented in this work can greatly facilitate the measurement process, determining the spin-exchange rate in several minutes. Moreover, this method is more suitable for vapor cells with long longitudinal relaxation time.
The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the magnetic field gradient cannot be ignored in the case of small static magnetic fields. Our study provides a theoretical and experimental basis for eliminating magnetic gradient relaxation in atomic sensors in the SERF region.
The sensitivity and bandwidth (BW) as the primary criteria of spin-exchange relaxation-free (SERF) atomic magnetometer (AM) are closely related to the transverse relaxation rate of atomic ensemble. In this study, we show an effective method for transverse relaxation rate measurement drawing on the golden Section algorithm. With no requirement of offline fitting, the measuring time of this method is only 7 s. This is especially advantageous for reducing measurement errors due to the drifts of spatial magnetic field and light intensity and using multiple measurements to better monitor the performance of magnetometers. Moreover, the reset after measuring is optimized as the sensitive axis magnetic field correction, ensuring the integrity of measurement while improving the accuracy of magnetic compensation. Furthermore, we find that the acquired experimental data using this method can be adopted for the evaluation of magnetic field compensation performance and the monitoring of magnetic anomaly states. This determination method for transverse relaxation rate is anticipated to play a greater important role in multichannel SERF AMs’ arrays.