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Optics Express

Optical magnetic combination method for suppressing the Rb polarization-induced magnetic gradient in Rb-Xe NMR co-magnetometers

Tengyue Wang, Jinpeng Peng, Zhanchao Liu, Yunkai Mao, Xuelei Wang

Published May 3, 2023• 13 citations
Publisher

Abstract

The Rb polarization-induced magnetic field gradient affects the Xe nuclear spin relaxation properties and degrades the long-term stability of the NMR co-magnetometers. This paper proposes a combination suppressing scheme, which uses the second-order magnetic field gradient coils to compensate for the Rb polarization-induced magnetic gradient under counter-propagating pump beams. Based on the theoretical simulation, we find that the spatial distribution of the Rb polarization-induced magnetic gradient and the magnetic field distribution generated by gradient coils are complementary. The experimental results indicate that the compensation effect is 10% higher under the counter-propagating pump beams scheme compared with the compensation effect under the conventional single beam. Besides, due to a more uniform spatial distribution of electronic spin polarization, the Xe nuclear spin polarizability is improved and the signal-to-noise ratio (SNR) of NMR co-magnetometers could be further enhanced. The study provides an ingenious method for suppressing magnetic gradient in the optically polarized Rb-Xe ensemble, which is expected to improve the performance of the atomic spin co-magnetometers.

DOI
10.1364/oe.489463

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The Rb polarization-induced magnetic field gradient affects the Xe nuclear spin relaxation properties and degrades the long-term stability of the NMR co-magnetometers. This paper proposes a combination suppressing scheme, which uses the second-order magnetic field gradient coils to compensate for the Rb polarization-induced magnetic gradient under counter-propagating pump beams. Based on the theoretical simulation, we find that the spatial distribution of the Rb polarization-induced magnetic gradient and the magnetic field distribution generated by gradient coils are complementary. The experimental results indicate that the compensation effect is 10% higher under the counter-propagating pump beams scheme compared with the compensation effect under the conventional single beam. Besides, due to a more uniform spatial distribution of electronic spin polarization, the Xe nuclear spin polarizability is improved and the signal-to-noise ratio (SNR) of NMR co-magnetometers could be further enhanced. The study provides an ingenious method for suppressing magnetic gradient in the optically polarized Rb-Xe ensemble, which is expected to improve the performance of the atomic spin co-magnetometers.
Abstract Nuclear Magnetic Resonance (NMR) co‐magnetometers are advanced quantum sensors capable of measuring angular velocity for inertial navigation. Within the Rb‐Xe atomic ensemble, Rb atoms absorb polarized light, decreasing electron spin polarization as the light travels greater distances. This attenuation causes a gradient in Rb spin polarization that severely affects atomic relaxation characteristics and degrades NMR sensors’ performance. Considering atomic diffusion motion, a theoretical simulation model is developed and the spatial distribution of electron spin polarization under the encounter‐propagating dual‐beam configuration is simulated. The simulation results demonstrate that the proposed dual‐beam scheme achieves a more uniform distribution of electron polarization within the atomic vapor cell. Experiments reveal an 18% enhancement in 129 Xe nuclear spin polarization using the dual‐beam scheme compared to the conventional single‐beam. Through the Fermi contact interaction between optically pumped Rb and Xe atoms, a more uniform spatial distribution of Rb spin polarization reduces the gradient relaxation of Xe atoms and its depolarization effects, thereby significantly enhancing the macroscopic Xe spin polarization and the signal‐to‐noise ratio (SNR) of NMR sensors. This study presents a new method for improving atomic polarization, significantly enhancing the performance of quantum sensors.
In spin-exchange-relaxation-free (SERF) atomic magnetometers (AMs), the attenuation of circularly polarized pump beam introduces spatial-dependent artificial magnetic fields in atomic spin polarization and light shift. In general, such a spatial-dependent effective magnetic field, especially prevalent in single-cell magnetometer arrays, cannot be uniformly compensated over the entire excitation region, resulting in a complex distributed compensation mask effect. In addition, any external effective-field-compensation methods inevitably complicate the final analysis of the unknown magnetic field measured. Here, we report a new excitation scheme that can substantially suppress spatial nonuniformity due to pump beam attenuation. Using counter-propagating pump beams with symmetrically red- and blue-detuning-paired frequencies, we demonstrate a significant reduction of spatial inhomogeneity in atomic spin polarization and light shift in comparison with the usual single-frequency unidirection and counter-propagating pump schemes. Employing this red-/blue-detuning-paired pump scheme, we construct a single-cell, four-channel SERF magnetometer array with high spatial homogeneity. We demonstrate ~10-fT/Hz1/2^{{1}/{2}}1/2sensitivity in the 5–45-Hz band for all four channels, with a gradient sensitivity of 2 fT/Hz1/21/2^{{1}/{2}}1/2/cm in the same band. This new scheme enables the development of single-vapor-cell, high-uniformity multichannel magnetic field mapping devices with potential applications in magnetoencephalography (MEG) and magnetocardiography (MCG), or in searching for exotic spin-dependent interactions.
Magnetic field gradients affect numerous modern sensitive measurements and physics studies using atomic ensemble, resulting in the loss of coherence. In some cases, the presence of small gradients can reduce the performance significantly. In this study, we propose an effectivein situmethod to compensate the triaxial magnetic field gradient for atomic magnetometers, based on multichannel atomic signals in one vapor cell. We design two novel gradient coils by solving closed-form equations to improve the accuracy and efficiency of compensation. We demonstrate the performance of the compensation method in a spin-exchange relaxation-free atomic magnetometer, which reduces the magnetic linewidth by 4.2%–7.6% under the same conditions at a compensation resolution of 50 pT/cm. The proposed coils generate gradients only along two axes and achieve triaxial decoupling, which compensates for the gradient completely without iteration. Our results may aid in suppressing atomic spin relaxations due to magnetic field gradients in several modern measurements and improve their performance, such as the detection of electric dipole moments, ultralow-field nuclear magnetic resonance, and biomagnetism.
Magnetic field gradients interfere with the coherence of the atomic ensemble and degrade the performance of the spin-exchange relaxation-free (SERF) atomic magnetometer. In this paper, the influence of magnetic field gradients is incorporated into the response model of single-beam atomic magnetometer, and the theoretical models of transverse relaxation rate, spin polarization and scale factor are established. Based on this model,we find that magnetic field gradients in different directions can cause varying degrees of degradation in the performance parameters of the magnetometer. The magnetic linewidths under different light power and magnetic field gradients are measured using the designed dedicated magnetic field gradients coils, and the spin polarization and magnetic field gradients relaxation rate are obtained through fitting. An unevenness and deviation of the curve are also observed in the experiments, indicating the presence of the magnetic field gradient of approximately 2 nT/cm in the magnetic shields. Furthermore, in a magnetic field gradient environment of 20 nT/cm, the mean of the long-term sensitivity decreased by 3.5 times and the standard deviation increased by 11 times. It shows that the magnetic field gradients will not only affect the signal-to-noise ratio, but also make the magnetometer more vulnerable to external interference when working. The sensitivity and stability of magnetometer will be greatly reduced. The research in this article provides a theoretical and experimental basis for eliminating the influence of magnetic field gradients and improving the accuracy of magnetic field measurements.
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.
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.