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Review of Scientific Instruments

Rapid atomic-signal-based in-situ magnetic compensation for unshielded spin-exchange relaxation-free atomic magnetometer

Jianjun Li, Haidong Wang, Teng Wu, Yuanxing Liu, Hong Mei Guo

Published Feb 1, 2026• 0 citations
Publisher

Abstract

We propose a novel method, which leverages the atomic signal itself, to achieve rapid in situ magnetic field compensation in an unknown geomagnetic environment for an unshielded spin-exchange relaxation-free atomic magnetometer. This method employs a strong bias magnetic field aligned with the pump beam direction to maximize the strength of the magnetic field resonance signal. The compensation process is divided into three sequential stages, each focusing on one of the x, y, and z axes by adjusting the magnitude of the bias magnetic field. As a result, the zero magnetic field is identified within 11 seconds, with compensation resolution smaller than 3.0 nT, primarily constrained by the laser power stability. This approach not only enhances the applicability of atomic magnetometers in unshielded environment but also proves effective for in situ magnetic field compensation in shielded environment, such as those used in magnetoencephalography, magnetocardiography, and space science.

DOI
10.1063/5.0302200

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We propose a novel method, which leverages the atomic signal itself, to achieve rapid in situ magnetic field compensation in an unknown geomagnetic environment for an unshielded spin-exchange relaxation-free atomic magnetometer. This method employs a strong bias magnetic field aligned with the pump beam direction to maximize the strength of the magnetic field resonance signal. The compensation process is divided into three sequential stages, each focusing on one of the x, y, and z axes by adjusting the magnitude of the bias magnetic field. As a result, the zero magnetic field is identified within 11 seconds, with compensation resolution smaller than 3.0 nT, primarily constrained by the laser power stability. This approach not only enhances the applicability of atomic magnetometers in unshielded environment but also proves effective for in situ magnetic field compensation in shielded environment, such as those used in magnetoencephalography, magnetocardiography, and space science.
The spin-exchange-relaxation-free (SERF) atomic magnetometer is an ultra-high sensitivity magnetometer, but it must be operated in a magnetic field with strength less than about 10 nT. Magnetic field compensation is an effective way to shield the magnetic field, and this paper demonstrates an in situ triaxial magnetic field compensation system for operating the SERF atomic magnetometer. The proposed hardware is based on optical pumping, which uses some part of the SERF atomic magnetometer itself, and the compensation method is implemented by analyzing the dynamics of the atomic spin. The experimental setup for this compensation system is described, and with this configuration, a residual magnetic field of strength less than 2 nT (±0.38 nT in the x axis, ±0.43 nT in the y axis, and ±1.62 nT in the z axis) has been achieved after compensation. The SERF atomic magnetometer was then used to verify that the residual triaxial magnetic fields were coincident with what were achieved by the compensation system.
In this study, we demonstrate a non-modulated triaxial magnetic field compensation method for spin-exchange relaxation-free (SERF) atomic magnetometers. We realize simultaneous compensation of the magnetic field along the pump and probe directions by maximizing the first order differential value of the zero-field resonance signal. The magnetic field perpendicular to the pump-probe plane is compensated by zeroing the DC response of the magnetometer. The zero-field resonance is obtained by applying a linearly changed magnetic field along the direction perpendicular to that of the pump-probe plane. The first order differential of the zero-field resonance is acquired using the second order central difference method in real time. Compared with the conventional compensation methods, this method does not involve modulation and demodulation. Therefore, it requires no lock-in amplifier, thus making it more applicable in miniature atomic devices. Moreover, this method compensates the magnetic field along the pump and probe directions using only one criterion. It avoids the cross-talk effect between the pump and probe directions in the presence of non-orthogonal coils. The experiment results show that the compensation resolution is 9 pT, 7 pT and 0.05 pT for the probe, pump, and direction perpendicular to the pump-probe plane, respectively.
The most efficient way to improve the sensitivity of an optically pumped magnetometer (OPM) is to maximize the spin-polarization lifetime of the atoms. The spin exchange relaxation-free (SERF) OPM that operates in zero field environment is one of the most sensitive magnetic sensors. This paper proposes a new three-axis magnetic field compensation method for the single-beam OPM, which zeroes the magnetic field around the sensor to make it work in SERF regime. The method simultaneously employs two modulation magnetic fields and two low-frequency sweeping magnetic fields along two directions perpendicular to the laser beam during the compensation process. The output signal of the photodiode is demodulated at different frequencies, resulting in two outputs that reflect the three components of the magnetic field. This method does not require iterate to tune the three-axis field, which increases the speed of the compensation process significantly. The magnetic field resolutions of the method for the x-axis, y-axis and z-axis are 0.6 pT, 0.6 pT and 5 pt, respectively. A compact OPM with noise density about 20 fT / Hz1/2@10 Hz is developed and tested, demonstrating the feasibility of the method.
A novel method to compensate the residual magnetic field for an atomic magnetometer consisting of two perpendicular beams of polarizations was demonstrated in this paper. The method can realize magnetic compensation in the case where the pumping rate of the probe beam cannot be ignored. In the experiment, the probe beam is always linearly polarized, whereas, the probe beam contains a residual circular component due to the imperfection of the polarizer, which leads to the pumping effect of the probe beam. A simulation of the probe beam's optical rotation and pumping rate was demonstrated. At the optimized points, the wavelength of the probe beam was optimized to achieve the largest optical rotation. Although, there is a small circular component in the linearly polarized probe beam, the pumping rate of the probe beam was non-negligible at the optimized wavelength which if ignored would lead to inaccuracies in the magnetic field compensation. Therefore, the dynamic equation of spin evolution was solved by considering the pumping effect of the probe beam. Based on the quasi-static solution, a novel magnetic compensation method was proposed, which contains two main steps: (1) the non-pumping compensation and (2) the sequence compensation with a very specific sequence. After these two main steps, a three-axis in situ magnetic compensation was achieved. The compensation method was suitable to design closed-loop spin-exchange relaxation-free magnetometer. By a combination of the magnetic compensation and the optimization, the magnetic field sensitivity was approximately 4 fT/Hz(1/2), which was mainly dominated by the noise of the magnetic shield.
The spin-exchange-relaxation-free(SERF)atomic magnetometer with ultrahigh sensitive becomes the most effective measurement to analyze the component of the planet in space exploration.In order to improve the sensitivity of the SERF atomic magnetometer,we must avoid the influence on the SERF regime disturbed by environment magnetic field.In this paper we design an active compensation system based on in-situ magnetic measurement to compensate the magnetic disturbance.Firstly,we get the three-axis magnetic field information according to the measurement of pop-laser,then control the current of coil to generate a magnetic field which is opposite and equal to the disturbance,to keep the SERF regime.Finally,we validat the method with the atomic magnetometer experimental setup.The result shows that compared with the manual compensation method,the system can track the magnetic compensation point in real time,reduce the signal noise and compensate the magnetic disturbance successfully,validating the efficiency of the active compensation system and establishing the technique foundation of the production.
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.