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Advanced Quantum Technologies

Optimal Magneto‐Optical Angle in Single‐Beam Optically Pumped Atomic Magnetometers Considering the Spatial Distribution of Spin Polarization

Yuntian Zou, Chi Fang, Jiali Liu, Jinghong Xu, Wenjing Tian, Haowen Tian, Bingchuan Shi, Xiangyang Zhou, Wei Quan, Liwei Jiang

Published Jan 1, 2026• 1 citation
Publisher

Abstract

ABSTRACT Single‐beam optically pumped atomic magnetometers (OPAMs) are promising tools for biomagnetic measurements in nonzero‐field environments owing to their high sensitivity and compactness. However, the angle between the external bias magnetic field and the pump light affects the spin polarization of the atomic ensemble, thereby influencing the magnetometer performance and making the optimization of the magneto‐optical angle essential. In this study, a spatially resolved steady‐state spin polarization model incorporating the magneto‐optical angle is established for the first time, and an analytical expression of the spin precession signal under RF magnetic field excitation is derived. It is demonstrated that an optimal angle exists at which the magnetometer exhibits the maximum response and the highest sensitivity, which is experimentally determined to be 26° in our single‐beam OPAM. At this angle, the combined influence of the steady‐state spin polarization, spin precession projection, transmitted light intensity, and transverse spin relaxation reaches an optimal overall state, leading to a 14% enhancement in sensitivity and a 32% reduction in detection noise compared with the conventional 45° configuration. These findings provide guidance for optimizing the experimental parameters of single‐beam OPAMs for weak magnetic signal detection in nonzero‐field environments.

DOI
10.1002/qute.202500902

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ABSTRACT Single‐beam optically pumped atomic magnetometers (OPAMs) are promising tools for biomagnetic measurements in nonzero‐field environments owing to their high sensitivity and compactness. However, the angle between the external bias magnetic field and the pump light affects the spin polarization of the atomic ensemble, thereby influencing the magnetometer performance and making the optimization of the magneto‐optical angle essential. In this study, a spatially resolved steady‐state spin polarization model incorporating the magneto‐optical angle is established for the first time, and an analytical expression of the spin precession signal under RF magnetic field excitation is derived. It is demonstrated that an optimal angle exists at which the magnetometer exhibits the maximum response and the highest sensitivity, which is experimentally determined to be 26° in our single‐beam OPAM. At this angle, the combined influence of the steady‐state spin polarization, spin precession projection, transmitted light intensity, and transverse spin relaxation reaches an optimal overall state, leading to a 14% enhancement in sensitivity and a 32% reduction in detection noise compared with the conventional 45° configuration. These findings provide guidance for optimizing the experimental parameters of single‐beam OPAMs for weak magnetic signal detection in nonzero‐field environments.
Abstract The single-beam optically pumped atomic magnetometer (OPAM) provides an effective solution for high-precision magnetic field measurements under nonzero-field conditions, including the geomagnetic field. However, the magnetometer performance is strongly influenced by the cell temperature, and the underlying mechanism by which the cell temperature affects the performance of single-beam OPAMs needs to be clarified. Based on the principle of the single-beam OPAM, a theoretical model for the scale factor is proposed, and the temperature dependence of the magnetometer signal-to-noise ratio (SNR) is investigated theoretically and experimentally. Both the theoretical analysis and experimental investigation indicate that an optimal temperature exists at which the SNR reaches its maximum, and this optimal temperature is determined for the constructed single-beam OPAM system. This work facilitates the optimization of magnetometer sensitivity and supports wearable magnetocardiography and magnetoencephalography measurements in geomagnetic field environments.
Abstract The optically pumped atomic magnetometer (OPAM) has emerged as an advanced instrument for magnetic field detection. However, the performance of conventional OPAMs is constrained by factors such as non‐uniform atomic polarization and insufficient precision in system polarization control. In this study, a bidirectional‐pump OPAM based on spin‐decoupled metasurface is presented and experimentally demonstrated. By designing a metasurface structure with a predefined phase distribution, the system generates two circularly polarized (CP) lights with distinct chirality and one linearly polarized (LP) light. Two beams of the CP light are directed in opposite directions along both sides of the cell, which effectively mitigates the polarization gradient within the cell and enhances the stability of the system. Additionally, the use of zero‐order LP light as the probe light for the system enhances energy utilization efficiency while simplifying the system architecture. The experimental results demonstrate that the OPAM based on metasurface successfully realizes precise control of atomic spin polarization. The proposed OPAM is capable of performing high‐precision magnetic field measurements, and the sensitivity has been improved to 1.85 pT/Hz 1/2 . This research not only advances the application of metasurfaces in quantum precision measurement but also offers a novel approach for enhancing the performance of atomic magnetometers.
ABSTRACT The Bell‐Bloom‐type optically‐pumped atomic magnetometers are well‐suited for weak geomagnetic field detection. However, conventional single‐beam pumping introduces an atomic spin polarization gradient, which limits the measurement accuracy and sensitivity. To address this issue, this paper proposes and experimentally demonstrates a Bell‐Bloom‐type rubidium FID magnetometer scheme integrating orthogonally polarized counter‐propagating pumping and multi‐pass probe detection. This design homogenizes the atomic spin polarization distribution and suppresses light shifts and power broadening. This uniform polarization, in turn, allows the five‐pass probe configuration to achieve a significantly larger signal amplitude. Experimental results reveal that, compared with the traditional single‐beam pumping and single‐pass detection scheme, the proposed magnetometer achieves a remarkable improvement in magnetic field measurement accuracy, and the magnetic field sensitivity is improved from 18.9 to 3.1 pT/. Notably, the two techniques work synergistically, yielding a 6.1‐fold sensitivity enhancement that far exceeds their individual gains. This work provides an effective technical approach and reference for optimizing the performance of atomic magnetometers and their applications in precision measurement fields.
Single-beam optically pumped atomic magnetometers (OPAMs) represent promising candidates for applications in geomagnetic field monitoring, biomedicine, and various other precision sensing domains owing to their exceptional sensitivity, high measurement accuracy, and cost-effectiveness. However, in orientation-based OPAMs utilizing circularly polarized light (CPL) pumping, the off-resonant light introduces light shift. The light shift acts as an additional magnetic field, reducing the measurement accuracy of OPAMs. Moreover, the dependence of the light shift on light intensity and frequency facilitates the conversion of light noise into magnetic noise, thereby degrading the sensitivity of OPAMs. In this study, we propose an optical pumping scheme based on space division multiplexing light (SDML) to suppress the light shift in OPAM. The approach employs two orthogonally polarized linearly polarized beams with a controlled phase difference to generate a spatially periodic distribution of left- and right-handed CPL. Optical pumping with the SDML induces oppositely directed light shifts in distinct spatial regions that effectively cancel each other. Finally, it is verified by experiments that the light shift is greatly suppressed based on SDML. Under detuned SDML pumping, both the light shift and light intensity noise are suppressed by over 50%, while maintaining a sensitivity comparable to that achieved with conventional CPL pumping. The proposed method demonstrates significant potential for magnetometers in light-noise-prone environments, and it can be generalized to other orientation-based magnetometers that rely on CPL pumping.
In a compact optically pumped atomic magnetometer (OPAM), there is a plateau in the sensitivity where the dependence of the sensitivity on pumping power is small compared with that predicted by the uniform polarization model. The mechanism that generates this plateau was explained by numerical analysis. The distribution of spin polarization in the alkali metal cell of an OPAM was modeled using the Bloch equation incorporating a diffusion term and an equation for the attenuation of the pump beam. The model was well-fitted to the experimental results for a module with a cubic cell with 20 mm sides and pump and probe beams with 8 mm diameter. On the plateau, strong magnetic response was generated at the regions that were not illuminated directly by the intense pump beam, while at the same time spin polarization as large as 0.5 was maintained due to diffusion of the spin-polarized atoms. Thus, the sensitivity of the magnetometer monitored with a probe beam decreases only slightly with increasing pump beam intensity because the spin polarization under an intense pump beam is saturated. This plateau, which is characteristic of this type of magnetometer using a narrow pump and probe beams, can be used in arrays of magnetometers because it enables stable operation with little sensitivity fluctuation from changes in pump beam power.
This paper investigates the performance optimization of optically pumped magnetometers under zero-field parametric modulation. Based on the analytical solutions of the Bloch equation, both longitudinal and transverse modulations are studied experimentally. To estimate the nonuniform polarization distribution of alkali-metal atoms in the vapor cell, an average pumping rate model is proposed. Furthermore, the accuracy of this model and the measurement of the transverse relaxation rate are verified via the agreement between experimental and theoretical values. The results indicate that optimal performance can be achieved by employing a suitable modulation field, the selection of which is related to the modulation index uuu in the Bessel series and the pump-light intensity. Although both operating modes show similar responses to weak magnetic fields, their effects on pump-light intensity are different due to the means of detecting atomic polarization. An optimal value of the pump-light intensity on the response strength exists in longitudinal modulation. However, with regard to transverse modulation, the sensitivity under weak pump-light intensity is better. This research has far-reaching significance for cases when the parametric modulation is manipulated.