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arXiv:2506.23160

Sub-Ensemble Isolation in SERF Magnetometry Enabled by Micrometer-Scale Polarization Control

Zihua Liang, Yuhao Zhang, Lu Liu, Jinsheng Hu, Peng Zhou, Gen Hu, Hou, Gaopu, Mao Ye

Published Jun 29, 2025• 0 citations
arXivHTMLPDF

Abstract

Conventional understanding of spin-exchange relaxation-free (SERF) atom ensemble pertains to the common perception that the rapid exchange of atom state finally results in uniform time evolution of the whole ensemble. However, in this study, we demonstrate that by manipulation of pumping polarization in micro-meter level, misalignment between the time evolution of different sub-ensemble can be created within single SERF ensemble with unprecedent independency. A novel pumping system consists of a miniaturized $^{87}$Rb vapor cell and a space-variant polarization metasurface is developed for the prove of concept. Our method induces position-dependent atomic anisotropy in both pumping and absorption into the thermal atomic ensemble. By constructing calculated Zeeman-sublevel populations in SERF regime, distinct sensing channels are generated with 0.22 $V\ nT^{-1}$ average scale factor, which is comparable with single channel generated by single SERF ensemble. Average crosstalk ratio between adjacent channels (between micron scale sub-ensembles) are measured up to 32 dB, through the excitation of fictitious magnetic field (3.5 nT, 30 Hz), which is measured as 20 dB without sub-ensemble isolation in same experimental condition. Our work demonstrates unprecedented spatial resolution in SERF magnetometry which hold new promises for applications including high-spatial resolution neural biomagnetism mapping and portable magnetism measurement device.

DOI
10.48550/arxiv.2506.23160

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ABSTRACT Conventional understanding of a spin‐exchange relaxation‐free (SERF) atom ensemble pertains to the common perception that the rapid exchange of atom state finally results in uniform time evolution of the whole ensemble. However, in this study, we demonstrate that by manipulating pumping polarization in a micrometer level, misalignment between the time evolution of different sub‐ensembles can be created within a single SERF ensemble with unprecedented independency. A novel pumping system consisting of a miniaturized 87 Rb vapor cell and a space‐variant polarization metasurface is developed for the proof of concept. Our method induces position‐dependent atomic anisotropy in both pumping and absorption into the thermal atomic ensemble. By constructing calculated Zeeman‐sublevel populations in the SERF regime, distinct sensing channels are generated with 0.22 V/nT average scale factor, which is comparable with a single channel generated by a single SERF ensemble. The average crosstalk ratios between adjacent channels are measured up to 32 dB, through the excitation of the fictitious magnetic field (3.5 nT, 30 Hz), which is measured as 20 dB without sub‐ensemble isolation in the same experimental condition. Our work demonstrates unprecedented crosstalk reduction in single‐cell multichannel SERF magnetometry, which holds new promise for applications including high‐spatial resolution neural biomagnetism mapping and portable magnetism measurement devices.

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Cover

The cover illustrates a spatially variant polarization metasurface for single-cell multi-channel spin-exchange relaxation-free (SERF) magnetometry. By generating opposite-handed circular polarizations, the metasurface partitions a single atomic ensemble into independently polarized sub-ensembles. Opposite-handed neighboring channels, assisted by linear polarization barriers, suppress diffusion-induced inter-channel crosstalk and demonstrate a promising route toward improved spatial resolution. More details can be found in the Research Article by Mao Ye and co-workers (DOI: 10.1002/nap2.70213).

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Conventional understanding of spin-exchange relaxation-free (SERF) atom ensemble pertains to the common perception that the rapid exchange of atom state finally results in uniform time evolution of the whole ensemble. However, in this study, we demonstrate that by manipulation of pumping polarization in micro-meter level, misalignment between the time evolution of different sub-ensemble can be created within single SERF ensemble with unprecedent independency. A novel pumping system consists of a miniaturized 87^{87}87Rb vapor cell and a space-variant polarization metasurface is developed for the prove of concept. Our method induces position-dependent atomic anisotropy in both pumping and absorption into the thermal atomic ensemble. By constructing calculated Zeeman-sublevel populations in SERF regime, distinct sensing channels are generated with 0.22 V nT−1V\ nT^{-1}V nT−1 average scale factor, which is comparable with single channel generated by single SERF ensemble. Average crosstalk ratio between adjacent channels (between micron scale sub-ensembles) are measured up to 32 dB, through the excitation of fictitious magnetic field (3.5 nT, 30 Hz), which is measured as 20 dB without sub-ensemble isolation in same experimental condition. Our work demonstrates unprecedented spatial resolution in SERF magnetometry which hold new promises for applications including high-spatial resolution neural biomagnetism mapping and portable magnetism measurement device.
ABSTRACT Conventional understanding of a spin‐exchange relaxation‐free (SERF) atom ensemble pertains to the common perception that the rapid exchange of atom state finally results in uniform time evolution of the whole ensemble. However, in this study, we demonstrate that by manipulating pumping polarization in a micrometer level, misalignment between the time evolution of different sub‐ensembles can be created within a single SERF ensemble with unprecedented independency. A novel pumping system consisting of a miniaturized 87 Rb vapor cell and a space‐variant polarization metasurface is developed for the proof of concept. Our method induces position‐dependent atomic anisotropy in both pumping and absorption into the thermal atomic ensemble. By constructing calculated Zeeman‐sublevel populations in the SERF regime, distinct sensing channels are generated with 0.22 V/nT average scale factor, which is comparable with a single channel generated by a single SERF ensemble. The average crosstalk ratios between adjacent channels are measured up to 32 dB, through the excitation of the fictitious magnetic field (3.5 nT, 30 Hz), which is measured as 20 dB without sub‐ensemble isolation in the same experimental condition. Our work demonstrates unprecedented crosstalk reduction in single‐cell multichannel SERF magnetometry, which holds new promise for applications including high‐spatial resolution neural biomagnetism mapping and portable magnetism measurement devices.
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.
The cover illustrates a spatially variant polarization metasurface for single-cell multi-channel spin-exchange relaxation-free (SERF) magnetometry. By generating opposite-handed circular polarizations, the metasurface partitions a single atomic ensemble into independently polarized sub-ensembles. Opposite-handed neighboring channels, assisted by linear polarization barriers, suppress diffusion-induced inter-channel crosstalk and demonstrate a promising route toward improved spatial resolution. More details can be found in the Research Article by Mao Ye and co-workers (DOI: 10.1002/nap2.70213).
Abstract The chip-scale hybrid optical pumping spin-exchange relaxation-free (SERF) atomic magnetometer with a single-beam arrangement has prominent applications in biomagnetic measurements because of its outstanding features, including ultrahigh sensitivity, an enhanced signal-to-noise ratio, homogeneous spin polarization and a much simpler optical configuration than other devices. In this work, a miniaturized single-beam hybrid optical pumping SERF atomic magnetometer based on a microfabricated atomic vapor cell is demonstrated. Although the optically thin Cs atoms are spin-polarized, the dense Rb atoms determine the experimental results. The enhanced signal strength and narrowed resonance linewidth are experimentally proven, which shows the superiority of the proposed magnetometer scheme. By using a differential detection scheme, we effectively suppress optical noise with an approximate five-fold improvement. Moreover, the cell temperature markedly affects the performance of the magnetometer. We systematically investigate the effects of temperature on the magnetometer parameters. The theoretical basis for these effects is explained in detail. The developed miniaturized magnetometer has an optimal magnetic sensitivity of 20 fT/Hz 1/2 . The presented work provides a foundation for the chip-scale integration of ultrahighly sensitive quantum magnetometers that can be used for forward-looking magnetocardiography (MCG) and magnetoencephalography (MEG) applications.
Emerging biomagnetic imaging requires compact optical pumping magnetometer (OPM) arrays that enable high spatial resolution with remarkable sensitivity. In such devices, atomic pumping fundamentally depends on conventional optical systems, yet realizing diverse pumping modalities persists as a critical challenge─particularly in integrated applications. In this study, a space-variant polarization distribution (SPD) pumping source is achieved and integrated with an atomic magnetometer for the first time. Our method enables modification of optical polarization within a 3 × 3 × 3 mm 3 87 Rb micro-vapor cell to spatially separate the atomic pumping and detection regions, resulting in a 56% mean (75% maximum) reduction in the AC Stark effect compared with the conventional scheme. Experiment results demonstrate a 20.1 fT/Hz 1/2 sensitivity and a 52 Hz bandwidth with a dynamic response range of ±3 nT, which is comparable to the state-of-the-art chip-scale OPMs. By leveraging the SPD pumping method, various atomic devices with a diffusion-pumping scheme are able to achieve chip integration, which further paves the way for the development of high-spatial-resolution biomagnetism imaging and portable atomic sensing applications.
Ultra-sensitive multi-channel optically pumped atomic magnetometers based on the spin-exchange relaxation-free (SERF) effect are powerful tools for applications in the field of magnetic imaging. To simultaneously achieve ultra-high spatial resolution and ultra-high magnetic field sensitivity, we proposed a high-resolution multi-channel SERF atomic magnetometer for two-dimensional magnetic field measurements based on a digital micro-mirror device (DMD) as the spatial light modulator for a single vapor cell. Under the optimal experimental conditions obtained via spatial and temporal modulation of the probe light, we first demonstrated that the average sensitivity of the proposed 25-channel magnetometer was approximately 25fT/Hz 1/2 with a spatial resolution of 216µm. Then, we measured the magnetic field distribution generated by a gradient coil and compared the experimentally obtained distributions with those calculated via finite element simulation. The obtained g value of 99.2% indicated good agreement between our experimental results and the theoretical calculations, thereby confirming that our proposed multi-channel SERF magnetometer was effective at measuring magnetic field distributions with an ultra-high spatial resolution.