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IEEE International Conference on Electronic Measurement & Instruments

Miniaturized Dual-Beam SERF Magnetometer Using Natural Abundance Rubidium

Jianwei Sheng, Ziao Liu, Xiaoyu Li, Zhongyu Wang, Shushan Gao, Jixi Lu

Published Aug 22, 2025• 0 citations
Publisher

Abstract

Due to high sensitivity and compactness, spin-exchange relaxation-free (SERF) atomic magnetometers have found widespread applications. These devices rely on alkali atoms to sense magnetic fields, with87Rb being the commonly used isotope. However,87Rb must be extracted from natural abundance rubidium (natRb) through isotopic separation, making it expensive. Replacing87Rb with natRb reduces the cost to approximately just 1%. Nevertheless, it is generally believed that the isotopic mixture of87Rb and85Rb in natRb affects the coherence of the atomic ensemble. Using a dual-beam SERF magnetometer, we systematically compared 3 mm inner-size cubic vapor cells filled with natRb and87Rb. By optimizing the buffer gas pressure and optical parameters, we demonstrate that the natRb magnetometer can achieve performance comparable to that of the87Rb magnetometer. Experimental results show that both cells achieved a sensitivity of 4 1T/Hz1/2. These findings demonstrate that natRb can effectively replace87Rb in miniaturized dual-beam SERF magnetometers, enabling low-cost, high-sensitivity magnetic field measurements.

DOI
10.1109/icemi66537.2025.11306847

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IEEE Transactions on Components Packaging and Manufacturing Technology•Sep 3, 2018•22 citations
Due to high sensitivity and compactness, spin-exchange relaxation-free (SERF) atomic magnetometers have found widespread applications. These devices rely on alkali atoms to sense magnetic fields, with87Rb being the commonly used isotope. However,87Rb must be extracted from natural abundance rubidium (natRb) through isotopic separation, making it expensive. Replacing87Rb with natRb reduces the cost to approximately just 1%. Nevertheless, it is generally believed that the isotopic mixture of87Rb and85Rb in natRb affects the coherence of the atomic ensemble. Using a dual-beam SERF magnetometer, we systematically compared 3 mm inner-size cubic vapor cells filled with natRb and87Rb. By optimizing the buffer gas pressure and optical parameters, we demonstrate that the natRb magnetometer can achieve performance comparable to that of the87Rb magnetometer. Experimental results show that both cells achieved a sensitivity of 4 1T/Hz1/2. These findings demonstrate that natRb can effectively replace87Rb in miniaturized dual-beam SERF magnetometers, enabling low-cost, high-sensitivity magnetic field measurements.
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.
We present a theoretical and experimental study of a single-beam spin-exchange relaxation-free magnetometer in 87Rb vapor cells under different nitrogen gas pressures. The spin relaxation rate is a key component to limit the magnetic sensitivity, and the zero-field resonance method was used to measure the spin relaxation rates of different alkali metal cells. Simultaneously, in a single-beam spin-exchange-relaxation-free (SERF) magnetometer, we demonstrated that the fundamental magnetic field sensitivity was also limited by the pumping light intensity. Based on our theoretical analysis and experimental results, we determined the optimal pumping light intensity and optimal gas pressure. We experimentally demonstrated that the magnetic field sensitivity was 8.89 fT / Hz in the single-beam configuration, with an active measurement volume of 3 × 3 × 3~mm3.
In the spin-exchange relaxation-free (SERF) magnetometer of a perpendicular pump-probe configuration, the pump and probe beam characteristics significantly affect the performance. In this paper, an efficient evaluation of optical parameters to improve the sensitivity of a miniature magnetometer has been presented. We have determined the pump light’s optimal intensity and wavelength through theoretical analysis and the zero-field resonance experiments. Chirp signals are applied to measure the optical rotations at different probe intensities and frequencies. Through theoretical and experimental analysis of noise source characterization under different beam intensities and wavelengths, we demonstrate that dual-beam magnetometer performance is mainly limited by photon shot noise. Based on the optimum pump and probe beam parameters, we demonstrate magnetic field sensitivity of 6.3 fT/ Hz in an 87Rb vapor cell filled with nitrogen gas, with an active measurement volume of 3 × 3 × 3 mm3.
Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed cross-talk effect, achieved by applying a rotating modulation field and using of the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 × 4 × 4 mm 87Rb cell, we achieve magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis to the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the cross-talk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for the noninvasive biomagnetic imaging.
Abstract The hybrid optical pumping spin exchange relaxation free (SERF) atomic magnetometers can realize ultrahigh sensitivity measurement of magnetic field and inertia. We have studied the 85 Rb polarization of two types of hybrid optical pumping SERF magnetometers based on 39 K- 85 Rb- 4 He and 133 Cs- 85 Rb- 4 He respectively. Then we found that 85 Rb polarization varies with the number density of buffer gas 4 He and quench gas N 2 , pumping rate of pump beam and cell temperature respectively, which will provide an experimental guide for the design of the magnetometer. We obtain a general formula on the fundamental sensitivity of the hybrid optical pumping SERF magnetometer due to shot-noise. The formula describes that the fundamental sensitivity of the magnetometer varies with the number density of buffer gas and quench gas, the pumping rate of pump beam, external magnetic field, cell effective radius, measurement volume, cell temperature and measurement time. We obtain a highest fundamental sensitivity of 1.5073 aT / Hz 1/2 (1 aT = 10 −18 T ) with 39 K- 85 Rb- 4 He magnetometer between above two types of magnetometers when 85 Rb polarization is 0.1116. We estimate the fundamental sensitivity limit of the hybrid optical pumping SERF magnetometer to be superior to 1.8359 × 10 −2 aT / Hz 1/2 , which is higher than the shot-noise-limited sensitivity of 1 aT / Hz 1/2 of K SERF atomic magnetometer.
The designs of microfabricated alkali vapor cells play an important role when the atomic magnetometers are developing toward on-chip integration. This paper presents microfabricated spherical alkali vapor cells with improved sensitivity for chip-scale atomic magnetometers. A microfabricated spherical rubidium vapor cell and a traditional microfabricated planar rubidium vapor cell are designed for comparison, which are of the same volume and interconnected by a microchannel to acquire equal inner buffer gas pressure. In chip-scale spin-exchange relaxation-free (SERF) atomic magnetometers, the rubidium polarization lifetimes of three pairs of spherical and planar vapor cells at different buffer gas pressures of 0.35, 0.73, and 2.00 amg are measured, respectively. The sensitivity of SERF atomic magnetometers using the vapor cells at a 0.35-amg buffer gas pressure is also measured. The results show that the transverse polarization lifetime of the spherical vapor cell is 2.46, 1.44, and 1.05 times that of the planar one at 0.35-, 0.73-, and 2.00-amg buffer gas pressures, respectively. Combining the improved polarization lifetime and increased optical path length, chip-scale SERF atomic magnetometer using the microspherical vapor cell has the sensitivity of 60 fT/Hz1/2from 5 to 15 Hz, while that based on the planar one is demonstrated with the sensitivity of 400 fT/Hz1/2from 5 to 15 Hz.