Spin-exchange Relaxation-free Atomic Magnetometer Research Articles

Study of Magnetic Noise of a Multi-Annular Ferrite Shield

Ferrites are promising nonmetallic materials used for the fabrication of low-noise magnetic shields because they possess high permeability and high electrical resistivity. However, large-sized ferrite components are difficult to fabricate or machine. In this study, we develop a cylindrical ferrite shield that consists of five annuli and two lids with an inner volume of φ11.2 cm × 22.5 cm. Although this structure contains gaps between different components, it eases considerably the fabrication and machining process as compared to the entire module. The magnetic noise is measured by a spin-exchange relaxation-free atomic magnetometer, and the detrimental effects of the gaps are analyzed quantificationally using the finite element method. Our research results indicate that compared with the ferrite shield without gaps, the magnetization noise increases by 34.1%. Nonetheless, the magnetic noise at the center of the ferrite shield achieves 5.5f -1/2 fT, which is much lower than that of μ-metal shields with a similar size. If the gap width can be reduced to be smaller than 0.01 mm, the increase of the magnetization noise will be less than 4.9%, which can be negligible in practical applications. Our study provides a low-cost, readily available, and low-noise ferrite shield structure.

Multi-objective optimization of multilayer passive magnetic shield based on genetic algorithm

The performance of a multilayer magnetic shield directly affects and limits the sensitivity improvement of an atomic magnetometer. To better meet the requirements of spin-exchange relaxation free atomic magnetometer for the environmental magnetic field, the magnetic shield should be optimized. At present, the optimizations have focused only on a single objective, such as the axial shielding factor. However, the importance of other goals should not be neglected. In this paper, multiobjective optimization of the shield is carried out to obtain a better comprehensive performance. First, according to the structural characteristics of the multilayer shield, a multiobjective optimization model is established. Then, a multiobjective genetic algorithm is utilized to optimize the shield. After optimization, a Pareto optimal solution set is obtained. Furthermore, depending on the desired design requirements, two sets of optimal combinations of target values and variable parameters are selected, based on the fuzzy comprehensive evaluation method and the lowest magnetic noise. This method can obtain a balance between different optimization objectives and effectively improve the comprehensive performance of the shield.

A New Method for Reduction of Atomic Magnetometer Noise Based on Multigene Genetic Programming

An ultra-high precision magnetic field measurement is of great scientific and economic significance. An atomic magnetometer that operates in a spin-exchange relaxation-free (SERF) regime has superior sensitivity and, thus, is of great significance for the ultra-high precision magnetic field measurement. In order to reduce the noise of a SERF atomic magnetometer and further improve its sensitivity, a method for noise reduction based on the multigene genetic programming (MGGP) is presented. Different from the existing methods, in this method, the model of magnetometer noise is established based on the experimental data. Namely, the noise of the SERF atomic magnetometer is first modeled by the MGGP algorithm and then reduced by the obtained model. Besides, in this way, the sensitivity is improved. The experimental results indicate that our MGGP model can adequately reflect the characteristics of the SERF magnetometer noise. Moreover, after applying the proposed method, the sensitivity of the SERF magnetometer is improved about 13 times at 1 Hz, and there is also a significant sensitivity increase at the frequencies less than 10 Hz. Therefore, the proposed method can effectively reduce the noise and improve the sensitivity of the SERF magnetometer in the low-frequency band.

SERF Atomic Magnetometer–Recent Advances and Applications: A Review

This paper aims to review the technology, developments, and applications of spin exchange relaxation free (SERF) atomic magnetometers. The generalized comparisons between the SERF magnetometer and the other types of weak magnetic sensors are elaborated. Further, the fundamental composition and operational principle of the SERF atomic magnetometer are elucidated. The development history and recent technological developments with respect to several aspects, including the alkali metal vapor source, atom heating method, pump and probe lasers, and magnetic shielding system, are presented. The classification and respective advantages and disadvantages of these technologies are described in detail. Finally, the existing and potential applications, as well as the prospective future development trends are presented.

Improvement of Sensitivity by Using Microfabricated Spherical Alkali Vapor Cells for Chip-Scale Atomic Magnetometers

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/Hz1/2 from 5 to 15 Hz, while that based on the planar one is demonstrated with the sensitivity of 400 fT/Hz1/2 from 5 to 15 Hz.

Suppression Method of AC-Stark Shift in SERF Atomic Magnetometer

In a spin-exchange-relaxation-free (SERF) atomic magnetometer with ultrahigh sensitivity, the ac-Stark shift generated by circularly polarized light results in the attenuation of the SERF atomic magnetometer response, and the virtual magnetic field gradient induced by the ac-Stark shift can reduce the sensitivity of the SERF atomic magnetometer. In this paper, we use a hybrid optical pumping method to address the ac-Stark shift problem. By switching between left-hand and right-hand circularly polarized lights and compensating the residual magnetic field of the magnetometer, the ac-Stark shift could be reduced and then suppressed effectively.

Transverse relaxation determination based on light polarization modulation for spin-exchange relaxation free atomic magnetometer**Project supported by the National Natural Science Foundation of China (Grant No. 61227902), the National Key R&D Program of China (Grant No. 2017YFB0503100), and the Natural Science Foundation of Beijing Municipality, China (Grant No. 4162038).

A transverse relaxation determination of spin-exchange relaxation free (SERF) magnetometer based on polarization modulation technique is proposed. Compared with the radio-frequency (RF) excitation and light intensity excitation methods used in SERF magnetometer, the light polarization modulation method has a high stability in low-frequency range, which indicates a more accurate transverse relaxation measurement.

Field optimization method of a dual-axis atomic magnetometer based on frequency-response and dynamics

The frequency-response and dynamics of a dual-axis spin-exchange-relaxation-free (SERF) atomic magnetometer are investigated by means of transfer function analysis. The frequency-response at different bias magnetic fields is tested to demonstrate the effect of the residual magnetic field. The resonance frequency of alkali atoms and magnetic linewidth can be obtained simultaneously through our theoretical model. The coefficient of determination of the fitting results is superior to 0.995 with 95% confidence bounds. Additionally, step responses are applied to analyze the dynamics of the control system and the effect of imperfections. Finally, a noise-limited magnetic field resolution of 15 has been achieved for our dual-axis SERF atomic magnetometer through magnetic field optimization.

Accuracy enhancement of magnetic field distribution measurements within a large cell spin-exchange relaxation-free magnetometer

The factorial design technique is implemented to achieve greater accuracy in the determination of magnetic field distribution within a single cell of spin-exchange relaxation-free atomic magnetometer. Three-dimensional magnetic field distribution within a single vapor cell can be found by consecutively pumping, layer by layer, all the cell volumes perpendicular to the probe laser beam, detected by a photodiode array. Thus each element of the array collects information about the magnetic field in the small volume (voxel) which forms when the corresponding part of the probe beam and optically pumped layer cross. One of the most effective ways to enhance measurement accuracy is repeated pumping of the layers and averaging the measured results. However, the measurement time is multiplied several times due to the repeated scanning of the cell volume. The suggested technique enables increased measurement accuracy of each voxel while preserving the number of measurements. Magnetic field distribution is determined by the illumination of the cell layers one by one or simultaneously, according to a special algorithm, with subsequent multifactorial analysis of the obtained results.

A dual-axis, high-sensitivity atomic magnetometer**Project supported by the National Natural Science Foundation of China (Grant Nos. 61227902, 61673041, and 61374210), the National Key R & D Program of China (Grant No. 2016YFB0501601), and the Academic Excellence Foundation of Beihang University (BUAA) for PhD Students.

Atomic magnetometer (AM) operated in a spin-exchange relaxation-free (SERF) regime features superior sensitivity and non-cryogenic operation, and thus is competitive with the best superconducting quantum interference devices. Previously, SERF AM with fT/Hz1/2 level sensitivity commonly acted as a single-axis sensor. Here we demonstrate a dual-axis SERF AM capable of simultaneously and independently detecting x- and y-field components with a sensitivity of 20 fT/Hz1/2. As there is no necessity to worry about the cross-talk effects arising from field modulations, the dual-axis scheme proposed here is of particular interest to AM array and hence the biomagnetic applications.

The polarization and the fundamental sensitivity of 39K (133Cs)-85Rb-4He hybrid optical pumping spin exchange relaxation free atomic magnetometers

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 85Rb polarization of two types of hybrid optical pumping SERF magnetometers based on 39K-85Rb-4He and 133Cs-85Rb-4He respectively. Then we found that 85Rb polarization varies with the number density of buffer gas 4He and quench gas N2, 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/Hz1/2 (1 aT = 10−18T) with 39K-85Rb-4He magnetometer between above two types of magnetometers when 85Rb polarization is 0.1116. We estimate the fundamental sensitivity limit of the hybrid optical pumping SERF magnetometer to be superior to 1.8359 × 10−2aT/Hz1/2, which is higher than the shot-noise-limited sensitivity of 1 aT/Hz1/2 of K SERF atomic magnetometer.

Ingenious Method for Measuring the Non-Orthogonal Angle of the Saddle-Shaped Coils of an SERF Atomic Magnetometer System

The spin exchange relaxation free (SERF) atomic magnetometer has been a research hotspot due to its ultra-high sensitivity. The SERF atomic magnetometer's sensitivity is subject to the accuracy of its compensating magnetic field and the calibration signal. These two processes need a tri-axial coil system to generate the required magnetic fields, which are directly affected by the non-orthogonality of the tri-axial coil system, especially the non-orthogonality between the x- and y-axes coils. In order to solve this problem, an ingenious and effective method for measuring the non-orthogonal angle between the x- and y-axes in a magnetic coil system has been demonstrated in this paper for the first time. This measurement method was only based on the dynamics of the SERF atomic magnetometer and did not need any additional calibration equipment. A large dc magnetic field B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> was applied to the y-axis, such that the magnetic field B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">yx</sub> projected from B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> on to the x-axis could be measured. By then solving the relevant trigonometric function, we could obtain the non-orthogonal angle. Using this method, we experimentally obtained the non-orthogonal angle of about 84.08° between the x- and y-axes saddle-shaped coils, with a variance of about 0.14°. Finally, the correctness of this method for calculating the non-orthogonal angle was analyzed.

Flat-response spin-exchange relaxation free atomic magnetometer under negative feedback.

We demonstrate that the use of negative feedback extends the detection bandwidth of an atomic magnetometer in a spin-exchange relaxation free (SERF) regime. A flat-frequency response from zero to 190 Hz was achieved, which is nearly a three-fold enhancement while maintaining sensitivity, 3 fT/Hz1/2 at 100 Hz. With the extension of the bandwidth, the linear correlation between measured signals and a magne-tocardiographic field synthesized for comparison was increased from 0.21 to 0.74. This result supports the feasibility of measuring weak biomagnetic signals containing multiple frequency components using a SERF atomic magnetometer under negative feedback.

In situ triaxial magnetic field compensation for the spin-exchange-relaxation-free atomic magnetometer

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.

Magnetocardiography with a modular spin-exchange relaxation-free atomic magnetometer array

We present a portable four-channel atomic magnetometer array operating in the spin-exchange relaxation-free regime. The magnetometer array has several design features intended to maximize its suitability for biomagnetic measurement, specifically foetal magnetocardiography, such as a compact modular design and fibre-coupled lasers. The modular design allows the independent positioning and orientation of each magnetometer. Using this array in a magnetically shielded room, we acquire adult magnetocadiograms. These measurements were taken with a 6–11 fT Hz−1/2 single-channel baseline sensitivity that is consistent with the independently measured noise level of the magnetically shielded room.

Sensitivity Improvement of Spin-Exchange Relaxation Free Atomic Magnetometers by Hybrid Optical Pumping of Potassium and Rubidium

An optically pumped atomic magnetometer using a hybrid cell of potassium and rubidium atoms was demonstrated to yield high sensitivity to magnetic fields. We operated the magnetometer with the four possible combinations of optically pumped and optically probed atoms and found that the combination of optically pumped potassium and optically probed rubidium showed the highest sensitivity among the four combinations because the rubidium atoms were denser than those of potassium. Furthermore, we investigated the dependence of the sensitivity on the power densities of the pump and probe beams and the wavelength of the probe beam. The magnetometer using the hybrid cell required higher pump-beam power and had narrower magnetic linewidth than those of the single alkali-metal cell. However, the magnetic linewidth was larger than the theoretical value, ignoring the spin relaxation caused by the spin-exchange collisions. By adjusting the laser conditions, the highest sensitivity approached 30 fTrms/Hz1/2.

High-temperature alkali vapor cells with antirelaxation surface coatings

Antirelaxation surface coatings allow long spin relaxation times in alkali-metal cells without buffer gas, enabling faster diffusion of the alkali atoms throughout the cell and giving larger signals due to narrower optical linewidths. Effective coatings were previously unavailable for operation at temperatures above 80 °C. We demonstrate that octadecyltrichlorosilane (OTS) can allow potassium or rubidium atoms to experience hundreds of collisions with the cell surface before depolarizing, and that an OTS coating remains effective up to about 170 °C for both potassium and rubidium. We consider the experimental concerns of operating without buffer gas and with minimal quenching gas at high vapor density, studying the stricter need for effective quenching of excited atoms and deriving the optical rotation signal shape for atoms with resolved hyperfine structure in the spin-temperature regime. As an example of a high-temperature application of antirelaxation coated alkali vapor cells, we operate a spin-exchange relaxation-free atomic magnetometer with sensitivity of 6 fT/Hz and magnetic linewidth as narrow as 2 Hz.

A low-noise ferrite magnetic shield

Ferrite materials provide magnetic shielding performance similar to commonly used high permeability metals but have lower intrinsic magnetic noise generated by thermal Johnson currents due to their high electrical resistivity. Measurements inside a ferrite shield with a spin-exchange relaxation-free atomic magnetometer reveal a noise level of 0.75fTHz−1∕2, 25 times lower than what would be expected in a comparable μ-metal shield. The authors identify a 1∕f component of the magnetic noise due to magnetization fluctuations and derive general relationships for the Johnson current noise and magnetization noise in cylindrical ferromagnetic shields in terms of their conductivity and complex magnetic permeability.