Spin-exchange Relaxation-free Magnetometer Research Articles

Measurement of the spatial magnetic field distribution in a single large spin-exchange relaxation-free vapor cell

This article presents a method to determine the magnetic field distribution within the vapor cell of a spin-exchange relaxation-free (SERF) atomic magnetometer with a sensitivity of the order of 10 femtoTesla and a bandwidth of DC to 100 Hz, in the presence of an uncompensated ambient magnetic field of up to several nanoTesla. The method is based on the analysis of the atomic polarization in a multichannel pump–probe configuration, in which a spatially selective optical pumping enables to probe specific layers of the atomic vapor contained in a gas-buffered cell. An SERF magnetometer is inherently sensitive to one component of the magnetic field, orthogonal to the pump and probe laser beams. The sensor’s performance can be drastically degraded by the other uncompensated components of the magnetic field. Typically, SERF magnetometry requires very good magnetic shielding and active compensation of residual magnetic field to properly function; this is commonly achieved by applying a complex design of Helmholtz coils in a sophisticated compensation procedure. The method suggested in this article eliminates the influence of non-uniform residual magnetic fields on the accuracy of measurements without precise compensation of the interfering field components. This procedure is used to simplify the measurements of magnetic field distribution in a large vapor cell with required accuracy. This is critically important for precise multichannel magnetic field mapping used for localization of the magnetic dipole-field source.

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.

Experimental Constraint on an Exotic Spin- and Velocity-Dependent Interaction in the Sub-meV Range of Axion Mass with a Spin-Exchange Relaxation-Free Magnetometer.

We conducted a search for an exotic spin- and velocity-dependent interaction for polarized electrons with an experimental approach based on a high-sensitivity spin-exchange relaxation-free (SERF) magnetometer, which serves as both a source of polarized electrons and a magnetic-field sensor. The experiment aims to sensitively detect magnetic-fieldlike effects from the exotic interaction between the polarized electrons in a SERF vapor cell and unpolarized nucleons of a closely located solid-state mass. We report experimental results on the interaction with 82h of data averaging, which sets an experimental limit on the coupling strength around 10^{-19} for the axion mass m_{a}≲10^{-3} eV, within the important axion window.

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.

Influence of Buffer-Gas Pressure Inside Micro Alkali Vapor Cells on the Performance of Chip-Scale SERF Magnetometers

Spin-exchange-relaxation-free (SERF) optical atomic magnetometers are facing the challenges of dramatic decrease of sensitivity when the alkali vapor cells become small. This paper focuses on the sensitivity improvement of the chip-scale atomic magnetometer (CSAM) by changing buffer-gas pressures inside the microfabricated vapor cells. The rubidium vapor cells with three different buffer-gas pressures were tested in the SERF magnetometer. The influences of vapor cell temperature and pumping laser power on the device sensitivity are also characterized for further optimization. Results show that the three atomic vapor cells, with $N_{2}$ buffer-gas pressure of 0.20, 0.88, and 2.35 amg, have sensitivities of 900, 500, and 150 fT/Hz1/2, respectively. Results indicated that when the alkali vapor cells continue to be smaller, high buffer-gas pressure would provide an effective way to reduce the wall collision and improve the performance of CSAMs.

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.

Polarization Measurement of Cs Using the Pump Laser Beam

In the optical pumping systems based on the pump-probe arrangement, the spin polarization of the atoms is generally monitored utilizing the probe laser beam, in which way an extra perturbation must be introduced and thus affects the normal operation of the sensors. By investigating the absorption rate of the circularly polarized pump laser, here we demonstrate the feasibility of extracting the electron-spin polarization from the transmitted pump laser intensity. We experimentally validate the method in a spin-exchange relaxation free (SERF) magnetometer and the results are in excellent agreement with the theory. The scheme operates in a silent mode and features a real-time observation. We also study the corresponding magnetic field response of the SERF magnetometer and a term arising from the diffusion effects has been added to the original model to explain the discrepancy of the response.

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.

Search for exotic spin-dependent interactions with a spin-exchange relaxation-free magnetometer

We propose a novel experimental approach to explore exotic spin-dependent interactions using a spin-exchange relaxation-free (SERF) magnetometer, the most sensitive non-cryogenic magnetic-field sensor. This approach studies the interactions between optically polarized electron spins located inside a vapor cell of the SERF magnetometer and unpolarized or polarized particles of external solid-state objects. The coupling of spin-dependent interactions to the polarized electron spins of the magnetometer induces the tilt of the electron spins, which can be detected with high sensitivity by a probe laser beam similarly as an external magnetic field. We estimate that by moving unpolarized or polarized objects next to the SERF Rb vapor cell, the experimental limit to the spin-dependent interactions can be significantly improved over existing experiments, and new limits on the coupling strengths can be set in the interaction range below 0.01 m.

Spin-exchange relaxation-free magnetometer with nearly parallel pump and probe beams

We constructed a spin-exchange relaxation-free (SERF) magnetometer with a small angle between the pump and probe beams facilitating a multi-channel design with a flat pancake cell. This configuration provides almost complete overlap of the beams in the cell, and prevents the pump beam from entering the probe detection channel. By coupling the lasers in multi-mode fibers, without an optical isolator or field modulation, we demonstrate a sensitivity of 10 f T for frequencies between 10 Hz and 100 Hz. In addition to the experimental study of sensitivity, we present a theoretical analysis of SERF magnetometer response to magnetic fields for small-angle and parallel-beam configurations, and show that at optimal DC offset fields the magnetometer response is comparable to that in the orthogonal-beam configuration. Based on the analysis, we also derive fundamental and probe-limited sensitivities for the arbitrary non-orthogonal geometry. The expected practical and fundamental sensitivities are of the same order as those in the orthogonal geometry. We anticipate that our design will be useful for magnetoencephalography (MEG) and magnetocardiography (MCG) applications.

Effects of AC magnetic field on spin-exchange relaxation of atomic magnetometer

By operating at high alkali-metal densities and in low magnetic fields, the spin-exchange relaxation of atomic magnetometers can be eliminated, allowing construction of ultra-high sensitive spin-exchange relaxation-free magnetometers. Significant AC magnetic fields are usually introduced in the magnetometer by the magnetic field modulation technique or the cell heater, whose effects on the spin-exchange relaxation have not been evaluated. In this paper, we study experimentally the spin-exchange relaxation rate as a function of the magnetic field frequency, the magnetic field amplitude and the spin-exchange rate in low magnetic fields. The experimental results indicate that for low atomic polarization the spin-exchange relaxation rate decays exponentially with the magnetic field frequency, and the reciprocal of the decay constant is proportional to the spin-exchange rate.

Spin dynamics of the potassium magnetometer in spin-exchange relaxation free regime**Project supported by the National Natural Science Foundation of China (Grant No. 61227902).

The laser-pumped potassium spin-exchange relaxation free (SERF) magnetometer is the most sensitive detector of magnetic field and has many important applications. We present the experimental results of our potassium SERF magnetometer. A pump–probe approach is used to identify the unique spin dynamics of the atomic ensemble in the SERF regime. A single channel sensitivity of 8 f·THz−1/2 is achieved with our SERF magnetometer.

Suppression of vapor cell temperature error for spin-exchange-relaxation-free magnetometer.

This paper presents a method to reduce the vapor cell temperature error of the spin-exchange-relaxation-free (SERF) magnetometer. The fluctuation of cell temperature can induce variations of the optical rotation angle, resulting in a scale factor error of the SERF magnetometer. In order to suppress this error, we employ the variation of the probe beam absorption to offset the variation of the optical rotation angle. The theoretical discussion of our method indicates that the scale factor error introduced by the fluctuation of the cell temperature could be suppressed by setting the optical depth close to one. In our experiment, we adjust the probe frequency to obtain various optical depths and then measure the variation of scale factor with respect to the corresponding cell temperature changes. Our experimental results show a good agreement with our theoretical analysis. Under our experimental condition, the error has been reduced significantly compared with those when the probe wavelength is adjusted to maximize the probe signal. The cost of this method is the reduction of the scale factor of the magnetometer. However, according to our analysis, it only has minor effect on the sensitivity under proper operating parameters.

Study of the operation temperature in the spin-exchange relaxation free magnetometer.

We study the influence of the cell temperature on the sensitivity of the spin-exchange relaxation free (SERF) magnetometer and analyze the possibility of operating at a low temperature. Utilizing a 25 × 25 × 25 mm(3) Cs vapor cell with a heating temperature of 85 °C, which is almost half of the value of potassium, we obtain a linewidth of 1.37 Hz and achieve a magnetic field sensitivity of 55 fT/Hz(1/2) in a single channel. Theoretical analysis shows that fundamental sensitivity limits of this device with an active volume of 1 cm(3) could approach 1 fT/Hz(1/2). Taking advantage of the higher saturated vapor pressure, SERF magnetometer based on Cs opens up the possibility for low cost and portable sensors and is particularly appropriate for lower temperature applications.

In-situ measurement of magnetic field gradient in a magnetic shield by a spin-exchange relaxation-free magnetometer**Project supported by the National Natural Science Foundation of China (Grant Nos. 61227902, 61374210, and 61121003).

A method of measuring in-situ magnetic field gradient is proposed in this paper. The magnetic shield is widely used in the atomic magnetometer. However, there is magnetic field gradient in the magnetic shield, which would lead to additional gradient broadening. It is impossible to use an ex-situ magnetometer to measure magnetic field gradient in the region of a cell, whose length of side is several centimeters. The method demonstrated in this paper can realize the in-situ measurement of the magnetic field gradient inside the cell, which is significant for the spin relaxation study. The magnetic field gradients along the longitudinal axis of the magnetic shield are measured by a spin-exchange relaxation-free (SERF) magnetometer by adding a magnetic field modulation in the probe beam’s direction. The transmissivity of the cell for the probe beam is always inhomogeneous along the pump beam direction, and the method proposed in this paper is independent of the intensity of the probe beam, which means that the method is independent of the cell’s transmissivity. This feature makes the method more practical experimentally. Moreover, the AC-Stark shift can seriously degrade and affect the precision of the magnetic field gradient measurement. The AC-Stark shift is suppressed by locking the pump beam to the resonance of potassium’s D1 line. Furthermore, the residual magnetic fields are measured with σ+- and σ–-polarized pump beams, which can further suppress the effect of the AC-Stark shift. The method of measuring in-situ magnetic field gradient has achieved a magnetic field gradient precision of better than 30 pT/mm.

A fast determination method for transverse relaxation of spin-exchange-relaxation-free magnetometer.

We propose a fast and accurate determination method for transverse relaxation of the spin-exchange-relaxation-free (SERF) magnetometer. This method is based on the measurement of magnetic resonance linewidth via a chirped magnetic field excitation and the amplitude spectrum analysis. Compared with the frequency sweeping via separate sinusoidal excitation, our method can realize linewidth determination within only few seconds and meanwhile obtain good frequency resolution. Therefore, it can avoid the drift error in long term measurement and improve the accuracy of the determination. As the magnetic resonance frequency of the SERF magnetometer is very low, we include the effect of the negative resonance frequency caused by the chirp and achieve the coefficient of determination of the fitting results better than 0.998 with 95% confidence bounds to the theoretical equation. The experimental results are in good agreement with our theoretical analysis.

Precision limit of atomic magnetometers in the presence of spin-destruction collisions

Ultra sensitive detection and high precision measurement of magnetic fields have been of great importance for both fundamental physics and practical applications. Recent developments in the technology of atomic magnetometers have enabled the spin-exchange relaxation free (SERF) atomic magnetometers as the most sensitive devices for detecting and measuring static or quasi-static magnetic fields. In this paper, we analyze the effect of the most prominent relaxation mechanism (i.e., the spin-destruction collisions) on the precision of SERF magnetometers. For static magnetic fields, it is explicitly demonstrated how the spin-destruction collisions degrade the precision of the SERF magnetometers from the Heisenberg limit to the standard limit even with quantum resources being employed.

Optimizations of spin-exchange relaxation-free magnetometer based on potassium and rubidium hybrid optical pumping.

The hybrid optical pumping atomic magnetometers have not realized its theoretical sensitivity, the optimization is critical for optimal performance. The optimizations proposed in this paper are suitable for hybrid optical pumping atomic magnetometer, which contains two alkali species. To optimize the parameters, the dynamic equations of spin evolution with two alkali species were solved, whose steady-state solution is used to optimize the parameters. The demand of the power of the pump beam is large for hybrid optical pumping. Moreover, the sensitivity of the hybrid optical pumping magnetometer increases with the increase of the power density of the pump beam. The density ratio between the two alkali species is especially important for hybrid optical pumping magnetometer. A simple expression for optimizing the density ratio is proposed in this paper, which can help to determine the mole faction of the alkali atoms in fabricating the hybrid cell before the cell is sealed. The spin-exchange rate between the two alkali species is proportional to the saturated density of the alkali vapor, which is highly dependent on the temperature of the cell. Consequently, the sensitivity of the hybrid optical pumping magnetometer is dependent on the temperature of the cell. We proposed the thermal optimization of the hybrid cell for a hybrid optical pumping magnetometer, which can improve the sensitivity especially when the power of the pump beam is low. With these optimizations, a sensitivity of approximately 5 fT/Hz(1/2) is achieved with gradiometer arrangement.

In situ magnetic compensation for potassium spin-exchange relaxation-free magnetometer considering probe beam pumping effect.

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.