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Review of Scientific Instruments

Spin alignment based Rb-87 magnetometry with free spin precession

Yang Li, Lulu Zhang, Yongbiao Yang, Junye Zhao, Yanhua Wang, Baodong Yang, Junmin Wang

Published Mar 1, 2025• 0 citations
Publisher

Abstract

Spin alignment based Rb-87 magnetometry with free spin precession has been proposed. A 795-nm linearly polarized laser beam, serving as the pump beam and the probe beam, propagates through a cylindrical Rb-87 enriched atomic vapor cell along the Z axis, with the polarization orientation aligned along the Y axis. Simultaneously, the static magnetic field B0⃗ is applied along the Y axis, and the π/2 pulse radio-frequency (RF) magnetic field BRF⃗ is applied along the Z axis. Given that the laser polarization direction aligns with the static magnetic field, the pump beam exhibits π polarization. This induces a symmetric population distribution of ground-state atoms across the Zeeman sublevels. This leads to the generation of a magnetic quadrupole moment, thereby facilitating the formation of the spin alignment state. The π/2 pulse RF magnetic field aligns the magnetic quadrupole moment with the direction of the RF magnetic field. Upon the cessation of the RF magnetic field, the magnetic quadrupole moment undergoes precession around the static magnetic field. The linearly polarized probe beam, in conjunction with a balanced polarimeter comprising a true zero-order half-wave plate, a Wollaston prism, and a balanced differential photodiode, is employed to measure the free precession decay signal. In this magnetometry system, both the pump power and the RF magnetic field strength have been optimized. The sensitivity of the spin alignment based Rb-87 magnetometry with free spin precession is about 1.7 pT/Hz1/2.

DOI
10.1063/5.0242881

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Spin alignment based Rb-87 magnetometry with free spin precession has been proposed. A 795-nm linearly polarized laser beam, serving as the pump beam and the probe beam, propagates through a cylindrical Rb-87 enriched atomic vapor cell along the Z axis, with the polarization orientation aligned along the Y axis. Simultaneously, the static magnetic field B0⃗ is applied along the Y axis, and the π/2 pulse radio-frequency (RF) magnetic field BRF⃗ is applied along the Z axis. Given that the laser polarization direction aligns with the static magnetic field, the pump beam exhibits π polarization. This induces a symmetric population distribution of ground-state atoms across the Zeeman sublevels. This leads to the generation of a magnetic quadrupole moment, thereby facilitating the formation of the spin alignment state. The π/2 pulse RF magnetic field aligns the magnetic quadrupole moment with the direction of the RF magnetic field. Upon the cessation of the RF magnetic field, the magnetic quadrupole moment undergoes precession around the static magnetic field. The linearly polarized probe beam, in conjunction with a balanced polarimeter comprising a true zero-order half-wave plate, a Wollaston prism, and a balanced differential photodiode, is employed to measure the free precession decay signal. In this magnetometry system, both the pump power and the RF magnetic field strength have been optimized. The sensitivity of the spin alignment based Rb-87 magnetometry with free spin precession is about 1.7 pT/Hz1/2.
In this paper, we present a triaxial alignment magnetometer based on free-spin precession deployed in the geomagnetic range. Existing vector measurement methods often require complex optical setups, heating structures, and laser modulation. This study addresses this challenge by employing a linearly polarized probe beam to induce atomic alignment and subsequently detecting the optical polarization rotation caused by the pulsed radio frequency field. The experiment is conducted in a paraffin-coated cell without buffer gas at room temperature, containing rubidium with natural abundance. We report triaxial measurements with a static magnetic field amplitude of approximately 50 μTμ{\text{T}}μT (close to Earth's magnetic field), where the noise levels for each axis are approximately 5.3 pT/Hz{\text{pT/}}\sqrt{\text{Hz}}pT/Hz​, 4.7 pT/Hz{\text{pT/}}\sqrt{\text{Hz}}pT/Hz​, and 9.3 pT/Hz{\text{pT/}}\sqrt{\text{Hz}}pT/Hz​ respectively. The proposed method demonstrates a simple structure suitable for cost-effective and versatile applications.
We report a rubidium atomic magnetometer based on pump-probe nonlinear magneto-optical rotation. The rubidium vapor cell is placed in a five-layer magnetic shield with inner coils that can generate uniform magnetic fields along the direction of pump beam, and the cell is also placed in the center of a Helmholtz coil that can generate an oscillating magnetic field perpendicular to the direction of pump beam. The atoms are optically pumped by circularly polarized pump beam along a constant magnetic field in a period of time, then the pump beam is turned off and a /2 pulse of oscillating magnetic field for 87Rb atoms is applied. After the above process, the individual atomic magnetic moments become phase coherent, resulting in a transverse magnetization vector precessing at the Larmor frequency in the magnetic field. The linearly polarized probing beam is perpendicular to the direction of magnetic field, and can be seen as a superposition of the left and right circularly polarized light. Because of the different absorptions and dispersions of the left and right circularly polarized light by rubidium atoms, the polarization direction of probing beam rotates when probing beam passes through rubidium vapor cell. The rotation of the polarization is subsequently converted into an electric signal through a polarizing beam splitter. Finally, the decay signal related to the transverse magnetization vector is measured. The Larmor frequency proportional to magnetic field is obtained by the Fourier transform of the decay signal. The value of magnetic field is calculated from the formula:B=(2/) f, where and f are the gyromagnetic ratio and Larmor frequency, respectively. In order to measure the magnetic field in a wide range, the tracking lock mode is proposed and tested. The atomic magnetometer can track the magnetic field jump of 1000 nT or 10000 nT, indicating that the atomic magnetometer has strong locking ability and can be easily locked after start-up. The main performances in different magnetic fields are tested. The results show that the measurement range of the atomic magnetometer is from 100 nT to 100000 nT, the extreme sensitivity is 0.2 pT/Hz1/2, and the magnetic field resolution is 0.1 pT. The transverse relaxation times of the transverse magnetization vector in different magnetic fields are obtained, and the relaxation time decreases with the increase of the magnetic field. When the measurement range is from 5000 nT to 100000 nT, the magnetic field sampling rate of the atomic magnetometer can be adjusted in a range from 1 Hz to 1000 Hz. The atomic magnetometer in high sampling rate can measure weak alternating magnetic field at low frequency. This paper provides an important reference for developing the atomic magnetometer with large measurement range, high sensitivity and high sampling rate.
Free-induction-decay (FID) magnetometers have evolved as simple magnetic sensors for sensitive detection of unknown magnetic fields. However, these magnetometers suffer from a fundamental problem known as a “dead zone,” making them insensitive to certain magnetic field directions. Here, we demonstrate a simple experimental scheme for the dead-zone-free operation of a FID atomic magnetometer. Using a single laser beam containing equal strength of linear- and circular-polarization components and amplitude modulation at a low-duty cycle, we have synchronously pumped the rubidium (87Rb) atoms with both first- and second-order frequency harmonics. Such a pumping scheme has enabled us to observe the free Larmor precession of atomic spins at a frequency of ΩL (orientation) and/or 2ΩL (alignment) in a single FID signal, depending on the direction of the external magnetic field. We observed that the amplitude of the FID signal does not go to zero for any magnetic field direction, proving the absence of dead zones in the magnetometer. The magnetometer has a sensitivity in the range of 3.2–8.4 pT/Hz in all directions. Our experimental scheme can be crucial in developing miniaturized atomic magnetometers for various practical applications, including geomagnetic applications.
Abstract An alignment magnetometer based on free‐spin precession (FSP) has been deployed in the geomagnetic range. A linearly polarized probe beam is utilized to create alignment in a buffer‐gas‐free paraffin‐coated natural abundance rubidium vapor cell at room temperature exposed to a pulsed radio frequency field. Furthermore, the utilization of hyperfine‐selective optical repumping employing linearly polarized beams amplifies the proportion of responsive atoms. As a consequence, this leads to a remarkable enhancement in the optical rotation signal. This paper reports measurements with a minor static magnetic field of about 1 inside a four‐layer ‐metal shield, achieving a system noise level of 0.16 ). With a static magnetic field amplitude of 48.5 (in proximity to Earth's magnetic field), the noise level is ).
Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled distributed feedback laser, we achieve magnetic field sensitivity of 7 fT/Hz7\text{ }\text{fT}/\sqrt{\text{Hz}}7 fT/Hz​ with a miniature $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{mm}\text{ }\text{Rb}$ vapor cell.
The dead zones of atomic magnetometers bring great challenges for the mobile applications in high-sensitivity magnetic field measurement. To solve this fundamental problem, we present a dead-zone-free orientation-based atomic magnetometer employing free-induction-decay (FID) scheme. A modulation scheme combining synchronous optical pumping and radio frequency pulse manipulation is utilized to achieve the free precession of spin polarization in all magnetic field orientations, and the FID signals are detected in two directions with an orthogonal pump-probe light configuration. We perform a comprehensive theoretical analysis of spin polarization evolution for arbitrary magnetic field orientations, and both simulation and experimental results demonstrate that the fusion of information from the pump and probe beams enables the dead-zone-free measurement of the magnetometer, with the sensor exhibiting a sensitivity in the range of 0.6 – 1.4 pT / Hz 1 / 2 across all directions. The proposed scheme is of great significance for the advancement of pulsed atomic magnetometers and their various applications in mobile platforms.