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Advanced Quantum Technologies

A Room‐Temperature Operational Alignment Magnetometer Utilizing Free‐Spin Precession

Ge Jin, Tao Shi, Yan Zhao, Hong Zhang, Sheng Zou

Published Jan 10, 2024• 16 citations
Publisher

Abstract

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 ).

DOI
10.1002/qute.202300377

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APS Open Science•Jun 8, 2026•0 citations
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 ).
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 describe a room-temperature alkali-metal atomic magnetometer for detection of small, high-frequency magnetic fields. The magnetometer operates by detecting optical rotation due to the precession of an aligned ground state in the presence of a small oscillating magnetic field. The resonance frequency of the magnetometer can be adjusted to any desired value by tuning the bias magnetic field. Based on experimentally measured signal-to-noise ratio, we demonstrate a sensitivity of 100pG∕Hz100\phantom{\rule{0.3em}{0ex}}\mathrm{pG}∕\sqrt{\mathrm{Hz}}100pG∕Hz​ (rms) in a $3.5\text{\ensuremath{-}}\mathrm{cm}$-diameter paraffin coated cell. Assuming detection at the photon shot-noise limit, we project a sensitivity as low as 25pG∕Hz25\phantom{\rule{0.3em}{0ex}}\mathrm{pG}∕\sqrt{\mathrm{Hz}}25pG∕Hz​ (rms).
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.
Abstract We have constructed a transverse modulated magnetometer based on spin alignment in a paraffin-coated 85 Rb cell operated in a geophysical magnetic field of 47.1 µT. When an orthogonal driving magnetic field ( x ^ axis) is resonant on the Larmor frequency ( z ^ axis), we have proposed a new method to zero the static residual magnetic fields in the transverse plane and achieved a sensitivity of 1.8 pT Hz − 1 with bandwidth of 200 Hz. The repump light ( F g = 2 → F e = 2 ) redistributes the populations in the ground state, rendering the state | F g = 2 ⟩ dark. This effect significantly amplifies the optical rotation signals nearly fourfold. The numerical solution of the Liouville equation is in good agreement with the experimental results. By using perturbation treatment and employing appropriate approximations, the derived analytical expressions for optical rotation are deduced to succinctly elucidate the dynamics of atomic alignment under parametric modulation. These outcomes could be extended for the advancement of an alignment magnetometer that has been designed to detect a weak magnetic signal in the geophysical range.
We demonstrate an atomic magnetometry using amplitude-modulated pumping and hyperfine repumping techniques in a paraffin-coated cell. By exploiting the constructive interference between spins polarized by the pump beam and an additional repump beam, we observe a three-fold increase in the amplitude of magnetic resonance, along with a reduction in linewidth by approximately two times. The implementation of the repump beam effectively narrows the linewidth, demonstrating successful suppression of spin-exchange relaxation. This reduction in relaxation rate, combined with the enhanced signal, significantly improves the sensitivity of the magnetometer. Consequently, our technique offers a promising approach for achieving SERF-like magnetometry with sub-fT-level sensitivity in Earth-field range and room-temperature environment.
We present an implementation of an atomic magnetometer based on free spin precession of optically pumped Rb 87 atoms. The design features a microfabricated vapor cell with an effective volume of 7.1 mm 3 containing isotopically enriched Rb 87 and nitrogen buffer gas. The magnetometer is designed to operate in the Earth-field range, exhibiting a magnetic noise floor of about 147 fT / Hz at field strengths comparable to Earth's magnetic field, i.e., around 50 μ T . This is in close agreement with the Cramér-Rao lower bound of 118 fT / Hz . The good sensitivity as well as the high dynamic range enables a wide range of applications, including magnetocardiography and geomagnetic measurements.