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Journal of Physics D Applied Physics

A 85Rb transverse modulation magnetometer in the geophysical range based on atomic alignment states

Tao Shi, Ge Jin, Hong Lin Zhang, Sheng Zou

Published Aug 22, 2024• 2 citations
Publisher

Abstract

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.

DOI
10.1088/1361-6463/ad726f

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Journal of Physics D Applied Physics•Oct 24, 2022•22 citations
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.
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.
A longitudinal radio-frequency (rf) magnetic field resonating at the Larmor frequency along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ direction in the presence of a static transverse magnetic field s investigated. Optical-radio-frequency double resonance in 85Rb^{85}\mathrm{Rb}85Rb characterizes the strength of an external magnetic field through the precession frequency of 85Rb^{85}\mathrm{Rb}85Rb atoms. The measurements are carried out in a paraffin-coated 85Rb^{85}\mathrm{Rb}85Rb cell with a spin alignment configuration. An intriguing discovery of magnetic resonance peaks is observed at subharmonics of the Larmor frequency, accompanied by the emergence of nonlinear effects in the vicinity of the resonance. Notably, when the Larmor frequency reaches zero, the optical rotation signals induced by the rf magnetic field demonstrate a superposition of high-order harmonics of the rf frequency. Based on the simplified density matrix formalism and perturbation treatment, analytical expressions for the optical rotation are consistent with experimental outcomes. These findings hold the potential of extending our understanding on unveiling the dynamics of population and coherence among Zeeman sublevels, thereby advancing our knowledge in the field of alignment magnetometry.
A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.
Optically pumped magnetometers are important instruments for imaging biological magnetic signals without the need for cryogenic cooling. These magnetometers are presently available in the commercial market and utilize the principles of atomic alignment or orientation, enabling remarkable sensitivity and precision in the measurement of magnetic fields. This research focuses on utilizing a spin-aligned atomic ensemble for magnetometry at zero field. An approach is introduced which involves evaluating how the linear polarization of light rotates as it passes through the atomic vapor to null the magnetic field. Analytical expressions are derived for the resulting spin alignment and photodetection signals. Experimental results are provided, demonstrating good agreement with the theoretical predictions. The sensitivity and bandwidth of the magnetometer are characterized based on the detected polarization rotation signal. Lastly, the practical utility of the magnetometer for medical applications is demonstrated by successfully detecting a synthetic cardiac signal.
Zero-field atomic ensembles enable the development of ultrahigh-sensitivity magnetometry. However, their intrinsic insensitivity to longitudinal magnetic fields along the pumping direction poses a fundamental limitation, restricting precise weak-field quantification and complex system decoupling. Herein, we propose a novel zero-field weak parametric modulation scheme capable of ultrahigh-sensitivity extraction of longitudinal magnetic fields. Transverse modulation is employed to manifest the electron spin polarization projection induced by the longitudinal fields. Based on the perturbation model established, first-order parametric resonance associated with the longitudinal magnetic field yields significant signal enhancement under low-frequency and weak modulation conditions, while simultaneously suppressing spin-exchange relaxation caused by magnetic field modulation. We experimentally achieved a longitudinal measurement sensitivity of 16 fT / Hz 1 / 2 , more than an order of magnitude improved over traditional schemes, with a bandwidth of 70 Hz. Benefiting from low spin relaxation, the noise floor of the transverse magnetic field measurement is preserved at 4 fT / Hz 1 / 2 . The proposed scheme pushes the boundaries of existing weak magnetic field measurement, providing critical capabilities for state-of-the-art precision experiments including new physical quantity detection.
Abstract We display a single-beam triaxial spin-exchange relaxation-free (SERF) atomic magnetometer by utilizing two orthogonal transverse modulation fields perpendicular to the light. The magnetic field’s three-axis vector components can be extracted simultaneously and independently from the transmitted light by lock-in amplifiers with reference to the different demodulated frequencies. We find that the residual magnetic field is a significant factor that could affect triaxial coupling, and specially the influence of the z -axis remanence on xy -axis coupling is studied in detail. Under the optimal parameters, the three-axis sensitivities are 20 fT Hz − 1 / 2 in the x - and y - axis, and 230 fT Hz − 1 / 2 in the z -axis. Meanwhile, the three-axis bandwidths are 110 Hz, 138 Hz and 211 Hz in the x -, y - and z -axis, respectively. The proposed single-beam triaxial SERF atomic magnetometer could be used to construct a compact and low-cost atomic sensor that is particularly suitable for the vector measurement of the biomagnetic such as magnetoencephalography and magnetocardiography.