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arXiv:2605.01460

FID Magnetometer Based on Paraffin-Coated Planar Reflective Multipass Cells

Xiangyu Li, Tingxuan Guo, Yang Li, Kaifeng Zhao

physics.opticsphysics.atom-phquant-phSubmitted May 2, 2026• Announced May 5, 2026• 0 citations
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Abstract

We demonstrate a paraffin-coated planar reflective multipass vapor cell for compact optical atomic magnetometry. The cell has an internal volume of $12 \times 12 \times 8~\mathrm{mm}^3$ and supports 20 optical passes with a total transmittance exceeding $65\%$, while maintaining a longitudinal spin-relaxation time of $^{87}\mathrm{Rb}$ longer than $1~\mathrm{s}$. The planar geometry provides spatially separated input and output beams, enabling compact optical integration. A single-cell free-induction-decay (FID) magnetometer reaches $10~\mathrm{pT}/\sqrt{\mathrm{Hz}}$ in the geomagnetic-field range, presently limited by current-source noise in the field coils. A two-cell differential configuration achieves a sensitivity of $\sim 28~\mathrm{fT}/\sqrt{\mathrm{Hz}}$ over the $1$--$15~\mathrm{Hz}$ band for bias fields of $0.3$--$0.7~\mathrm{G}$. These results establish that paraffin-coated planar multipass cells offer high optical depth, long coherence times, and an integration-friendly platform for ultrasensitive magnetometry.

AI Summary

The paper investigates whether internal planar multipass optics can be combined with a paraffin anti-relaxation coating to increase atom–light interaction length in compact vapor-cell magnetometers without sacrificing spin lifetime or optical transmission. Dielectric mirrors are deposited on two opposing inner windows of a $12\times12\times8~\mathrm{mm}^3$ cell, with separate entrance and exit ports. The fabrication combines optical-contact bonding, evaporative tetratetracontane coating, and $^{87}\mathrm{Rb}$ filling. Completed cells support 20 optical passes with 65–75% transmission. Dark-relaxation measurements on two cells give longitudinal relaxation times $T_1>1.1~\mathrm{s}$ at room temperature, corresponding to approximately $4.4\times10^4$ wall collisions, decreasing to about $0.6~\mathrm{s}$ at $50~^\circ\mathrm{C}$. Magnetometry is demonstrated at $50~^\circ\mathrm{C}$ inside a four-layer magnetic shield. Optical pumping followed by a $\pi/12$ spin-tip pulse produces FID signals measured over $27.5~\mathrm{ms}$ per cycle; the small tilt suppresses contributions from additional nonlinear-Zeeman-split components. Single-channel sensitivity is $10~\mathrm{pT}/\sqrt{\mathrm{Hz}}$, limited by bias-coil current-source noise. Differential frequency-ratio readout of two cells yields $28~\mathrm{fT}/\sqrt{\mathrm{Hz}}$ at $0.5~\mathrm{G}$ over approximately 0.5–15 Hz, with comparable results at 0.3 and 0.7 G. A split-signal reference measurement gives a frequency-counter noise floor of $19~\mathrm{fT}/\sqrt{\mathrm{Hz}}$; estimated photon-shot- and spin-projection-noise limits are approximately 3 and 7 $\mathrm{fT}/\sqrt{\mathrm{Hz}}$. Residual noise is attributed to channel mismatch and non-common-mode environmental perturbations. Coating preparation requires one to three trials, and relaxation increases markedly above $80~^\circ\mathrm{C}$, consistent with coating melting.

Little sign of AI-generated textIncrementalMissing comparison

Error found: The spin-projection-noise appendix omits the measurement-to-spectral normalization and channel bookkeeping: its parameters imply approximately 6.5 fT/√Hz single-cell or 9.2 fT/√Hz differential sensitivity under the usual one-sided convention. The photon-shot-noise parameters imply approximately 1.0 fT/√Hz differential sensitivity after bandwidth normalization, rather than the quoted 2.9 fT/√Hz.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

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8 pages, 7 figures

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We demonstrate a paraffin-coated planar reflective multipass vapor cell for compact optical atomic magnetometry. The cell has an internal volume of 12×12×8 mm312 \times 12 \times 8~\mathrm{mm}^312×12×8 mm3 and supports 20 optical passes with a total transmittance exceeding 65%65\%65%, while maintaining a longitudinal spin-relaxation time of 87Rb^{87}\mathrm{Rb}87Rb longer than 1 s1~\mathrm{s}1 s. The planar geometry provides spatially separated input and output beams, enabling compact optical integration. A single-cell free-induction-decay (FID) magnetometer reaches 10 pT/Hz10~\mathrm{pT}/\sqrt{\mathrm{Hz}}10 pT/Hz​ in the geomagnetic-field range, presently limited by current-source noise in the field coils. A two-cell differential configuration achieves a sensitivity of ∼28 fT/Hz\sim 28~\mathrm{fT}/\sqrt{\mathrm{Hz}}∼28 fT/Hz​ over the 111--15 Hz15~\mathrm{Hz}15 Hz band for bias fields of 0.30.30.3--0.7 G0.7~\mathrm{G}0.7 G. These results establish that paraffin-coated planar multipass cells offer high optical depth, long coherence times, and an integration-friendly platform for ultrasensitive magnetometry.
The paper investigates whether internal planar multipass optics can be combined with a paraffin anti-relaxation coating to increase atom–light interaction length in compact vapor-cell magnetometers without sacrificing spin lifetime or optical transmission. Dielectric mirrors are deposited on two opposing inner windows of a 12×12×8 mm312\times12\times8~\mathrm{mm}^312×12×8 mm3 cell, with separate entrance and exit ports. The fabrication combines optical-contact bonding, evaporative tetratetracontane coating, and 87Rb^{87}\mathrm{Rb}87Rb filling.
Completed cells support 20 optical passes with 65–75% transmission. Dark-relaxation measurements on two cells give longitudinal relaxation times T1>1.1 sT_1>1.1~\mathrm{s}T1​>1.1 s at room temperature, corresponding to approximately 4.4×1044.4\times10^44.4×104 wall collisions, decreasing to about 0.6 s0.6~\mathrm{s}0.6 s at 50 ∘C50~^\circ\mathrm{C}50 ∘C. Magnetometry is demonstrated at 50 ∘C50~^\circ\mathrm{C}50 ∘C inside a four-layer magnetic shield. Optical pumping followed by a π/12\pi/12π/12 spin-tip pulse produces FID signals measured over 27.5 ms27.5~\mathrm{ms}27.5 ms per cycle; the small tilt suppresses contributions from additional nonlinear-Zeeman-split components. Single-channel sensitivity is 10 pT/Hz10~\mathrm{pT}/\sqrt{\mathrm{Hz}}10 pT/Hz​, limited by bias-coil current-source noise. Differential frequency-ratio readout of two cells yields 28 fT/Hz28~\mathrm{fT}/\sqrt{\mathrm{Hz}}28 fT/Hz​ at 0.5 G0.5~\mathrm{G}0.5 G over approximately 0.5–15 Hz, with comparable results at 0.3 and 0.7 G.
A split-signal reference measurement gives a frequency-counter noise floor of 19 fT/Hz19~\mathrm{fT}/\sqrt{\mathrm{Hz}}19 fT/Hz​; estimated photon-shot- and spin-projection-noise limits are approximately 3 and 7 fT/Hz\mathrm{fT}/\sqrt{\mathrm{Hz}}fT/Hz​. Residual noise is attributed to channel mismatch and non-common-mode environmental perturbations. Coating preparation requires one to three trials, and relaxation increases markedly above 80 ∘C80~^\circ\mathrm{C}80 ∘C, consistent with coating melting.
Optically pumped magnetometers (OPMs) based on the free-induction-decay (FID) configuration have recently attracted interest owing to their advantages, such as being calibration-free, easy to operate, and showing reduced light shifts induced by the pump light. Currently, alkali-metal vapor cells have been employed in most of the FID magnetometers. However, the relatively narrow linewidth of the magnetic resonance signal and nonlinear Zeeman (NLZ) effects generate bandwidth limitation and heading errors of these magnetometers in the Earth's field, respectively. In this work, we propose a 4He{}^{4}\mathrm{He}4He-based FID magnetometer, which not only has the advantages of being NLZ-free and showing a high bandwidth, but also maintains an enhanced sensitivity assisted by a multipass cell. This magnetometer demonstrates a magnetic field noise floor of 0.34pT/Hz1/20.34\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/{\mathrm{Hz}}^{1/2}0.34pT/Hz1/2 with a Nyquist-limited bandwidth of 555 kHz, which opens another route for atomic magnetometry using FID signals, and exhibits potential for applications in high-frequency magnetic field detection, such as biomagnetic measurements and magnetic communications in the geomagnetic field.
Miniaturized high-sensitivity atomic magnetometers play a crucial role in biomagnetic measurements. The utilization of multipass cells enhances the sensitivity of atomic magnetometers. This study explores the benefits of employing a multipass cell in spin-exchange relaxation-free (SERF) atomic magnetometers through theoretical and experimental analysis. A signal-to-noise ratio (SNR) model was established by accounting for the optimal detuning frequency of the probe light and low-frequency 1/f noise. This enabled the optimal number of probe light passes through the vapor cell to be determined. Additionally, compact and symmetric reflective optical paths were designed to mitigate polarization gradients in atomic magnetometers with high atomic number density. Experimental results confirmed that the magnetic field sensitivity of the triple-pass configuration (1.8 fT/Hz1/2^{1/2}1/2) was 1.89 times that of the single-pass configuration (3.4 fT/Hz1/2^{1/2}1/2), consistent with the theoretical analysis. This study provides valuable theoretical guidance for analyzing the SNR performance of SERF atomic magnetometers and demonstrates the potential of multipass cells in enhancing their magnetic field sensitivity. The utilization of multipass cells presents a viable approach toward the realization of highly sensitive and portable miniaturized SERF atomic magnetometers.
Abstract We have manufactured more than 250 nominally identical paraffin-coated Cs vapor cells (30 mm diameter bulbs) for multi-channel atomic magnetometer applications. We describe our dedicated cell characterization apparatus. For each cell we have determined the intrinsic longitudinal, Γ01, and transverse, Γ02, relaxation rates. Our best cell shows Γ01/2π ≈ 0.5 Hz, and Γ02/2π ≈ 2 Hz. We find a strong correlation of both relaxation rates which we explain in terms of reservoir and spin exchange relaxation. For each cell we have determined the optimal combination of rf and laser powers which yield the highest sensitivity to magnetic field changes. Out of all produced cells, 90 % are found to have magnetometric sensitivities in the range of 9 to 30 fT / √ Hz. Noise analysis shows that the magnetometers operated with such cells have a sensitivity close to the fundamental photon shot noise limit. 1
This letter reports light narrowing in paraffin-coated vapor cells from room temperature 27℃ to 59℃, where spin-exchange relaxation is suppressed. By means of a coating lock and eliminating the reservoir effect, an ultra-narrow magnetic resonance linewidth of 0.36 Hz and an atomic coherence lifetime of T₂ = 0.9 s are achieved. In cells free of buffer gas, the narrow linewidth over this broad temperature range is a result of enhanced spin polarization, which is facilitated by the effective suppression of radiation trapping benefiting from the stability of the vapor density. Using such cells in atomic magnetometers, the photon shot noise limit is estimated as 0.2 fT/rtHz and the spin-projection noise limit is estimated as 1.1 fT/rtHz. Also, a magnetometer system with the stable coated cell is identified, which demonstrates the potential for achieving relatively stable magnetometer sensitivity without precisely controlling the cell temperature. The long coherence lifetime and broad operating temperature range expand the potential applications of quantum memory and other quantum sensors such as atomic clocks.
A free-induction-decay (FID) type optically-pumped rubidium atomic magnetometer driven by a radio-frequency (RF) magnetic field is presented in this paper. Influences of parameters, such as the temperature of rubidium vapor cell, the power of pump beam, and the strength of RF magnetic field and static magnetic field on the amplitude and the full width at half maximum (FWHM) of the FID signal, have been investigated in the time domain and frequency domain. At the same time, the sensitivities of the magnetometer for the single-pass and the triple-pass probe beam cases have been compared by changing the optical path of the interaction between probe beam and atomic ensemble. Compared with the sensitivity of ∼21.2 pT/Hz1/2 in the case of the single-pass probe beam, the amplitude of FID signal in the case of the triple-pass probe beam has been significantly enhanced, and the sensitivity has been improved to ∼13.4 pT/Hz1/2. The research in this paper provids a reference for the subsequent study of influence of different buffer gas pressure on the FWHM and also a foundation for further improving the sensitivity of FID rubidium atomic magnetometer by employing a polarization-squeezed light as probe beam, to achieve a sensitivity beyond the photo-shot-noise level.
The detection dead zone is an important systematic limitation in scalar atomic magnetometers, constraining their practical utility. In this work, we demonstrate a sensitive dead zone free scalar magnetometer by integrating previously established techniques into a FID magnetometer based on Bell Bloom optical pumping. The dead zone is eliminated by inserting a reflecting mirror within a multipass cavity assisted atomic cell, which folds the optical beam into orthogonal paths. Our analysis reveals that interregion cross talk is essential for interpreting the resulted experimental signals. The sensor exhibits oscillation signal amplitude variations within a factor of three across all orientations in three dimensional space, and a field sensitivity better than 80 fT/Hz^(1/2) over the full space. We further characterize the heading error in the geomagnetic field range, where experimental results agree with theoretical predictions within 0.7 nT. Additionally, the sensor can operate in a closed loop mode by feeding back the real time frequency extracted from the FID signal to modulate the pump beam, we characterize the magnetic field slew rate of the sensor. This work paves the way towards a sensitive vector FID magnetometer.