Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2408.00898

Investigating the hyperfine systematic error and relative phase in low spin-polarization alkali FID magnetometers

D. P. Hewatt, Michaela Ellmeier, Christopher Kiehl, Thanmay S. Menon, J. W. Pollock, Cindy A. Regal, Svenja A. Knappe

Published Aug 1, 2024• 0 citations
arXivHTMLPDF

Abstract

Alkali-metal optically-pumped magnetometers are prone to inaccuracies arising from the overlap of the average F = I + 1/2 and F = I - 1/2 ground-state Zeeman resonances. We employ density-matrix simulations and experiments to investigate how this hyperfine systematic error varies with spin polarization in a $^{87}$Rb free-induction-decay (FID) magnetometer. At low spin polarizations, ($P \leq 0.5$), this effect causes single-frequency magnetic-field extraction techniques to exhibit inaccuracies up to approximately 3.5 nT. Density-matrix simulations reveal that this bias can be traced to the relative amplitude and phase between the F = I $\pm$ 1/2 hyperfine ground-state manifolds in the FID spin precession signal. We show that this systematic error can be mitigated using either a double-frequency fitting model that accounts for the relative amplitude and phase or synchronous-pulse pumping, that minimizes the F = 1 contribution to the FID signal. Theoretical simulations predict accuracies within 0.5 nT for both techniques across a wide range of spin polarizations, suggesting a sevenfold enhancement over single-frequency extraction methods. Our experiments validate this, showcasing a variation in the extracted field below 1 nT, a 3.5-fold improvement compared to single-frequency extraction methods. Furthermore, the mitigation techniques demonstrate agreement of the extracted magnetic field within 1.5 nT.

DOI
10.48550/arxiv.2408.00898

Related papers

Investigating the hyperfine systematic error and relative phase in low-spin-polarization alkali-metal free-induction-decay magnetometers

D. P. Hewatt, Michaela Ellmeier, Christopher Kiehl +4 more

The authors present model as well as experimental results for free-induction-decay magnetometer accuracy in the presence of variable ground-state spin polarization, an important step in quantifying, reducing, or removing significant systematic shifts in finite-magnetic-field sensing. The techniques described in the paper are expected to significantly improve the accuracy of such sensors under limited optical power conditions.

Physical Review A•Mar 10, 2025•8 citations

Free-induction-decay 4 He magnetometer using a multipass cell

Kaiwen Yi, Yang Liu, Bowen Wang +4 more

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 ${}^{4}\mathrm{He}$-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 $0.34\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ with a Nyquist-limited bandwidth of $5$ 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.

Physical Review Applied•Jul 31, 2024•25 citations

Suppression of heading errors in Bell-Bloom optically pumped free-induction-decay alkali-metal atomic magnetometers

S.-Q. Liu, X.-K. Wang, Xiangdong Zhang +2 more

Heading errors of atomic magnetometers refer to the dependence of measurement results on the sensor orientation with respect to the external magnetic field. There are three main sources of such errors: the light shift effect, the linear nuclear-spin Zeeman effect, and the nonlinear Zeeman effect. In this work, we suppress the first two effects by using the Bell-Bloom optical pumping method and probe the atomic signals while the pumping beam is off, and focus on the heading error induced by nonlinear Zeeman effect while the sensor operates in the geomagnetic field range. We demonstrate several schemes to suppress this remaining heading error within 1 nT using a single magnetometer or a comagnetometer. In the magnetometer system, two schemes are developed to average out the horizontal atomic polarization in space or in time, respectively. In the comagnetometer system, we combine the simultaneously measured Larmor frequencies of two different alkali atoms to either suppress the heading error or extract the orientation of the pumping beam relative to the bias field.

arXiv:2502.13414•Physical Review A•Feb 26, 2025•6 citations

Correcting heading errors in optically pumped magnetometers through microwave interrogation

Christopher Kiehl, Thanmay S. Menon, D. P. Hewatt +3 more

We demonstrate how to measure in situ for heading errors of optically pumped magnetometers (OPMs) in the challenging parameter regime of compact vapor cells with imperfect optical pumping and high buffer gas pressure. For this, we utilize microwave-driven Ramsey and Rabi frequency spectroscopy (FS) to independently characterize scalar heading errors in free-induction-decay (FID) signals. Both of these approaches suppress 5-nT inaccuracies in geomagnetic fields caused by nonlinear Zeeman shifts in FID measurements to below 0.6 nT. For Ramsey FS, we implement short periods of microwave interrogation within a $\ensuremath{\pi}/2\text{\ensuremath{-}}{t}_{R}\text{\ensuremath{-}}3\ensuremath{\pi}/2$ Ramsey interferometry sequence, effectively circumventing systematic errors from off-resonant driving. Conversely, Rabi FS leverages an atom-microwave Hamiltonian for accurate modeling of Rabi oscillation frequencies, achieving a measurement precision down to $80\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\text{Hz}}$ that is limited primarily by technical microwave noise. We show that the fundamental sensitivity of Rabi FS is $30\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\text{Hz}}$ with our vapor cell parameters through a Cram\'er-Rao lower-bound analysis. This work paves the way for future investigations into the accuracy of hyperfine structure magnetometry and contributes to the broader applicability of OPMs in fields ranging from navigation and geophysics to space exploration and unexploded ordinance detection, where heading error mitigation is essential.

arXiv:2310.11017•Physical Review Applied•Jul 2, 2024•12 citations

Spin projection noise and the magnetic sensitivity of optically pumped magnetometers

K. Mouloudakis, V. Koutrouli, Iannis K. Kominis +2 more

Present protocols for obtaining the ultimate magnetic sensitivity of optically pumped magnetometers (OPMs) utilizing alkali-metal ensembles rely on uncorrelated atoms in stretched states. A new approach for calculating the spin projection noise (SPN)-limited signal to noise ratio (SNR) and the magnetic sensitivity of OPMs is proposed. Our model is based solely on the mean-field density matrix dynamics and in contrast to previous models, it applies to both low and high field regimes, it takes into account the degree of spin polarization, the intra- and interhyperfine correlations, the decoherence processes, the atom-light coupling and the effects of the spin dynamics on the spin-noise spectra. Fine tuning of the probe frequency allow us to explore different hyperfine states and ground-state correlations. Especially in the spin-exchange-relaxation-free (SERF) regime, alongside the magnetic resonance narrowing and the increased number density, hallmarks of SERF magnetometers, we report on a new SERF feature; the reduction of spin-projection noise at the spin precession frequency as a consequence of strongly-correlated hyperfine spins that attenuate and redistribute SPN when properly probed.

arXiv:2402.10746•Feb 16, 2024•0 citations

Optical pumping enhancement of a free-induction-decay magnetometer

Dominic Hunter, Marcin S. Mrozowski, Allan McWilliam +4 more

Spin preparation prior to a free-induction-decay (FID) measurement can be adversely affected by transverse bias fields, particularly in the geophysical field range. A strategy that enhances the spin polarization accumulated before readout is demonstrated, by synchronizing optical pumping with a magnetic field pulse that supersedes any transverse fields by over two orders of magnitude. The pulsed magnetic field is generated along the optical pumping axis using a compact electromagnetic coil pair encompassing a micro-electromechanical systems (MEMS) vapor cell. The coils also resistively heat the cesium vapor to the optimal atomic density without spurious magnetic field contributions as they are rapidly demagnetized to approximately zero field during spin readout. The demagnetization process is analyzed electronically, and directly with a FID measurement, to confirm that the residual magnetic field is minimal during detection. The sensitivity performance of this technique is compared to existing optical pumping modalities across a wide magnetic field range. A noise floor sensitivity of 238fT/Hz was achieved in a field of approximately 50 µT, in close agreement with the Cramér–Rao lower bound predicted noise density of 258fT/Hz.

arXiv:2307.11600•Journal of the Optical Society of America B•Sep 18, 2023•22 citations
Alkali-metal optically-pumped magnetometers are prone to inaccuracies arising from the overlap of the average F = I + 1/2 and F = I - 1/2 ground-state Zeeman resonances. We employ density-matrix simulations and experiments to investigate how this hyperfine systematic error varies with spin polarization in a 87^{87}87Rb free-induction-decay (FID) magnetometer. At low spin polarizations, (P≤0.5P \leq 0.5P≤0.5), this effect causes single-frequency magnetic-field extraction techniques to exhibit inaccuracies up to approximately 3.5 nT. Density-matrix simulations reveal that this bias can be traced to the relative amplitude and phase between the F = I ±\pm± 1/2 hyperfine ground-state manifolds in the FID spin precession signal. We show that this systematic error can be mitigated using either a double-frequency fitting model that accounts for the relative amplitude and phase or synchronous-pulse pumping, that minimizes the F = 1 contribution to the FID signal. Theoretical simulations predict accuracies within 0.5 nT for both techniques across a wide range of spin polarizations, suggesting a sevenfold enhancement over single-frequency extraction methods. Our experiments validate this, showcasing a variation in the extracted field below 1 nT, a 3.5-fold improvement compared to single-frequency extraction methods. Furthermore, the mitigation techniques demonstrate agreement of the extracted magnetic field within 1.5 nT.
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.
Heading errors of atomic magnetometers refer to the dependence of measurement results on the sensor orientation with respect to the external magnetic field. There are three main sources of such errors: the light shift effect, the linear nuclear-spin Zeeman effect, and the nonlinear Zeeman effect. In this work, we suppress the first two effects by using the Bell-Bloom optical pumping method and probe the atomic signals while the pumping beam is off, and focus on the heading error induced by nonlinear Zeeman effect while the sensor operates in the geomagnetic field range. We demonstrate several schemes to suppress this remaining heading error within 1 nT using a single magnetometer or a comagnetometer. In the magnetometer system, two schemes are developed to average out the horizontal atomic polarization in space or in time, respectively. In the comagnetometer system, we combine the simultaneously measured Larmor frequencies of two different alkali atoms to either suppress the heading error or extract the orientation of the pumping beam relative to the bias field.
We demonstrate how to measure in situ for heading errors of optically pumped magnetometers (OPMs) in the challenging parameter regime of compact vapor cells with imperfect optical pumping and high buffer gas pressure. For this, we utilize microwave-driven Ramsey and Rabi frequency spectroscopy (FS) to independently characterize scalar heading errors in free-induction-decay (FID) signals. Both of these approaches suppress 5-nT inaccuracies in geomagnetic fields caused by nonlinear Zeeman shifts in FID measurements to below 0.6 nT. For Ramsey FS, we implement short periods of microwave interrogation within a $\ensuremath{\pi}/2\text{\ensuremath{-}}{t}_{R}\text{\ensuremath{-}}3\ensuremath{\pi}/2$ Ramsey interferometry sequence, effectively circumventing systematic errors from off-resonant driving. Conversely, Rabi FS leverages an atom-microwave Hamiltonian for accurate modeling of Rabi oscillation frequencies, achieving a measurement precision down to 80pT/Hz80\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\text{Hz}}80pT/Hz​ that is limited primarily by technical microwave noise. We show that the fundamental sensitivity of Rabi FS is 30pT/Hz30\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\text{Hz}}30pT/Hz​ with our vapor cell parameters through a Cram\'er-Rao lower-bound analysis. This work paves the way for future investigations into the accuracy of hyperfine structure magnetometry and contributes to the broader applicability of OPMs in fields ranging from navigation and geophysics to space exploration and unexploded ordinance detection, where heading error mitigation is essential.
Present protocols for obtaining the ultimate magnetic sensitivity of optically pumped magnetometers (OPMs) utilizing alkali-metal ensembles rely on uncorrelated atoms in stretched states. A new approach for calculating the spin projection noise (SPN)-limited signal to noise ratio (SNR) and the magnetic sensitivity of OPMs is proposed. Our model is based solely on the mean-field density matrix dynamics and in contrast to previous models, it applies to both low and high field regimes, it takes into account the degree of spin polarization, the intra- and interhyperfine correlations, the decoherence processes, the atom-light coupling and the effects of the spin dynamics on the spin-noise spectra. Fine tuning of the probe frequency allow us to explore different hyperfine states and ground-state correlations. Especially in the spin-exchange-relaxation-free (SERF) regime, alongside the magnetic resonance narrowing and the increased number density, hallmarks of SERF magnetometers, we report on a new SERF feature; the reduction of spin-projection noise at the spin precession frequency as a consequence of strongly-correlated hyperfine spins that attenuate and redistribute SPN when properly probed.
Spin preparation prior to a free-induction-decay (FID) measurement can be adversely affected by transverse bias fields, particularly in the geophysical field range. A strategy that enhances the spin polarization accumulated before readout is demonstrated, by synchronizing optical pumping with a magnetic field pulse that supersedes any transverse fields by over two orders of magnitude. The pulsed magnetic field is generated along the optical pumping axis using a compact electromagnetic coil pair encompassing a micro-electromechanical systems (MEMS) vapor cell. The coils also resistively heat the cesium vapor to the optimal atomic density without spurious magnetic field contributions as they are rapidly demagnetized to approximately zero field during spin readout. The demagnetization process is analyzed electronically, and directly with a FID measurement, to confirm that the residual magnetic field is minimal during detection. The sensitivity performance of this technique is compared to existing optical pumping modalities across a wide magnetic field range. A noise floor sensitivity of 238fT/Hz was achieved in a field of approximately 50 µT, in close agreement with the Cramér–Rao lower bound predicted noise density of 258fT/Hz.