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Optics Letters

Accurate optically pumped magnetometer based on Ramsey-style interrogation

Dominic Hunter, T. Dyer, Erling Riis

Published Jan 24, 2022• 29 citations
Publisher

Abstract

Light-atom interactions during spin preparation and readout in optically pumped magnetometers can lead to inaccuracies. We demonstrate a novel, to the best of our knowledge, detection strategy that exploits an interrogation sequence in the pulsed free-induction-decay modality to suppress these systematic errors. The technique is predicated on monitoring the dynamics of preoriented atomic spins as they evolve unperturbed during a dark interval, by subsequently applying a time-delayed optical pulse to infer the spin state's phase. This detection mode reduced light shift inaccuracies to within 0.6 nT, and could be employed in a wide variety of high-precision atomic magnetometry experiments.

DOI
10.1364/ol.449180

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Review of Scientific Instruments•Nov 1, 2016•6 citations
Light-atom interactions during spin preparation and readout in optically pumped magnetometers can lead to inaccuracies. We demonstrate a novel, to the best of our knowledge, detection strategy that exploits an interrogation sequence in the pulsed free-induction-decay modality to suppress these systematic errors. The technique is predicated on monitoring the dynamics of preoriented atomic spins as they evolve unperturbed during a dark interval, by subsequently applying a time-delayed optical pulse to infer the spin state's phase. This detection mode reduced light shift inaccuracies to within 0.6 nT, and could be employed in a wide variety of high-precision atomic magnetometry experiments.
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.
Free induction decay (FID) optically pumped magnetometers are among the most sensitive quantum sensors available for high-precision magnetic field measurements. This review summarizes recent advances in FID magnetometers, which represent a critical technological route that suppresses systematic errors by temporally separating atomic polarization from magnetic field detection. This review covers light-modulated optical pumping schemes (such as the Bell-Bloom mode), radio-frequency excitation schemes, and magnetic-field-enhanced pumping schemes. Key technological developments, including chip-scale MEMS fabrication, optical multipass architectures, advanced frequency estimation algorithms, and error suppression methods, are also discussed. Finally, emerging trends toward higher integration, environmental robustness, and intelligent signal demodulation are highlighted. These trends provide insights into the development of next-generation high-performance quantum sensors and their growing role in biomedical and geophysical applications, as well as in fundamental scientific research.
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.
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.
A high-sensitivity atomic magnetometer based on broad linewidth laser pumping is demonstrated. The broad linewidth laser was obtained by broadening a narrow linewidth laser utilizing an external phase modulator driven by a pseudo-random-bit-sequence (PRBS) waveform. This ingenious optical pumping scheme can significantly enhance the signal response and suppress light shift noise, as well as can reduce the gradient of electron spin polarization and improve the stability of magnetometers. Compared with a narrow linewidth laser pumping scheme, the response of the magnetometer is increased by 56%, experimentally. The relative polarization gradient is reduced from 9.8%/cm to better than 4%/cm by setting PRBS with a bit rate of 16 GHz. This scheme can also be applied to many other fields such as noninvasive bio-magnetic imaging and atomic spin gyroscopes.
We report a laser-pumped 4He atomic magnetometer with light shift suppression through the atomic sensor itself. A linearly polarized light is used to optically align the 4He metastable atoms and we monitor the magneto-optical double resonance (MODR) signals produced by the left- and right-circularly orthogonal components. It is shown that light shift leads to the atomic alignment to orientation conversion effect, and thus, the difference between the two MODR signals. One of these two MODR signals is locked at the Larmor frequency and is used to measure the ambient magnetic field, while the differential signal is, simultaneously, fed back to suppress the light shift. The scheme could be of the advantage to the design of compact magnetometers by reducing the systematic errors due to light shift.