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Physical Review A

Theory of a He4 parametric-resonance magnetometer based on atomic alignment

François Beato, Élie Belorizky, Étienne Labyt, Matthieu Le Prado, Agustin Palacios-Laloy

Published Nov 26, 2018• 37 citations
Publisher

Abstract

In parametric-resonance magnetometers (PRMs), optically pumped atoms subject to rf fields allow one to measure the components of a very low magnetic field. Here, instead of using a circularly polarized light for creating atomic orientation, as in previous works, a linearly polarized light is used for creating atomic alignment. A dressed-atom formalism introduced by Polonsky and Cohen-Tannoudji is extended and allows one to map this situation to a simpler Hanle effect on aligned atoms. Analytical expressions for all alignment tensor components and photodetection signals are obtained for both the Hanle magnetometer and PRMs. A two-rf field alignment PRM is shown to display several improvements from its orientation counterpart, providing a secular third axis sensitivity and a lower degradation from the one-axis sensitivity obtained by using a single rf field. These specificities are shown to result from the apparent depolarization of the pumping light acting on the atom dressed by two-rf fields. Experimental measurements showing a good agreement with the theoretical predictions are presented.

DOI
10.1103/physreva.98.053431

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In parametric-resonance magnetometers (PRMs), optically pumped atoms subject to rf fields allow one to measure the components of a very low magnetic field. Here, instead of using a circularly polarized light for creating atomic orientation, as in previous works, a linearly polarized light is used for creating atomic alignment. A dressed-atom formalism introduced by Polonsky and Cohen-Tannoudji is extended and allows one to map this situation to a simpler Hanle effect on aligned atoms. Analytical expressions for all alignment tensor components and photodetection signals are obtained for both the Hanle magnetometer and PRMs. A two-rf field alignment PRM is shown to display several improvements from its orientation counterpart, providing a secular third axis sensitivity and a lower degradation from the one-axis sensitivity obtained by using a single rf field. These specificities are shown to result from the apparent depolarization of the pumping light acting on the atom dressed by two-rf fields. Experimental measurements showing a good agreement with the theoretical predictions are presented.
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For measurements in near-zero magnetic fields, vector atomic magnetometers are promising alternatives to scalar versions, but generally suffer from inconsistent performance over three axes. We present a single-beam vector magnetometer with three-axis isotropic sensitivity based on 4He^{4}\mathrm{He}4He atomic alignment. Longitudinal parametric resonance with atoms pumped by linearly polarized light is proposed for the zzz-axis measurement, while the Hanle effect in transverse geometry is used for xxx- and yyy-axis measurements. The evolution of atomic alignment multipole components and expressions of the three-axis magnetic responses are analyzed in detail. We describe the experimental operation of the proposed magnetometer with a magnetic-field noise floor of 150fT/Hz1/2150\phantom{\rule{0.16em}{0ex}}\mathrm{fT}/{\mathrm{Hz}}^{1/2}150fT/Hz1/2 for the three axes, which is attractive for biomagnetism requiring vector characterization and source reconstruction.
Abstract Optically-pumped magnetometers (OPM) based on parametric resonance allow real-time tri-axial measurement of very small magnetic fields with a single optical access to the gas cell. Most of these magnetometers rely on circularly polarized pumping light. We focus here on the ones relying on linearly polarized light, yielding atomic alignment. For these magnetometers we investigate three second order effects which appear in the usual regimes of operation, so to clarify if they translate to metrological problems like systematic errors or increased noise. The first of these effects is the breakdown of the three-step approach when the optical beam has a large intensity. The second one is the breakdown of the rotating wave approximation when the frequencies of the RF fields are not much larger than the rates of other atomic processes. The third one is the tensor light-shift which appears when the light is slightly detuned from resonance. This work should help to clarify the accuracy reachable with OPM, which is an important question notably for medical imaging applications.
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.
We present an experimental study of the intrinsic magnetometric sensitivity of an optical or rf-frequency double-resonance magnetometer in which linearly polarized laser light is used in the optical pumping and detection processes. We show that a semiempirical model of the magnetometer can be used to describe the magnetic resonance spectra. Then, we present an efficient method to predict the optimum operating point of the magnetometer, i.e., the light power and rf Rabi frequency providing maximum magnetometric sensitivity. Finally, we apply the method to investigate the evolution of the optimum operating point with temperature. The method is very efficient to determine relaxation rates and thus allowed us to determine the three collisional disalignment cross sections for the components of the alignment tensor. Both first and second harmonic signals from the magnetometer are considered and compared.
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.