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Review of Scientific Instruments

Laser pumped 4He magnetometer with light shift suppression

Zaisheng Lin, He Wang, Xiang Peng, Teng Wu, Hong Mei Guo

Published Nov 1, 2016• 6 citations
Publisher

Abstract

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.

DOI
10.1063/1.4967304

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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.
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Parametric-resonance magnetometer is a high-sensitivity quantum sensor characterized by applying the non-resonant radio-frequency (RF) fields to the atomic ensemble. The RF fields lead to crosstalk in the multi-sensor design, thus disturbing the magnetic-field measurement results. We propose an optically modulated alignment-based 4He parametric-resonance magnetometer. By using the fictitious field generated by the modulated light shift, parametric resonance is realized, and crosstalk caused by the magnetic RF field is prevented. The relative intensity noise of the lasers is suppressed to optimize the sensitivity of the magnetometer. Our magnetometer experimentally demonstrates a magnetic-field noise floor of 130 fT/Hz1/2 in both open- and closed-loop operations and has the potential to reach 70 fT/Hz1/2 when compared with the optimized magnetic RF scheme. It provides near-zero magnetic-field measurements with a 2 kHz bandwidth at room temperature, which is useful for high-bandwidth measurements in biomagnetic applications.
We demonstrate a suppression of the light shift in a Coherent-Population-Trapping (CPT) atomic magnetometer by using linearly polarized light and a differential measurement between magnetic resonances. The radio frequency that creates the optical sidebands for CPT quickly switches between two magnetic sensitive transitions, and the magnetic field is extrapolated from the difference of the center frequencies of the magnetic resonances. Light shifts and common drifts like collisional shifts can be suppressed through careful choice of measured resonances, and we show the light shift reduction by more than a factor of 20 compared to excitation with circularly-polarized light. Various limitations of the method are discussed.
Optically pumped magnetometers usually rely on optical pumping using circularly or linearly polarized light. We study here zero-field magnetometers pumped with elliptically polarized light, preparing both atomic orientation and alignment with complementary geometries. We start by extending the three-step approach for elliptically polarized pumping. This allows us to study the Hanle effect in elliptical polarization by comparing the analytical absorption signals with experiments made on 4He^{4}\mathrm{He}4He metastable state. We then study parametric resonance magnetometers based on elliptical polarization by using the dressed-atom formalism with one and two radio-frequency fields. The results show good agreement with the experimental measurements and interesting perspectives for magnetometry where symmetry breaking by pumping light is mitigated.
The light shift represents a major source of measurement error and system noise in optically pumped atomic magnetometers (OPAMs). Tuning the laser frequency to a specific value is a common and effective approach to suppressing light shift in many optical-pumping systems. However, when this method is applied to OPAMs, a significant issue arises: at the zero-light-shift frequency, the magnetometer’s sensitivity decreases markedly. Consequently, it is not possible to optimize simultaneously both the measurement accuracy affected by the light shift and the overall sensitivity of the magnetometer. To address this contradiction, the underlying mechanism was analyzed using the theory of light absorption and the principles governing light–atom interactions, leading to the proposal of a dual-frequency optical-pumping scheme. Experimental results demonstrate that this approach can suppress the light shift by more than 95% while maintaining the optimal sensitivity of the OPAM. The proposed dual-frequency scheme can also be extended to other circularly polarized optical-pumping systems, thereby facilitating both the enhancement of OPAM performance and its practical deployment in real-world applications.
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.