Optically-pumped magnetometers constitute a valuable tool for imaging biological magnetic signals without cryogenic cooling. Nowadays, numerous developments are being pursued using alkali-based magnetometers, which have demonstrated excellent sensitivities in the spin-exchange relaxation free (SERF) regime that requires heating to >100 °C. In contrast, metastable helium-4 based magnetometers work at any temperature, which allows a direct contact with the scalp, yielding larger signals and a better patient comfort. However former ⁴He magnetometers displayed large noises of >200 fT/Hz^1/2 with 300-Hz bandwidth. We describe here an improved magnetometer reaching a sensitivity better than 50 fT/Hz^1/2, nearly the photon shot noise limit, with a bandwidth of 2 kHz. Like other zero-field atomic magnetometers, these magnetometers can be operated in closed-loop architecture reaching several hundredths nT of dynamic range. A small array of 4 magnetometers operating in a closed loop has been tested with a successful correction of the cross-talks.

MagnetoEncephaloGraphy (MEG) provides a measure of electrical activity in the brain at a millisecond time scale. From these signals, one can non-invasively derive the dynamics of brain activity. Conventional MEG systems (SQUID-MEG) use very low temperatures to achieve the necessary sensitivity. This leads to severe experimental and economical limitations. A new generation of MEG sensors is emerging: the optically pumped magnetometers (OPM). In OPM, an atomic gas enclosed in a glass cell is traversed by a laser beam whose modulation depends on the local magnetic field. MAG⁴Health is developing OPMs using Helium gas (⁴He-OPM). They operate at room temperature with a large dynamic range and a large frequency bandwidth and output natively a 3D vectorial measure of the magnetic field. In this study, five ⁴He-OPMs were compared to a classical SQUID-MEG system in a group of 18 volunteers to evaluate their experimental performances. Considering that the ⁴He-OPMs operate at real room temperature and can be placed directly on the head, our assumption was that ⁴He-OPMs would provide a reliable recording of physiological magnetic brain activity. Indeed, the results showed that the ⁴He-OPMs showed very similar results to the classical SQUID-MEG system by taking advantage of a shorter distance to the brain, despite having a lower sensitivity.

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.

The usual Hanle-effect magnetometers allow the measurement of two components of a very-low magnetic field using three orthogonal light beams. Here we present a scheme based on atomic alignment that requires only a single optical access for both a pump beam and a probe beam with a small angle between them. The results of experimental tests of this configuration are in close agreement with the theoretical predictions. We show that measurements of the third component should be possible by instrumentation of a partial depolarization of the pump beam. Such compact architectures open interesting perspectives for magnetometer arrays for medical imaging.

Optically pumped magnetometers (OPMs) are new, room-temperature alternatives to superconducting quantum interference devices (SQUIDs) for measuring the brain’s magnetic fields. The most used OPM in MagnetoEncephaloGraphy (MEG) are based on alkali atoms operating in the spin-exchange relaxation-free (SERF) regime. These sensors do not require cooling but have to be heated. Another kind of OPM, based on the parametric resonance of ⁴He atoms are operated at room temperature, suppressing the heat dissipation issue. They also have an advantageous bandwidth and dynamic range more suitable for MEG recordings. We quantitatively assessed the improvement (relative to a SQUID magnetometers array) in recording the magnetic field with a wearable ⁴He OPM-MEG system through data simulations. The OPM array and magnetoencephalography forward models were based on anatomical MRI data from an adult, a nine-year-old child, and 10 infants aged between one month and two years. Our simulations showed that a ⁴He OPMs array offers markedly better spatial specificity than a SQUID magnetometers array in various key performance areas (e.g., signal power, information content, and spatial resolution). Our results are also discussed regarding previous simulation results obtained for alkali OPM.

Low intrinsic noise, high bandwidth, and high accuracy vector magnetometers are key components for many ground or space geophysical applications. Here, we report the design and the test of a ⁴He vector optically pumped magnetometer specifically dedicated to these needs. It is based on a parametric resonance magnetometer architecture operated in the Earth magnetic field with closed-loop compensation of the three components of the magnetic field. It provides offset-free vector measurements in a ±70 μT range with a DC to 1 kHz bandwidth. We demonstrate a vector sensitivity up to 130 fT/√Hz, which is about ten times better than the best available fluxgate magnetometers currently available for the same targeted applications.