Optically Pumped Magnetometers (SERF regime) and Magnetoencephalography 

Optical magnetometry based on the measurement of the Faraday rotation using polarimetry

Applications in OPM-MEG  and remote magnetic anomaly detection

There have been important developments in the field of magnetoencephalography (MEG) with the use of optically-pumped magnetometers (OPM). OPM-MEG has been considered to represent the next generation of functional neuroimaging [*]. A most important development was a wearable system for OPM-MEG described in the publication "Moving magnetoencephalography towards real-world applications with a wearable system" [*]. The effort led by the University of Nottingham using sensors by the U.S. company QuSpin (launched with the help of NIH-funded research) had been also cited in the U.S. NIH Director's blog [*] [*]. The U.S. Sandia National Laboratories had presented a similar line of research in the report "Atomic Magnetometer for Human Magnetoencephalography" [*]. Issues linked to the interference of the Earth's magnetic field have been addressed by other efforts, including the U.S. DARPA AMBIIENT program [*] which led to related studies using gradiometer systems [*] [*] [*] [*]. A gradiometer consists of a system of two magnetometers in close proximity, one near the subject or object to be measured and one further away, providing a setting that allows to subtract the background magnetic field.

There have also been notable developments in remote optical magnetometry which can be conducted for magnetic anomaly detection, including for the detection of underwater or underground objects [*].  

How an optically-pumped magnetometer works

Brief description: Atomic spins, which have been polarized by a pump wave, are deflected by a magnetic field. A wave transversing the spins will have its polarization rotated. 

Pumping or excitation leads to atomic spin polarization

Certain atoms or molecules demonstrate a behavior that can be close to magnetic, meaning paramagnetic ("para" being synonymous to "next to"). Paramagnetic atoms or molecules are those that have unpaired electrons. Alkali atoms, such as sodium, potassium or rubidium, as well as molecules such as oxygen (O2) are paramagnetic. An optically-pumped magnetometer consists of a cell of alkali atoms, which are often rubidium atoms.

The electrons, protons and neutrons which compose an atom have spins which act like tiny magnets, meaning magnetic dipoles. Their sum gives the atomic spin. In the above atoms, all spins cancel out with the exception of the spin of the outer electron. Therefore, this spin determines the atomic spin, including, figuratively-speaking, the direction to which the atomic spin will point.

Similarly to a compass which points to the direction of the Earth’s magnetic field, atomic spins also point to the direction of the magnetic field in which they are found, under one condition: that the magnetic field is stronger than their thermal motion. This motion is generally strong enough to randomize the direction that the spin points to. Figure 2 on the left hand-side shows atomic spins pointing in random directions.

When we irradiate a rubidium atom with an electromagnetic wave which has a wavelength of 795 nm and circular polarization, we will energetically pump, or in other words, excite its outer unpaired electron, thereby changing its energetic level and also its spin state, including the direction of the spin. The new direction will be the direction of the magnetic field of the electromagnetic wave. Given that the outer electron spin points to that direction, the atom spin will also point to that. The atomic spin will be polarized.

Figure 1 : Irradiation of a rubidium atom with an electromagnetic wave which has a wavelength of 795 nm and circular polarization induces outer electron excitation and spin alignment, thereby leading to atomic spin alignment.

Αtomic spin polarization leads to medium magnetization

Let us consider the optical magnetometer cell, which corresponds to a material medium composed of rubidium atoms. If we irradiate similarly, with an electromagnetic wave which has a wavelength of 795 nm and circular polarization, such a medium, the atomic spins which point to random directions will now all point to the direction of the electromagnetic wave (Figure 2, left hand-side and middle). The atomic spins will be aligned, like tiny magnets which align their poles (Figure 2, middle). In this manner, we are inducing atomic-spin polarization; as a result, the medium will be (atomic-spin) polarized. As tiny magnets aligned in the same direction are equivalent to a strong magnet, the medium will behave as one. A magnetic current density is created; we are inducing medium magnetization. As a net magnetization will be grown, the medium will be magnetized. 

The polarized atomic spins of a medium are deflected by a magnetic field: a probe wave transversing the medium has its polarization rotated

What will occur if an electromagnetic wave transverses such a material medium whose atomic spins are aligned? In OPMs, a probe wave transverses the atomic cell. The wave will be subjected to an influence that will result in the rotation of its polarization. The rotation will be proportional to the strength of the magnetic field in which the atoms reside. Specific instruments called polarimeters can measure the degree of rotation of polarization of a wave and calculate the strength of the transversed magnetic field.

What will occur if the medium and more specifically the OPM cell is brought into proximity with the head of human subject whose brain generates a magnetic field? The atomic spins, given that spins acts like magnets, will receive an influence that will tilt them or rotate them and have them point to a new direction as shown in Figure 3 on the right-hand side. In other words, the atomic spins will be deflected. If an electromagnetic wave (probe wave) transverses the medium, its polarization will be rotated because of the deflected spins. The polarimeter will measure the new degree of rotation of polarization and will determine the strength of the magnetic field generated by the subject.