Inside every rubidium-87 atom there is a tiny magnet — the nuclear spin and the electron spin. In a strong external field, these little magnets stop influencing each other and align independently, like two separate compasses. It is in this regime that the new sensor, SASHMAG, operates: it catches the moment when atoms absorb laser light and determines the magnetic field from the frequency of that absorption.
The laser beam passes through a cloud of rubidium vapor twice — in the forward and backward directions. Atoms flying toward the beam see the light as slightly more "blue," while those fleeing see it as slightly more "red." Normally, this blurs the signal, but the counter-propagating beams compensate for the Doppler shift, leaving only sharp absorption peaks. Each peak corresponds to a specific orientation of the atomic magnet.
To understand exactly how atoms behave in a strong field, the scientists built a model based on the Bloch equations. It accounts for how atoms transition between energy levels, how the laser light saturates the transitions, and how thermal motion broadens the lines. The model matched the experiment to within the linewidth — meaning that spectra can now be predicted in advance for any conditions.
The field is not calculated by hand but automatically: the program searches through possible field values and finds the one where the calculated line frequencies best match the measured ones. This approach already yields reliable results, and in the future it will allow training neural networks on synthetic data — then the sensor could work even faster and more autonomously.
Magnetometry is needed not only in laboratories. In MRI machines, the field reaches 1.5–3 tesla, and it must be controlled with high precision. In fusion reactors, magnetic fields confine the superheated plasma, and the slightest deviation could disrupt the reaction. A sensor based on rubidium-87 is a step toward making such measurements cheaper and more accessible.
Rubidium-87 is the very atom that powers the world's most accurate clocks. Its transition frequency is used to define the second, and now it also helps measure magnetic fields.
In Andy Weir's novel "The Martian," the protagonist uses magnetic fields for navigation on Mars — accurate magnetometers would make such a task a reality.