You can make a light beam look like a flower with several petals. If you send such a beam through a cloud of cold rubidium atoms and switch on a magnetic field, the petals start to rotate. The angle of rotation is directly linked to the field's strength.
Usually, measuring magnetic fields with light requires polarizers — special plates that only let through waves with a specific oscillation direction. Here, they weren't needed: the rotation of the petals is visible without them. You just look at how much the pattern has shifted.
This approach turns measuring light's polarization into measuring geometry — the angle of a picture's rotation. It paves the way for magnetic field sensors that see the field's distribution in space, not just at a single point.
Rubidium is a soft metal that melts at just 39 degrees Celsius, slightly above body temperature.
In science fiction, magnetic fields are often depicted as invisible lines of force, but here light itself maps them by the rotation of its petals.