Magnetic fields are invisible, but they can be heard. Rubidium atoms inside a glass cell respond to them like strings to a bow: radio waves make them oscillate between energy states. The frequency of these oscillations depends on the field's strength, and the amplitude on its direction. Spectroscopy turns these oscillations into a measurable signal.
Conventional magnetometers go blind when the field is aligned with the laser beam: the signal vanishes, and the device can't see the field. Here, that dead zone is bypassed: the radio waves are circularly polarized, like a corkscrew, and the atoms respond to them at any field orientation. The scientists calibrate the device by rotating the external field and recording how the response changes. This way, the sensor learns what each "corkscrew" means and how to interpret it.
Such precision paves the way for miniature sensors for GPS-free navigation, for mapping magnetic anomalies in geology, and for medical scanners that see the weak fields of the heart and brain. All this without bulky optics and without rotating the sensor itself.
The rubidium atoms in the sensor are rocked by radio waves at a frequency of about 100 kilohertz — lower than what the human ear can hear, but higher than a whale's pulse.
In Larry Niven's novel Ringworld, characters use ultra-precise magnetic compasses to navigate the depths of an artificial world, such a sensor could be their prototype.