In laboratories where atoms are cooled to near absolute zero, even a tiny change in the magnetic field throws off all the settings. Conventional sensors, Hall plates or coils with ferrite cores, sit a few centimeters away from the vacuum chamber because they themselves create interference. But that distance means they don't sense the field exactly where the atoms are.

Physicists at the Max Planck Institute of Quantum Optics took a different approach: they turned the cloud of rubidium-87 atoms itself into a measuring device. Two weak laser pulses gently 'tickle' the atoms without destroying the cloud. By how the atoms absorb light, one can calculate the splitting of energy levels in a magnetic field — the Zeeman effect. The stronger the field, the larger the splitting, and the more precisely the atoms 'report' on their environment.

But the atoms don't just measure: they also correct. To eliminate the slow drift of the field, which reached 70 nanoteslas per hour, the scientists used a Kalman filter — a mathematical algorithm that predicts the next value from previous measurements, much like an experienced driver anticipates a car's behavior on a slippery road. The filter controlled compensating coils, and the field stabilized. The price for precision is slightly more random shot-to-shot fluctuations: from 1.8 to 2.0 nanoteslas.

Now ultracold atoms are not just an object of study but a sensitive guardian of their own stability. The same approach could benefit atomic clocks and quantum simulators, where the magnetic field must remain perfectly uniform for hours on end.