A radio hisses if the antenna is pointed the wrong way. Atomic magnetometers have the same problem: at certain angles to the magnetic field, they almost stop hearing the signal. Engineers have figured out how to make them listen equally well from all directions. Inside a glass cell filled with rubidium vapor, light bounces off three mirrors and combines into two crossed beams, like two conversationalists who interrupt each other but together never fall silent.
Previously, researchers thought the two beams worked independently, but it turns out that atoms from one beam sometimes jump to the other. This cross-coupling, which accounts for 0.28 of the signal, explains why the signal never disappears no matter how you rotate the device. Tests across 266 orientations showed that the amplitude varies by no more than a factor of three, and the field measurement accuracy stays better than 0.7 nT. That's like measuring the length of a football field to within the thickness of a hair, even if you hold the tape measure at an angle.
Such a sensor will already be useful for geologists hunting for ores via magnetic anomalies, and for navigators without GPS. In the future, it could determine not just the field's strength but also its direction. But the main feat is already done: the radio no longer hisses, no matter how you turn it.
The diffusion length of rubidium atoms during the probing time is only 0.6 mm, so cross-talk arises not from long-range flights but from local mixing near the middle mirror.