Light passing through a gas leaves narrow dark lines on the spectrum — the fingerprints of absorption. Acetonitrile molecules, like tiny antennas, respond only to strictly defined frequencies. But when there are many such lines and they are closely spaced, the signals overlap, like voices in a noisy room. As a result, clocks based on these molecules start to drift.

Scientists used a clever trick: they made the signal sound like three notes played simultaneously, but with slightly different pitches. This "chord" helped isolate the desired line and suppress neighboring ones. The result was a clean signal that can stabilize frequency to within one part per billion per second.

Such precision is enough to detect time drift in navigation systems or to perform ultra-sensitive measurements. Moreover, the method allowed observation of an effect previously lost in the noise — the so-called Lamb dips. These are narrow peaks in the spectrum arising because molecules moving toward and away from the light interact differently with the wave.

Clocks that are not thrown off by interference are a step toward portable frequency standards. They could find a place in satellites, laboratories, and even telecommunications equipment, where every billionth of a second matters.