A calcium ion in a magnetic trap resembles a tiny ball that constantly trembles from heat. To make it freeze, physicists usually hit it with lasers from several sides. But in the new experiment, a single beam aimed along the trap's axis sufficed.

First, the ion is cooled the usual way — the laser slightly slows its motion, like a headwind slowing a runner. Then the laser frequency is nudged, and a dark resonance appears: the ion stops absorbing light, and its trembling abruptly weakens. In 800 microseconds, the thermal motion drops from 72 quanta of vibration to one and a half — almost a complete halt.

This method requires no extra lasers or complex optics. It works in a compact trap with a magnetic field of 0.91 tesla — about the same as in an MRI machine. That makes it convenient for future quantum devices, where every extra jiggling atom interferes with precise measurements.