Miners didn't take canaries down for their songs. The bird would fall off its perch before the miner noticed the carbon monoxide. In ultrahigh vacuum, the canary's role is played by a cloud of one hundred thousand lithium or rubidium atoms. A laser cools them to tens of microkelvins, and they hover in a shallow magnetic trap whose depth is only about one thousandth of a kelvin. Residual gas molecules, moving at room-temperature speeds, collide with the cold atoms and knock them out of the trap. The more molecules in the chamber, the faster the cloud empties.

Previously, pressure was measured with mercury columns and piston gauges, but in deep vacuum they lose accuracy. The new method counts molecular impacts on cold atoms directly. The rate of atom loss from the trap shows how many residual gas molecules are flying around the chamber, and thus what pressure they create. It's like counting raindrops hitting a puddle: the more drops, the faster the puddle loses its calm.

Today, the sensor measures pressure in the range from five nanopascals to fifty micropascals with an accuracy of one to three percent. It needs no calibration against another instrument because it relies on precise quantum calculations. This is a first step toward a standard for ultrahigh vacuum that is simple and reliable, like a canary in a mine.