Miners took canaries underground not for their songs. The bird would fall off its perch before the miner noticed 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 microkelvin, and they hover in a shallow magnetic trap whose depth is only about one thousandth of a kelvin. Residual gas molecules, flying 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.
For a century, metrologists measured pressure with mercury columns and piston gauges. These instruments work well near atmospheric pressure, but their accuracy fades as you move into deep vacuum. After the redefinition of units in 2019, the pascal can be realized through particle density and temperature, which opened the door to quantum sensors. Cold atoms here do not measure fields or time, as in neighboring quantum sensors. Instead, they become pressure probes, and their response requires no calibration against another standard. The atom loss rate equals the background gas density multiplied by a known loss-rate coefficient, which is calculated from first principles of quantum scattering mechanics.
Today, the sensor measures pressure from five nanopascals to fifty micropascals with a relative uncertainty of one to three percent. In one demonstration, a pressure of 42 nanopascals was measured with an uncertainty of just one nanopascal. This is the first technology capable of becoming a primary standard in ultrahigh vacuum without expensive flow systems. Calibrating ionization gauges will become simpler and cheaper, and the traceability chain to the pascal will shorten.
The canary in the mine warned of danger but did not say how much gas was in the air. Cold atoms speak precisely. They fall out of the trap one by one, and each departure leaves a trace in the remaining count. Thus a new vacuum standard is born, relying not on springs and pistons but on the laws of the quantum world.
The lithium source in the CAVS is so cold that its vapor pressure at room temperature is below $10^{-16}$ Pa, which is $10^{12}$ times lower than that of rubidium. One gram of rubidium in an atomic oven can provide an atom flux of $10^8$ per second for over a hundred years.
The idea of a sensor that detects individual collisions with molecules echoes the atmospheric scanners from Frank Herbert's "Dune," where sensitive instruments pick up the slightest changes in air composition.