A thermometer that doesn't lie for years. Ordinary resistance thermometers, even very accurate ones, start to err over time: the material ages, contacts oxidize, and readings drift. The new quantum thermometer works differently: it uses the rubidium atoms themselves as a reference.
Inside a tiny glass cell, there's rubidium vapor. Scientists shine a laser on it, the atoms get excited and emit light of two different colors. The brightness ratio of these two colors depends only on temperature, and nothing else. It's like measuring temperature by the height of a flame: as long as the same gas is burning, the height tells you about the heat, not the condition of the burner.
The main advantage is long-term stability. A platinum thermometer needs regular checking and calibration because its resistance changes with shocks, vibrations, and even how you handle it. A rubidium thermometer is immune to all that: the properties of rubidium atoms don't change after a year or after ten years. The accuracy of 30 millikelvins (0.03 degrees) holds without any adjustment.
Such a device is especially needed in space, where you can't bring a thermometer back to the lab for verification. Satellites work for years in vacuum and extreme temperature swings, and if a sensor starts lying, all the scientific instruments can fail. The quantum thermometer promises to be a reliable 'eternal' temperature standard in orbit.
Rubidium has been used in the most accurate atomic clocks, but its ability to glow differently at different temperatures has only now become the basis for a thermometer.