A laser beam passes through a glass cell containing rubidium vapor, and by how the atoms absorb light, you can determine their temperature. This is like measuring the speed of cars by the blur in a photograph: the faster the atoms move, the wider the absorption line becomes.
An ordinary thermometer shines at a single frequency and slowly sweeps it, like a person trying piano keys one by one. During that time, the atoms get 'used to' the light and distort the picture, as if the pianist held a key too long and the sound wavered. Instead of a single note, the new device plays a chord: a frequency comb sends thousands of frequencies into the vapor at once, and the atoms don't have time to distort the signal.
This thermometer needs no adjustment for a specific gas and works quickly. It could be placed in a pharmaceutical plant where drugs are sensitive to heat, or near containers of nuclear waste where constant monitoring is needed without human intervention.
A frequency comb is light that contains thousands of precisely spaced frequencies at once, like the teeth of a comb. Its invention earned the Nobel Prize in Physics in 2005.
In the series 'The Expanse', laser-based thermometers monitor the temperature of a ship's nuclear reactors to catch overheating in time.