New atomic absorption spectrometer helps redefine the kelvin

A team of Australian scientists has developed an atomic absorption spectrometer that provides a new way to determine the Boltzmann constant, a number that relates the motion of an individual atom to its temperature. The research, published in Nature Communication, contributes to a worldwide scientific effort to redefine the kelvin, the international unit of temperature. Zero kelvin, or absolute zero, is the absence of all thermal energy and equivalent to -273.15°C.

“Although temperature is a familiar concept to all of us, remarkably, it can only be measured accurately at a handful of locations around the globe,” says Professor Andre Luiten, director of the University of Adelaide’s Institute for Photonics and Advanced Sensing (IPAS). The experiments began in 2010 at IPAS, with researchers there joined by colleagues from University of Queensland and University of Western Australia.

The team used the spectrometer to measure the speed of individual atoms moving in a gas. “An atom sitting at rest will absorb light of a particular frequency or color, says Luiten. “If it is moving towards you or away from you then the absorbed light is very slightly changed because of the Doppler effect. This is exactly the same effect that makes a police siren sound different depending on whether the car is moving towards you or away. We use a pure laser to measure these changes in light absorption, from which we can infer the speeds of the atoms and the temperature of the gas.”

The experiment led to an unexpected realization: Light had an apparent effect on the atoms themselves, and the measurement itself ended up changing the result. One of the breakthroughs of the project was developing an explanation of how this happened and ensuring that it didn’t affect the result. The finding ensures that any laboratory in the world with appropriate skills and equipment can now accurately measure temperature. Further development could even deliver this capability to industry.

“Our work will bring a universally agreed temperature scale to the globe,” says Tom Stace, associate professor from the ARC Centre for Engineered Quantum Systems at the University of Queensland. “As with any upgrade, this one will be deemed successful if people hardly notice the transition on a day-to-day basis. But for those at the cutting edge, whether developing new metal alloys at very high temperatures or measuring the temperatures of the coldest substances, the need for absolute temperature is critical.”