Thermometers have come a long way. From the mercury tube models that would kill you if they broke to the fancy new versions that produce a reading in seconds without an uncomfortable probe, they have a fascinating history. Researchers from the University of Adelaide’s Institute for Photonics and Advanced Sensing (IPAS) have just added to that history, and in a big way: The world’s most sensitive thermometer uses light to measure temperature to within 30 billionths of a degree.

Best of all, it measures with such precision at incredible speeds – less than one second, in some cases.

“To emphasise how precise this is, when we examine the temperature of an object we find that it is always fluctuating. We all knew that if you looked closely enough you find that all the atoms in any material are always jiggling about, but we actually see this unceasing fluctuation with our thermometer, showing that the microscopic world is always in motion,” says project leader Professor Andre Luiten, Chair of Experimental Physics in IPAS and the School of Chemistry and Physics.

The mechanics behind the thermometer are genius. It uses two beams of light (one red, one green) and shoots them into a crystalline disk. Due to their different wavelengths, the beams move through the disk at slightly different speeds. Just how different, exactly, depends on the disk’s temperature.

“When we heat up the crystal we find that the red light slows down by a tiny amount with respect to the green light,” Professor Luiten says.

Though the differences are almost impossibly small, whipping the light beams around the disk thousands of times allows for the miniscule differences to be measured. Professor Luiten says the techniques they’ve developed are applicable to other fields that could benefit from very precise measurement, like humidity or force.

“Being able to measure many different aspects of our environment with such a high degree of precision, using instruments small enough to carry around, has the capacity to revolutionise technologies used for a variety of industrial and medical applications where detection of trace amounts has great importance,” he said.