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Written by Sarah Kellett

A whispering gallery of light has made the world’s most sensitive thermometer yet.

This thermometer sets a new record in precision. Made by an Australian team of researchers, it is three times better than the previous record holder, and can measure temperature differences of just 30 billionths of a degree. This is the smallest change which can currently be measured.

Although the thermometer is very precise, it is not accurate – it measures changes in temperature, not an absolute temperature. It can’t tell you that it is 20 degrees Celsius today, but it can tell you that a chemical reaction has produced energy! Scientists use these measurements to investigate how medicines react with chemicals in the body.

Curved echoes

The thermometer works like a whispering gallery. Maybe you’ve played with one before. A whispering gallery is a curved wall, and when you whisper to one end the sound carries all the way across to the other. There’s a great example at the Barossa Reservoir in South Australia.

Instead of sound and a wall, the thermometer uses light and a crystal. Lasers fire red and green light into a crystal shaped like a Frisbee. The light circles ‘round and ‘round the disc, like sound in a whispering gallery.

Heat makes the crystal expand. That changes the speed of the red and green light. If the lights were sounds, the two tones would be slightly out of tune with each other. Because they are lights, the change in speed as they circle means a change in colour, which can be measured.

Detecting everything

“We have built a very precise thermometer, and our technique could be used to measure other things,” explains Andre Luiten from the University of Adelaide. “Force, pressure or small quantities of molecules could be detected … We can use the power of light to do many useful things.”

The whispering gallery technique can detect a single virus in a drop of water. To do this, a round crystal is coated with chemicals that stick to a certain kind of virus. When a virus gets stuck, it changes the size of the crystal. Just like with the thermometer, this affects light in a way that can be measured.

One issue with the virus sensors is that they are sensitive to temperature. The thermometer team used this issue to their advantage – being sensitive to temperature is just what you want in a thermometer!

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One of the really exciting applications for me is adapting the device to measure force, pressure or small quantities of molecules, such as toxins in a river. What do you think we could use it for?

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