By exploiting the difference between the speed of two different beams of colored light when traveling through a heated crystalline disk, University of Adelaide researchers claim to have produced the world's most sensitive thermometer – with an accuracy of 30 billionths of a degree.

The thermometer works by using a laser to inject red and green light into a highly polished crystalline disk. As the refractive index (basically, how much the light is bent) of the crystal is dependent upon the ambient temperature, the two colors travel at slightly different speeds in the crystal and the researchers are able to measure that minute speed difference and extrapolate a very accurate temperature reading.

"By forcing the light to circulate thousands of times around the edge of this disk in the same way that sound concentrates and reinforces itself in a curve in a phenomena known as a 'whispering gallery' – as seen in St Paul's Cathedral in London – we can measure this minuscule difference in speed with great precision," said Professor Andre Luiten, Chair of Experimental Physics in IPAS and the School of Chemistry and Physics.

A "whispering gallery" is usually a circular or hemispherical structure where whispered acoustic communication is possible from any part of the internal circumference to any other part because waves form on the walls to carry the sound around. This phenomenon also exists for light, so the University of Adelaide team has taken advantage of this resonance to concentrate the beams around a disk.

Claimed to be three times more precise than any other thermometer currently available, the researchers from the school’s Institute for Photonics and Advanced Sensing (IPAS) are hopeful that the new technique could be redesigned to take a myriad other ultra-sensitive measurements including pressure, humidity, force, or looking for a specific chemical.

"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 revolutionize technologies used for a variety of industrial and medical applications where detection of trace amounts has great importance," Professor Luiten added.

According to the team, it was also able to control active suppression of temperature fluctuations in the system by controlling the intensity of the driving laser to further enhance the accuracy of the temperature readings.

The research has been published in the journal Physical Review Letters.

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