Using a small isotropic crystalline whispering-gallery resonator, a group of Australian researchers has demonstrated the most precise room-temperature-range thermometer in existence; the device measures temperature to a precision of 3x10-8°C (a resolution of 80 nK/(Hz)0.5).1
The approach is conceptually simple: prism-couple light of two different wavelengths into the resonator so that the mode frequency ratio is very close to 2 (±0.3 parts per million), and monitor the output. At the temperature changes, the frequency difference between the two modes changes. The approach takes advantage of the fact that the change in refractive index with respect to temperature is different for the two wavelengths (the researchers used red and green light).
"We believe this is the best measurement ever made of temperature—at room temperature," says project leader professor Andre Luiten, chair of experimental physics at the University of Adelaide's Institute for Photonics and Advanced Sensing (IPAS) and the School of Chemistry and Physics, who points out that it is possible to make more-sensitive measurements of temperature in cryogenic environments near absolute zero.
"We've been able to measure temperature differences to 30 billionths of a degree in one second," notes Luiten. "To emphasize how precise this is, when we examine the temperature of an object we find that it is always fluctuating." These fluctuations are below the limits set by the fundamental thermodynamic limit of the crystalline resonator material; as a result, the experiment is revealing the minute fluctuations that occur in even a perfectly temperature-stabilized environment.
Other types of ultraprecise sensing possible
Luiten adds that the resonator technique could be redesigned for ultrasensitive measurements of other things such as pressure, humidity, force, or the amount of a particular chemical in the surrounding environment.
The research is supported by the Australian Research Council and the South Australian Government's Premier's Science and Research Fund.
1. Wenle Weng et al., Physical Review Letters (2014); doi: http://dx.doi.org/10.1103/PhysRevLett.112.160801