Researchers at the Universitat Autònoma de Barcelona (UAB) in Spain and the University of Nottingham in the United Kingdom have developed a quantum thermometer so accurate that it can detect even the smallest changes in temperatures inside a cell.
Mathematicians from both universities present how they were able to formulate the smallest possible temperature fluctuation that can be measured. They analyzed the sensitivity of existing thermometers using a handful of atoms small enough to be able to show typical quantum-style behaviors.
The quantum thermometers the team produced are capable of reading variations of millikelvin temperature across nanoscale regions. They were built using single quantum dots, which are minute pieces of semiconductor contained within a larger solid object.
The thermometers have been used to measure the temperatures of semiconductor electrons and the variations of heat within a living cell.
The researchers use a method wherein they first allow the quantum thermometers to reach equilibrium with the sample temperature and then they proceed in reading the precise measurement of its spectrum. They also try to gauge the temperature-dependent fluorescence of the object.
When questions arose regarding the ultimate precision of these thermometers and how they could produce an object that could make the ideal nanoscale thermometer, the researchers formulated a theoretical approach that would allow them to combine mathematical tools used for quantum mechanics with concepts involving thermodynamics.
"At the end of the day, what we have to measure to estimate temperature is closely related to the energy of the thermometer," UAB researcher Luis Correa said.
The researchers were able to derive the maximum sensitivity of a nanoscale thermometer by mathematically stretching its heat capacity to its very limit. This depended on the configuration of the thermometers energy level and the number of quantum states available.
The team used a nanodiamond thermometer as an example to test their theory. They noted how this type of thermometer showed a single ground state and two excited states with the same energy during their experiments.
They arrived at the conclusion that in order to produce the most accurate thermometer, it must have two energy levels similar to that of the nanodiamond thermometer, but its upper energy must have a large number of energy states instead of just two.
Despite this, the researchers said there is a trade-off between the precision of a thermometer and its operable temperature range. To increase its precision, the thermometer's excited states must be increased as well. This, however, lessens the range of temperatures the thermometer can operate in efficiently.
The team recommends that a low precision thermometer with a wide temperature range be used for future experiments to get an estimate of the sample temperature. More accurate probes could then be used in different locations to derive smaller temperature readings across various regions of the sample.
Correa pointed out that the results of their study are crucial in producing accurate temperature readings for future research. Their findings could be used to help explain how the heat in nanoscale circuits dissipates and how the thermal processes inside cells work.
The findings of the Autonomous University of Barcelona and University of Nottingham study are featured in the journal Physical Review Letters.
Photo: Ged Carroll | Flickr