Quantum refrigerators: research is making progress
Thermodynamics and quantum mechanics are two independent physical theories. They were nevertheless associated in the beginning of the 20th century, in particular within early work of Albert Einstein1. In the second half of the 20th century, some practical applications emerged, with the invention of masers and lasers. Nevertheless, these devices were of large size.
Today, progress in the experimental control of small quantum systems is reviving interest in merging these two fields.
For example, an article published in January 2019 in Nature describes the experimentation by researchers at the University of Singapore of a quantum absorption refrigerator utilizing three modes of motion of trapped Ytterbium ions:
a radial “zig-zag” mode: it represents the heat body that is in contact with a cold environment to be refrigerated;
- an axial “zig-zag” mode, of higher frequency, which represents the ambient (hot) temperature;
- another radial “rocking” mode, serving as the heat source that drives the refrigeration of the cold mode. It replaces the work reservoir of a conventional refrigerator.
Three intermediate ions were placed in a metal chamber with no air at all. The ions were then manipulated with lasers until conditions were favorable for cooling.
According to the researchers, this experimental device allowed cooling to less than 40 µK of absolute zero.
Another article published in Nature last February indicates that a research team has, for the first time, succeeded in performing "photonic cooling" in the absence of light. Again, a quantum refrigerator was used.
Until now, photonic cooling could only be achieved thanks to a laser, and therefore light. In the experiment reported by Nature, photonic cooling was obtained without laser light, by using a custom-fabricated nanocalorimetric device and a photodiode placed a few nanometers from it. A transfer of photons could be observed between the two objects, thanks to “quantum tunelling”. It has thus been observed that the photodiode has absorbed more heat than it has returned to the calorimeter, lowering its temperature by a tenth of a thousand degrees Celsius, which is equivalent to an energy flow of 6 W per m2.
Such techniques could one day help cool microelectronic components.
1 EINSTEIN, Albert. Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt. Annalen der Physik. 1905, vol. 322, n.6, pp. 132-148.