Researchers at VTT Technical Research Centre of Finland have developed a new purely electrical refrigeration method where cooling and thermal isolation operate effectively through the same point like junction. In the experiment, the researchers suspended a piece of silicon from such junctions and refrigerated the object by feeding electrical current from one junction to another through the piece. The current lowered the thermodynamic temperature of the silicon object as much as 40 percent from that of the surroundings. This discovery can be used, for example, in the miniaturization of future quantum computers as it can simplify the required cooling infrastructure significantly. The discovery has been published in Science Advances magazine on April 10, 2020.

“We expect that this newly discovered electronic cooling method could be used in several applications from the miniaturization of quantum computers to ultra-sensitive radiation sensors of the security field,” said Research Professor Mika Prunnila from VTT Technical Research Centre of Finland.

Several sensitive electronic and optical devices require low temperature operation. One timely example is quantum computer built from superconductive circuits, which require refrigeration close to the absolute zero of thermodynamic temperature (-273.15 oC). 

Nowadays, superconductive quantum computers are cooled by so called dilution refrigerators, which are multi-stage coolers based on pumping of cryogenic liquids. The complexity of this refrigerator arises, especially from the coldest stage, the operation of which is based on pumping of a mixture of different isotopes of helium. Even though modern dilution refrigerators are commercial technology they are still expensive and large scientific instruments. The electronic cooling technology develop by the VTT researchers could replace the complex coldest parts and, thereby, lead to significant reductions in complexity, cost and size.  

“The demonstrated cooling effect can be used to actively cool quantum circuits on a silicon chip or in large scale refrigerators. Needless to say that we at Bluefors are following this new electrical refrigerator development with great interest,” said David Gunnarsson, chief sales officer at Bluefors Oy.  

The research team was searching for an efficient and practical method to drive heat from one place to another by electrical current. The most efficient solution would be provided by a solid junction, where the hottest electrons climb over a short atomic scale potential barrier. The challenge with this approach is that the heat is not carried only by the electrons, but the quanta of the atomic lattice vibrations—so called phonons—also carry a significant amount of the heat. The phonons travelling between the hot and the cold level out the temperature differences very effectively, especially, over a short distance. 

It seemed that the most efficient electronic cooling method always led to the worst possible phonon heat leak and, thereby, a nil result in terms of overall cooling. The VTT research team postulated that, in fact, a straightforward solution to this seemingly fundamental problem could exist: Certain material junctions could block the propagation of the phonons while the hot electrons pass it. 

The team demonstrated the effect by utilizing semiconductor-superconductor junctions to refrigerate a silicon chip. In these junctions the forbidden electronic states in the superconductor form a barrier, over which the electrons from the semiconductor have to climb to drive the heat away. At the same time the junction itself scatters or blocks the phonons so effectively that the electronic current can introduce a significant temperature difference over the junction. 

“We believe that this cooling effect can be observed in many different settings like for example in molecular junctions,” says Researcher Emma Mykkänen from VTT.

The findings are results of a VTT-coordinated EU-project EFINED. The VTT project team, which made the discovery, includes expertise from physics and cryogenics to microelectronics and metrology. The obtained results are a tangible example of multidisciplinary research and development collaboration.