Researchers at VTT Technical Research Centre of Finland have successfully demonstrated a new electronic refrigeration technology that can enable major leaps in the development of quantum computers. Current quantum computers require large, complicated cooling infrastructures based on mixtures of different isotopes of helium. The new electronic cooling technology could replace these cryogenic liquid mixtures and enable miniaturization of quantum computers. The discovery was published in Science Advances.

The researchers have developed a 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% from that of the surroundings. The researchers said the discovery can be used in the miniaturization of future quantum computers as it can simplify the required cooling infrastructure significantly.

“We expect that this newly discovered electronic cooling method could be used in several applications from the miniaturization of quantum computers to ultrasensitive radiation sensors of the security field,” said Mika Prunnila, a researcher from VTT.

Superconductive quantum computers are generally cooled by dilution refrigerators with cryogenic liquids pumping through multiple stages. Even though modern dilution refrigerators are commercial technology, they are still expensive and large scientific instruments. The electronic cooling technology developed by the VTT researchers could replace the complex coldest parts and lead to 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,” said David Gunnarsson, chief sales officer for Bluefors Oy, a refrigerator solutions company for quantum systems and computers.

The research team said their work started by looking for a method to drive heat from one place to another by electrical current. The most practical 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 heat is carried not only by the electrons, but by the quanta of the atomic lattice vibrations as well.

Instead, the team used 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 quanta so 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,” said researcher Emma Mykkänen.