Controlling temperature is crucial for the functioning of electronic devices. It’s even more so for highly complex quantum computers that rely on the ability to control quantum bits (also called qubits) in order to achieve processing capabilities far above the most powerful classical computer.
For a quantum computer to maintain its prowess, it must be cooled to a temperature close to absolute zero (–273.15oC) to keep the qubits in a state of coherence. However, keeping a quantum computer’s core temperature near absolute zero is not a simple feat and poses a major roadblock to the advancement of quantum computing. Often quantum computer producers keep the machines cool by using liquid helium as a refrigerant delivered in multiple stages. Nonetheless, this system is cumbersome and elaborate, and is not user-friendly.
Quantum computing’s temperature problem may now be solved by a new fully electronic refrigeration technology developed by researchers at VTT Technical Research Centre of Finland. Current electrical refrigeration technology functions by transferring heat from one place to another through an electric heat pump, though this method is generally expensive and inefficient. While this method may be suitable for certain cooling tasks, it’s impractical for cooling to the deep space conditions necessary for quantum computers to work.
Controlling Phonons
The latest technology is based on a discovery chronicled in Science Advances, in which researchers demonstrated the ability to make electrical refrigeration more efficient. They achieved this by effectively blocking phonons, quasiparticles that contain heat energy within solids. Because phonons behave erratically, they are difficult to control and can cause a reduction in the desired cooling effect.
The team used semiconductor-superconductor junctions to create a superconducting gap that facilitated the transfer of electrons under heated conditions, known as thermionic operation, while also creating a thermal boundary to block the flow of phonons. The blockage of phonons was achieved due to the nature of the materials used in the junctions and the energy from the electrical current, which combined to create a temperature gradient between the two junctions. A tiny silicon chip was suspended between the junctions, and by running the electrical current through it, the team was able to reduce the temperature of the chip by 40 percent compared to the ambient conditions.
“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 Research Professor Mika Prunnila from VTT.
Easier Absolute Zero
The team extrapolated its results using simulations to conclude that the method would be effective in achieving near absolute zero temperatures. This small-scale, efficient and practical method could be used to simplify the cooling process for quantum computers and could ultimately help make these machines smaller and less expensive.
“We believe that this cooling effect can be observed in many different settings like, for example, in molecular junctions,” said Researcher Emma Mykkänen from VTT. The team also stated that the cooling technology could be used in thermal energy harvesting as well as thermal photodetectors used in chemical sensing, security and radio astronomy.