New Method to Cool Superconducting Circuits in Quantum Computers

Researchers from the VTT Technical Research Center in Finland have proposed a solution to cool the superconducting circuits of the quantum computer to an operating temperature, which would be close to zero. The new method proposed does not require the use of a cryogenic heat removal system. Instead, it allows them to cool the superconducting circuits utilizing an electric current. Currently, the method used to cool superconducting circuits is by obtaining low temperatures via cryostats. Superconductors are ordinary materials cooled to extremely low temperatures, which damps the vibrations of their atoms, letting electrons zip past without collision. The cryostats are run on helium isotopes. Additionally, although there are eight known isotopes of helium (He) (standard atomic mass: 4.002602(2) u), only helium-3 (3He) and helium-4 (4He) are stable. Therefore, when the certain temperature is reached, the isotopes divide into two phases. The lighter helium isotope needs energy to cross the phase boundary. Taking it in the form of heat from the environment cools down the isotope system. The biggest disadvantage of the cryogenic method application is the exuberant costs and production challenges. The researchers used the principle of heat removal from cryostats. The electrons were supposed to take away the system’s thermal energy. Instead of the phase boundary of the isotope mixture, scientists planned to build an artificial barrier for the electric current in the form of an atomic potential barrier. The research work is entitled, “Thermionic junction devices utilizing phonon blocking.
Electrons from the semiconductor closest to the cold part of the system were sent to the part that needed to be cooled down. To pass the forbidden zone of the superconductor, the electrons had to pay almost a bounty, in order to give a share of energy. The funds for paying the “tax” were provided by the hot area of the cooling system, “impoverished” by several degrees. Photons, however, when they hit the barrier, dispersed and could no longer prevent cooling. This method is what could be needed to contain costs and also allow smaller sizes for quantum computers. It could also have broader application beyond just quantum computers.

A) Conceptual image of particle fluxes through solid-state thermionic barrier/interface connecting hot and cold reservoirs. The interface suppresses phonon heat flow between the reservoirs and acts as a thermionic junction that governs electron transport. (B and C) Electron (B) and phonon (C) interfaces between the reservoirs. Electron transport is limited by the energy gap of the superconducting lead and phonon transport by the phonon thermal boundary resistance RPTB.

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