Physicists Developing Superconducting Quantum Refrigerator

A research team led by University of Rochester physicists has conceived an idea for a superconducting quantum refrigerator, which would cool atoms to nearly absolute zero temperatures.

The superconducting quantum refrigerator is similar to a conventional refrigerator, in that it moves a material between hot and cold reservoirs. However, instead of a refrigerant that changes from a liquid state to a gas, the electrons in a metal change from the paired superconducting state to an unpaired normal state. Image credit: Michael Osadciw, University of Rochester illustration.

The superconducting quantum refrigerator is similar to a conventional refrigerator, in that it moves a material between hot and cold reservoirs. However, instead of a refrigerant that changes from a liquid state to a gas, the electrons in a metal change from the paired superconducting state to an unpaired normal state. Image credit: Michael Osadciw, University of Rochester illustration.

The superconducting quantum refrigerator uses the principles of superconductivity to operate and generate an ultra-cold environment, which is conducive to generating the quantum effects required to enhance quantum technologies.

The device would create an environment whereby researchers could change materials into a superconducting state — similar to changing a material to a gas, liquid, or solid.

“While superconducting quantum refrigerators would not be for use in a person’s kitchen, the operating principles are quite similar to traditional refrigerators,” said University of Rochester’s Professor Andrew Jordan.

“What your kitchen fridge has in common with our superconducting refrigerators is that it uses a phase transition to get a cooling power.”

In a conventional refrigerator, the refrigerant in a liquid state passes through an expansion valve. When the liquid is expanded, its pressure and temperature drop as it transitions into a gaseous state.

The now cold refrigerant passes through an evaporator coil on the inside of the fridge box, absorbing heat from the refrigerator’s contents. It is then re-compressed by a compressor powered by electricity, raising its temperature and pressure even more and turning it from a gas to a hot liquid.

The condensed hot liquid, hotter than the outside environment, flows through condenser coils on the outside of the fridge, radiating heat to the environment. The liquid then reenters the expansion valve and the cycle repeats.

The superconductor refrigerator is similar to a conventional one, in that it moves a material between hot and cold reservoirs.

However, instead of a refrigerant that changes from a liquid state to a gas, the electrons in a metal change from the paired superconducting state to an unpaired normal state.

“We are doing the exact same thing as a traditional fridge, but with a superconductor,” said Sreenath Manikandan, a graduate student at the University of Rochester.

In the superconducting quantum fridge, the researchers place a layered stack of metals in an already cold, cryogenic dilution refrigerator.

The bottom layer of the stack is a sheet of the superconductor niobium, which acts as a hot reservoir, akin to the environment outside a traditional refrigerator.

The middle layer is the superconductor tantalum, which is the working substance, akin to the refrigerant in a traditional refrigerator.

The top layer is copper, which is the cold reservoir, akin to the inside of a traditional fridge.

When the physicists slowly apply a current of electricity to the niobium, they generate a magnetic field that penetrates the middle tantalum layer, causing its superconducting electrons to unpair, transition to their normal state, and cool down.

The now cold tantalum layer absorbs heat from the now warmer copper layer.

The scientists then slowly turn off the magnetic field, causing the electrons in the tantalum to pair and transition back into a superconducting state, and the tantalum becomes hotter than the niobium layer. Excess heat is then transferred to the niobium. The cycle repeats, maintaining a low temperature in the top copper layer.

“This is similar to the refrigerant in a traditional refrigerator, transitioning from cycles of cold where it is expanded into a gas and hot where it is compressed into a fluid,” Manikandan said.

“But because the working substance in the quantum superconducting refrigerator is a superconductor, it’s instead the cooper pairs that unpair and get colder when you apply a magnetic field slowly at very low temperatures, taking the current state-of-the-art refrigerator as a baseline and cooling it even more.”

The team’s work was published in the journal Physical Review Applied.

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Sreenath K. Manikandan et al. 2019. Superconducting Quantum Refrigerator: Breaking and Rejoining Cooper Pairs with Magnetic Field Cycles. Phys. Rev. Applied 11 (5); doi: 10.1103/PhysRevApplied.11.054034

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