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John O | June 2019

Concept developed for superconducting quantum fridge to cool atoms to nearly absolute zero

By Josh Perry, Editor
[email protected]


Physicists from the university of Rochester (N.Y.) revealed a concept for a superconducting quantum refrigerator that could cool atoms to nearly absolute zero (around -273°C) to support the performance of quantum computers, according to a report from the university.


A depiction of the refrigeration cycle inside the proposed quantum fridge.
(University of Rochester illustrations/Michael Osadciw)


“The superconducting quantum refrigerator uses the principles of superconductivity to operate and generate an ultra-cold environment,” the report explained. “The cold environment then is conducive to generating the quantum effects required to enhance quantum technologies. The superconducting quantum refrigerator would create an environment whereby researchers could change materials into a superconducting state—similar to changing a material to a gas, liquid, or solid.”


In the superconducting fridge, materials would move between hot and cold reservoirs, much like they do in conventional refrigeration cycles, but rather than a phase-change from liquid to gas the change would be into a superconducting state.


Layered stacks of metals would be placed in a cryogenic dilution refrigerator. The bottom layer would be a sheet of niobium, a superconductor that acts as a hot reservoir. The middle layer is tantalum, another superconductor that acts as the working substance, similar to refrigerant in a conventional system. The top layer is copper, which acts as the cold reservoir.


“When the researchers 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 article continued. “The now cold tantalum layer absorbs heat from the now warmer copper layer. The researchers 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.”


The research was recently published in Physical Review Applied. The abstract stated:


“We propose a solid-state refrigeration technique based on repeated adiabatic magnetization and demagnetization cycles of a superconductor, which acts as the working substance. The gradual cooling down of a substrate (normal metal) in contact with the working substance is demonstrated for different initial temperatures of the substrate. Excess heat is given to a hot large-gap superconductor.


“The on-chip refrigerator works in a cyclic manner because of an effective thermal switching mechanism: heat transport between N-N versus N-S junctions is asymmetric because of the appearance of the energy gap. This switch permits selective cooling of the metal.


“We find that this refrigeration technique can cool down a 0.3 cm3 block of Cu by almost 2 orders of magnitude starting from 200 mK, and down to about 1 mK starting from the base temperature of a dilution fridge (10 mK). The corresponding cooling power at 200 and 10 mK for a 1×1cm2 interface are 25 and 0.06 nW respectively, which scales with the area of the interface.”

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