A conventional refrigerator operates by doing work to compress a fluid and letting it freely expand in a phase transition where it cools down and absorbs heat from a cold reservoir. The now hot fluid is then re-compressed, liquifying it, and then dumps the excess heat to a hot reservoir, which is usually the environment that allows the fluid to thermalize and reset to its initial temperature. The cycle repeats many times such that a stable low final temperature is achieved in the cold reservoir. The goal of building solid state refrigerators and heat engines working on quantum principles is an outstanding need for next generation quantum technologies. There were attempts in the past to try and implement adiabatic magnetization of a superconductor as an effective cooling technique. However, unlike a freely movable liquid-gas refrigerator where heat transfer to either a hot or cold reservoir can be made on demand, like a piston operating a heat-transfer switch (on/off type energy exchange), solid state systems do not have such freedom.
This invention is a superconducting quantum refrigerator where a magnetic field inducing the phase transition is applied quasi-statically and thus can be reversed quasi-statically to its initial value and reverse the superconducting to normal phase transition of the working substance. The 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. This cycle can be performed repeatedly, where the working substance (superconductor) is driven between two different temperatures (hot and cold), envisaging a refrigeration cycle.
This refrigeration technique can cool down a 0.3 cubic centimeter block of Cu (for example) by almost two orders of magnitude starting from 200mK, and down to about 1mK starting from the base temperature of a dilution fridge (10mK). The refrigerator has a maximum cooling power of 6mW per cubic centimeter at its optimal point of operation.
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