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I have read that Josephson junctions are used in supercomputers. I read that these superconduction circuits used a special material as a cunductor, and it also said in the article that the speed of these computers can be measured in 100s of gigahertz. My question is what is limiting these supercomputers from going into mass production? Does it have something to do with cooling or energy use? Does it have something to do with the size of these computers?
Answer 1:

Correct the first time -- Josephson junction work only at temperatures where the materials are superconducting --
For most materials this is a few degrees above absolute zero. Cooling requires liquid helium which is about $7-$10/liter and a thumbtack are room temperature will vaporize a quart of LiHe... The big issue is lower the cost of providing a cryogenic environment for such circuits to work. (The recent spate of high temperature superconductors (as high as 125K or -140C) lead to new research in this area, but as yet high temperature J.J's have not been realized. Even at these temperatures, the refrigeration is very substantial... and this is the basic issue with such circuits currently.
There is some hope that nano circuits can find related devices witch will function at higher temperatures.


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