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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