The cryotron is a switch that operates using superconductivity. The cryotron works on the principle that magnetic fields destroy superconductivity. This simple device consists of two superconducting wires ( e.g. tantalum and niobium) with different critical temperature (Tc). A straight wire of tantalum ( having lower Tc) is wrapped around with a wire of niobium in a single layer coil. Both wires are electrically isolated from each other. When this device is immersed in a liquid helium bath both wires become superconducting and hence offer no resistance to the passage of electric current. Tantalum in superconducting state can carry large amount of current as compare to its normal state. Now when current is passed through the niobium coil (wrapped around tantalum) it produces a magnetic field, which in turn reduces (kills) the superconductivity of the tantalum wire and hence reduces the amount of the current that can flow through the tantalum wire. Hence one can control the amount of the current that can flow in the straight wire with the help of small current in the coiled wire. We can think of the tantalum straight wire as a "gate" and the coiled niobium as a "control".
A planar cryotron was developed in 1957 by Dudley Allen Buck at Massachusetts Institute of Technology, made of thin films of lead and tin. This was one of the first integrated circuits, although using superconductors rather than semiconductors. In the next few years a demonstration computer was made and arrays with 2000 devices operated. A short history of this work is in the newsletter of the IEEE History Center, number 75, November 2007.
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... Laboratory Memorandum 6M-3843The Cryotron - A Superconductive Computer Component 1956 A Cryotron Catalog Memory System by Al Slade and Howard McMahon 1957 James W ...