cryotron

cryotron, magnetically controlled electronic switching device that operates at extremely low temperatures; it is designed to supplant, in part, the transistor in special electronic equipment (e.g., the computer). One type of cryotron consists of a straight wire (the gate) around which a wire coil (the control coil) is wound. Kept at temperatures near absolute zero, both wires become superconductors (see low-temperature physics; superconductivity), but when current is passed through the coil, a magnetic field is induced that causes the gate to lose its superconductivity, thus switching off the current through it. Another type of cryotron uses conducting films in place of the wires to increase the switching speed. The cryotron is so minute that many of these devices can be packed into a very small area, thereby greatly increasing the degree of control.

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 tantalum straight wire as a "Gate" and coiled niobium as a "control".

A planar cryotron was invented in 1957, made of thin films of lead and tin. This was one of the first integrated circuits, although using superconductivity rather than semiconductivity. In the next few years a demonstration computer was made and 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.