Simpler superconducting promised by nanowire device
MIT researchers have developed a circuit design that could make simple superconducting devices with zero electrical resistance much cheaper to manufacture, and which would be 50 to 100 times as energy efficient as today’s chips. Even though the circuits’ speed probably would not top that of today’s chips, they could solve the problem of reading out the results of calculations performed with Josephson junctions.
The MIT researchers – Adam McCaughan, a graduate student in electrical engineering, and his adviser, professor of electrical engineering and computer science Karl Berggren – call their device the nanocryotron, after the cryotron, an experimental computing circuit developed in the 1950s by MIT professor Dudley Buck (see “Dudley Buck’s Forgotten Cryotron Computer“). The cryotron was briefly the object of a great deal of interest – and federal funding – as the possible basis for a new generation of computers, but it was eclipsed by the integrated circuit.
“The superconducting-electronics community has seen a lot of new devices come and go, without any development beyond basic characterization,” said McCaughan. “But in our paper published in Nano Letters, we have already applied our device to applications that will be highly relevant to future work in superconducting computing and quantum communications.”
Superconducting circuits are used in light detectors that can register the arrival of a single light particle, or photon; that’s one of the applications in which the researchers tested the nanocryotron. McCaughan also wired together several of the circuits to produce a fundamental digital-arithmetic component called a half-adder.
Superconductors have no electrical resistance, meaning that electrons can travel through them completely unimpeded. Even the best standard conductors – like the copper wires in phone lines or conventional computer chips – have some resistance; overcoming it requires operational voltages much higher than those that can induce current in a superconductor. Once electrons start moving through an ordinary conductor, they still collide occasionally with its atoms, releasing energy as heat.
Superconductors are ordinary materials cooled to low temperatures, which damps the vibrations of their atoms, letting electrons zip past without collision. Berggren’s lab focuses on superconducting circuits made from niobium nitride, which has the relatively high operating temperature of 16 Kelvin, or minus 257°C. That is achievable with liquid helium, which, in a superconducting chip, would probably circulate through a system of pipes inside an insulated housing, like Freon in a refrigerator.
A liquid-helium cooling system would of course increase the power consumption of a superconducting chip. But given that the starting point is about one percent of the energy required by a conventional chip, the savings could still be enormous.
Cheap superconducting circuits could also make it much more cost-effective to build single-photon detectors, an essential component of any information system that exploits the computational speedups promised by quantum computing.
The nanocryotron – or nTron – consists of a single layer of niobium nitride deposited on an insulator in a pattern that looks roughly like a capital ‘T.’ But where the base of the T joins the crossbar, it tapers to only about one-tenth its width. Electrons sailing unimpeded through the base of the T are suddenly crushed together, producing heat, which radiates out into the crossbar and destroys the niobium nitrides superconductivity.
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