Cheaper Superconducting Computer Chips

Computer chips with superconducting circuits — circuits with zero electrical resistance — would be 50 to 100 times as energy-efficient as today’s chips, an attractive trait given the increasing power consumption of the massive data centers that power the Internet’s most popular sites.

Superconducting chips also promise greater processing power: Superconducting circuits that use so-called Josephson junctions have been clocked at 770 gigahertz, or 500 times the speed of the chip in the iPhone 6.

But Josephson-junction chips are big and hard to make; most problematic of all, they use such minute currents that the results of their computations are difficult to detect. For the most part, they’ve been relegated to a few custom-engineered signal-detection applications.

Nanoletters – A Superconducting-Nanowire Three-Terminal Electrothermal Device

MIT researchers present a new circuit design that could make simple superconducting devices much cheaper to manufacture. And while the circuits’ speed probably wouldn’t 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 advisor, 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.

Better MRAM memory and more single photon detectors

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.

“I think this is a great device,” says Oleg Mukhanov, chief technology officer of Hypres, a superconducting-electronics company whose products rely on Josephson junctions. “We are currently looking very seriously at the nTron for use in memory.”

“There are several attractions of this device,” Mukhanov says. “First, it’s very compact, because after all, it’s a nanowire. One of the problems with Josephson junctions is that they are big. If you compare them with CMOS transistors, they’re just physically bigger. The second is that Josephson junctions are two-terminal devices. Semiconductor transistors are three-terminal, and that’s a big advantage. Similarly, nTrons are three-terminal devices.”

“As far as memory is concerned,” Mukhanov adds, “one of the features that also attracts us is that we plan to integrate it with magnetoresistive spintronic devices, mRAM, magnetic random-access memories, at room temperature. And one of the features of these devices is that they are high-impedance. They are in the kilo-ohms range, and if you look at Josephson junctions, they are just a few ohms. So there is a big mismatch, which makes it very difficult from an electrical-engineering standpoint to match these two devices. NTrons are nanowire devices, so they’re high-impedance, too. They’re naturally compatible with the magnetoresistive elements.”

Abstract – A Superconducting-Nanowire Three-Terminal Electrothermal Device

Superconducting electronics based on Josephson junctions are used to sense and process electronic signals with minimal loss; however, they are ultrasensitive to magnetic fields, limited in their amplification capabilities, and difficult to manufacture. We have developed a 3-terminal, nanowire-based superconducting electrothermal device which has no Josephson junctions. This device, which we call the nanocryotron, can be patterned from a single thin film of superconducting material with conventional electron-beam lithography. The nanocryotron has a demonstrated gain of over 20, can drive impedances of 100 kΩ, and operates in typical ambient magnetic fields. We have additionally applied it both as a digital logic element in a half-adder circuit, and as a digital amplifier for superconducting nanowire single-photon detectors pulses. The nanocryotron has immediate applications in classical and quantum communications, photon sensing, and astronomy, and its input characteristics are suitable for integration with existing superconducting technologies.

SOURCES – MIT Technology Review, NanoLetters

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