First nanotube computer could spark carbon revolution

COMPUTING, like life, may soon be carbon-based. A functioning computer has been built from carbon nanotubes – complete with its own operating system and software.

It is a simple device, made of only 178 transistors compared with the billions in today's silicon computers. And it is not the first time a computer has been made from something other than silicon (see "March of the Machines").

But given the long-touted potential benefits of carbon nanotubes over silicon, it's a step that could spark a major revolution in computing, akin to the switch from vacuum tubes to silicon around 50 years ago.

"It's a simple computer, but it's not a trivial computer," says Subhasish Mitraof Stanford University in California, who led the development of the device with Philip Wong, also at Stanford.

The computer also represents a victory for much-hyped carbon nanotube transistors, created in 1998 by Cees Dekker and his group at Delft University of Technology in the Netherlands. "It is wonderful to see such a carbon nanotube computer realised, 15 years after our group discovered that carbon nanotube molecules could be used as the basic element of a computer," says Dekker.

Carbon nanotubes' electrical properties mean they make faster and more efficient transistors – the semiconducting switches that create logic gates and allow computation. But difficulties manipulating the tiny molecular rods left many asking if they would ever be useful.

Because they are so small, nanotubes can slip out of place and connect parts of a circuit that are not meant to touch. Mitra and his colleagues guided their tubes by growing them on a quartz wafer, aligning 99.5 per cent of them along the crystal's regular structure. Once the nanotubes were in place, they etched out any misaligned tubes.

The team also sent a large current through the circuit to burn out any useless metallic nanotubes and ensure that only semiconducting nanotubes were left behind (Nature, DOI: 10.1038/nature12502).

"Everybody says that nothing is manufacturable with nanotubes," says Mitra. "That question has been resolved."

Once it had a working chip, the team programmed it to run a counting program and a sorting algorithm. The computer can switch between the two programs, allowing it to multitask like more sophisticated machines. Its basic design is what is known as Turing complete, which means the carbon nanotube machine can theoretically compute anything a regular PC can – just much, much more slowly. It runs at a speed of 1 kilohertz, millions of times slower than modern machines.

This raw speed is deceptive, though, says Mitra, because the experimental chip is hooked up to measurement equipment that slows it down. "If you take out the measurement side of things you would get significant speed-up."

The computer's mere existence is more important than its complexity, saysAaron Franklin, a researcher at IBM in New York who was not involved in the work. "It is a key milestone on the path towards a competitive carbon nanotube computer," he says.

Even once the technology is ready, high costs mean you are unlikely to see a carbon chip inside your laptop or smartphone any time soon, Franklin adds. They might show up first in the enormous servers run by the likes of Google and Amazon. "Servers are always going to benefit from improvements that help them have higher performance and run at lower power," he says.

This article appeared in print under the headline "Nanotubes shake up computing"

March of the Machines

Computers have been redesigned over and over since their invention, shifting to more advanced materials as they become available.

l Mechanical: In 1837, Charles Babbage published designs for the Analytical Engine, a general-purpose computer based on rotating gears, but he died before he was able to build it.

l Electromechanical: Alan Turing developed the bombe, a device combining moving parts with electrical circuits, to analyse Nazi codes during the second world war.

l Vacuum tubes: The US army's Electronic Numerical Integrator And Computer (ENIAC), which contained more than 17,000 tubes, became the first fully electronic general-purpose computer in 1946.

l Silicon: Bell Labs developed the first silicon transistor in 1954, which led to the Intel 4004, the first commercially available microprocessor, released in 1971.

l Carbon nanotubes: Cees Dekker and colleagues at Delft University of Technology made the first practical carbon nanotube transistor in 1998, leading to the first carbon nanotube computer (see main story).

l Optical, DNA and more: Transistors using laser light, DNA molecules and other exotic materials all exist, but have yet to spawn general-purpose computers.