A SIMPLE computer that performs calculations by the molecular equivalent of flicking over a line of dominoes has been built in California. The device represents the ultimate in the computer industry鈥檚 ceaseless struggle for miniaturisation.
The molecular computer, developed by Andreas Heinrich and his colleagues at IBM鈥檚 Almaden Lab near San Jose, is far tinier than any traditional silicon chip makers will be able to achieve for a long while. 鈥淚f the density of today鈥檚 chips were to continue to double every 2.5 years, it would take 45 years to shrink to this size,鈥� says Heinrich.
His computer is unlike any desktop PC鈥檚 processor. For a start, it is a mechanical device that has more in common with the Difference Engine built by Charles Babbage in the 19th century than with today鈥檚 electronic chips.
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Heinrich鈥檚 group placed carbon monoxide molecules on an atomically flat copper surface. The copper atoms fit snugly together, like snooker balls in an undisturbed pack, and when the carbon monoxide molecules are put on top they settle into the dips between neighbouring atoms.
But Heinrich鈥檚 team found that when they used the fine tip of a scanning tunnelling microscope to push the carbon monoxide molecules around, certain arrangements would make molecules hop from one position to another. In particular, when a third molecule was pushed up to two others to form a flattened triangle, the molecule at the apex of the triangle was pushed forwards one position.
What鈥檚 more, they found that the molecules could be arranged in such a way that when one molecule jumped out of a triangle, it formed a new triangle with two others molecules, forcing one of them to jump forwards, and so on. The result was the molecular equivalent of flicking a line of dominoes to cause a cascade.
Heinrich realised the molecular cascades could be used to make the logic gates which perform calculations in computers. For example, he made an AND gate that gives an output of 1 only if both inputs are 1. When both input molecules are pushed forward one place 鈥� the equivalent of making both inputs a 1 鈥� the final molecule will jump forwards, representing an output of 1 (see Graphic). The group鈥檚 most advanced device uses over 500 molecules arranged into a number of AND and OR gates, the basic building blocks of a universal computer (Science, DOI: 10.1126/science.1076768).
Heinrich鈥檚 computer may have shown us the point where miniaturisation will have to stop. 鈥淭his is the limit,鈥� says James Heath, a chemist at the University of California, Los Angeles. 鈥淵ou can鈥檛 get any denser than this.鈥�
But it won鈥檛 be easy to turn the molecular cascades into a desktop computer. As well as needing an expensive, bulky microscope to start and read the result of each calculation, each molecule has to be pushed back into its original position to reset it after every run. Another possible show-stopper is that the devices, which take an hour to perform a calculation, need to be run at less than 鈭�260 掳C in an ultra-high vacuum.
But as high as these hurdles may seem, Heath says they may not matter. 鈥淪ilicon technology is 45 years behind them, so they have plenty of time to sort things out,鈥� he says.