It seems that the "cascading" concept is making inroads in technology. Do you remember "Tune in to terahertz: Quantum cascade lasers now work at new wavelengths," a column which appeared here last August?
Today, we'll speak about cascading molecules, not cascading lasers. These molecules are the heart of one of the latest IBM's research projects. It apparently took technology columnists by surprise, and many of them wrote an article last week. I chose R. Colin Johnson's story for you.
IBM researchers have created a simple computation engine that's more than 250,000 times smaller than the most advanced silicon circuitry. Called the world's smallest computer, the system relies on a "molecular cascade" that pushes a handful of carbon monoxide molecules across a copper surface to perform digital logic functions.
"Our molecular cascades are still research, but their small size is literally generations smaller than today's silicon circuitry," said Andreas Heinrich, a physicist at IBM's Almaden Research Center here. "Our 3-input sorter implemented in next-generation CMOS technology requires an area of over 50 square microns, but our molecular cascade implementation uses just 200 square nanometers. Even if CMOS density follows Moore's Law for 40 more years, molecular cascades are still going to be smaller."
The molecular cascades rely on the natural attraction that the carbon ends of CO molecules have towards copper. On the lattice of a single-crystal copper substrate, CO molecules are positioned in a way that's somewhat like trying to cram tennis balls into an egg carton. IBM has lined up these molecules in a staggered 0.25-nanometer grid. The CO spontaneously hops to adjacent grid sites, nudging one another in a chain reaction that performs a preset calculation.
Even high-tech journalists like simple expressions. This is why several articles about IBM's announcement chose to discuss the so-called "domino effect." But what exactly are domino calculations?
"Imagine two lines of dominoes that curve toward each other, and at their end there is a single domino that can be toppled by either line -- that's an OR gate," Heinrich said. "Your input is either a nudge to topple the first one in a given line of dominoes, or a zero is no nudge."
By placing CO molecules on a 0.25-nm grid of crystalline copper, IBM set up logical calculations in domino code. A scanning tunneling microscope (STM) was used to push a naturally occurring grid of CO molecules atop copper into a preset pattern needed to perform a given calculation. Then the STM supplied the "input" to the domino-coded circuit by manually nudging the first molecule in the cascade.
Finally, I want to give you another staggering number about IBM circuits, picked by Caroline Humer, in "IBM Builds Circuit with Carbon Monoxide Molecules."
One circuit is so small that 190 billion could fit on a standard pencil-top eraser, IBM said.
This is the kind of numbers I like.
Sources: R. Colin Johnson , EE Times, October 24, 2002; Caroline Humer, Reuters, October 24, 2002
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