Computing With Molecules

Computerworld - Several years ago, Hewlett-Packard Co. built a 256-processor computer, but 220,000 of its parts were defective. HP was thrilled with the results.

The computer was built by HP Labs, HP's central research operation, using ordinary but faulty silicon chips, as part of its program in molecular computing. It proved that clever software can allow a computer to work even when many of its components are defective. That ability will be necessary for the construction of computers whose parts are so tiny that their reliability can't be assured.

"We know [that] at the molecular scale, there will be defects," says R. Stanley Williams, director of quantum science research at HP Labs in Palo Alto, Calif. "We won't attempt to build perfect circuits like Intel does."

For three decades, silicon chips have doubled in transistor density and performance every 18 months. But experts say that the laws of quantum physics will make further improvements impractical in about 10 years.

Researchers at HP and elsewhere are betting that tiny switches built from single molecules - many thousands of times smaller than a silicon transistor - will save the day. Molecular technology could be used to build supercomputers the size of wristwatches and diagnostic sensors that could be injected into the bloodstream, they predict.

The machines HP Labs is working on use switches made of molecules of rotaxane, an organic chemical. The company has already demonstrated a simple, one-function molecular switch, and it's now working on a circuit that will combine two Boolean operators, such as "and" and "or."

In a molecular switch, a molecule is trapped between two wires. A voltage applied across the wires changes the shape of the molecule, which in turn alters its electrical resistance. When the resistance is low, the switch is considered closed, representing a logical 1. When the resistance is high, the switch is open, representing a logical 0.

Power to Match Intel 4004

Williams says he hopes to make a molecular processor as powerful as the Intel 4004 chip "in a few years time." (The 4004, developed in 1969, was a four-bit, 104-KHz silicon device with 2,300 transistors.) Microprocessors based on molecular-scale switches will pass silicon in capability in 10 to 15 years, Williams predicts.

Organic molecules can serve as storage devices as well as switches. The University of California, Los Angeles (UCLA), which is working with HP, recently demonstrated a primitive memory built from these molecules.

Williams says he hopes to build an experimental memory device made of molecular switches that can store 1 trillion bits in one square centimeter within seven years. That's about 1,000 times denser than today's silicon memory chips.