Fast option
computation might take a giant leap in its race against time, by storing data in single molecules that can transmit signals in femtoseconds (fs; 10 -15 seconds). Even the fastest of today's silicon-based computers rely on switches a million times slower, registering information in nanoseconds.
Although there are some experimental switches that operate in trillionths (10 -12 ) of a second, the system built by Michael Wasielewski and his colleagues at the Argonne National Laboratory in Illinois is a thousand times as fast. It operates by pushing an electron through a single molecule with two pulses of laser light. "To the best of our knowledge, it's the first time anyone has developed fs control of a charge shift device.' Each bit of information could potentially be stored in the molecule by a single electron. This is moved from zone to zone with separate laser pulses.
The first laser pulse, which has a wavelength of 416 nanometres (nm, 10 -9 ), pushes the electron from the storage zone to the central, priming zone. The second pulse, of wavelength 480 nm, shuttles it within just 400 fs into the final zone of the molecule, where it could be read.
In order to make all this possible, the electron has to be siphoned off into an electrical circuit. Such an action is impossible with the existing molecule, because within 600 fs the electron flips back to the original storage zone, bringing the molecule to its original stability. Wasielewski compares the laser pulses to "pushing an electron up a cliff' in two separate stages. He hopes to add a further zone that captures the electron in an energy "well', at the top of the cliff to make reading easier. This would allow more time for the electron's position to be detected.
For practical purposes Wasielewski is attempting to make arrays of the molecules that could serve as miniature electronic devices. One possibility is to anchor the molecules to metal or glass surfaces, which would feed electrons into electrical circuits. The experiment was described as "extremely elegant' by David Vass, leader of the applied optics group at the University of Edinburgh. But Vass does not expect practical applications to be possible for several years. "It requires parallel developments in laser technology,' he says. The light sources must be fast-acting enough to keep up with the speed of the electrons ( New Scientist, Vol 151, No 2049).
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