Bacteria give new meaning to 'computer bug'

A team of researchers has devised a scheme to use a light-sensitive protein made by a saltwater bacterium as the basic component of an optical computer. In an optical computer, information zips around as photons of light rather than as electrons. The protein, called bacteriorhodopsin, converts light energy into electric energy quickly and efficiently.

 Bacteriorhodopsin is an attractive material for optical computers because it exists in two stable forms, one purple and one yellow. Shining two lasers of different wavelengths alternately on the protein flips it back and forth between the two colors. Several groups have used bacteriorhodopsin as computer memory and as the light-sensitive element in artificial retinas.

 Aaron Lewis and his colleagues at the Hebrew University of Jerusalem impregnated a plastic film with equal concentrations of the purple and yellow forms of bacteriorhodopsin. As they report in the March 7 Science, they then illuminated the film with two lasers, modifying the light using an array of lenses with focal properties that differ slightly for the two wavelengths. Along the edge of an object placed above the film, the two lasers produced shadows slightly displaced from one another, creating a narrow region where the relative intensity of the two wavelengths varied.

 That varying intensity altered the relative concentrations of the purple and yellow forms of bacteriorhodopsin.

 In the system devised by Lewis and his colleagues, high concentrations of yellow represent negative values, high concentrations of purple indicate positive, and an equal amount designates zero. Because the state of the film can be quickly switched by light, it could, in principle, form the working element of an optical computer that would have an immediate electronic output.

 The device operates under a digital "trinary logic system," says Robert R. Birge, a chemist at Syracuse (N.Y.) University and director of the W.M. Keck Center for Molecular Electronics. In analog environments, the scheme also provides "the capability to both add and subtract."

 One of the main disadvantages, Lewis and his colleagues say, is that the experimental setup is complex and expensive -- a significant obstacle to making a practical device. Also, few commercially available lasers produce the wavelengths of light that most efficiently stimulate bacteriorhodopsin.