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Computing: The Next Generation *
by James G. Williams
The next generation of computers will more than likely have characteristics such as terabyte data storage, will emit almost no heat, will run for days from a battery, exhibit more intelligence than current AI methods and provide massively parallel computing magnitudes faster than today in a palm sized container. Such super systems may have little in common with those based on the semiconductor technology in use today. It is hypothesized that such ultra-fast and high volume storage systems will be based on molecular-sized devices grown from biological systems. Research being performed in this area is termed biomolecular electronics and operates in the realm of nanosecond technology using man-made molecular devices.
In 1992, the space shuttle Columbia carried into orbit a scientific payload consisting of a purplish looking, coastal swamp marsh bacteria found in the San Francisco Bay area. This bacteria, called Halobacterium, gets its purple color from a pigment found in its cell membrane called bacteriarhodopsin or simply bR. The bR protein absorbs light energy (photons) and converts it into cellular energy for the Halobacterium bacteria. Molecular computing specialists believe that this bacterium will be critical for building molecular computing devices since it can switch between alternate states in a way that is similar to the binary logic of today's semiconductor devices used in digital systems.
When a nanometer sized section of bR molecules are sufficiently cooled and then exposed to light from a green laser, they will alter their shape. When the altered shaped bR molecules are exposed to the light from a red laser they will quickly change back to their original form. This behavior can be used to emulate a molecular binary switch. It is estimated that this switching behavior can be used to store 480 gigabytes of data in five (5) cubic centimeters and can be written, read or erased in as little as five (5) pico seconds (five trillionth of a second) using current diode laser technology. This is approximately 1000 times faster that current semiconductor technology and has magnitudes greater storage density.
The space shuttle experimentation referred to previously was conducted by Dr. Robert Birge of the Center for Molecular Electronics at Syracuse University. He is not the only researcher experimenting with this technology. Dr Felix Hong of Wayne State University has shown that genetically engineered mutants of bR can be used as switching devices. Dr Hong uses chemistry to control the switching of his bR devices. By modifying the hydrogen ion concentration (pH) surrounding the protein he is able to easily modulate the electrical behavior of his bR device.
Both Hong and Birge use light as the input source to their bR systems but their approaches differ in that Hong's bR device outputs electrical current while Birge's molecular device outputs light as the information carrier. Digital computing is based on the ability to have materials quickly switch states and modulate a medium such as electricity or light. Thus, molecular devices provide a likely basis for computing technology. A few years ago AT&T bell labs demonstrated a photonic computer which went beyond a proof of concept for using light instead of electricity as a medium for computing.
Researchers are also exploring non-biological techniques for miniaturized molecular devices in the realm of nanosecond technology. Richard Potember of the Applied Physics Laboratory at John Hopkins University has patented a radically new device for storing data using a scanning tunneling microscope (STM). STM's have been used for exploring the atomic domain of materials. An STM consists of a sharp needle which is brought so close to the surface of a material that the electrons spinning in the atomic orbits of the domain of the needle overlap with those spinning in the domain of the material. When a small electrical charge is applied, it creates a potential difference between the STM needle and the material such that electrons tunnel from the needle's tip into the material or vice versa. It has been demonstrated that it is possible to use this to cause a field-induced, reversible-phase transition in certain types of materials. The material shifts from a high impedance state to a low impedance state yielding angstrom sized (an angstrom is one ten billionth of a meter) that can be detected by STM. These reversible states are similar to the operations performed by the read/write heads of a magnetic disk. The domains used by STM are in the 30 to 40 angstrom range compared to the magnitude larger domain (1 square micron) used by today's magnetic disk technology. The STM technology could store a terabyte of data in a 5.25 inch container similar to today's electromechanical devices.
Semiconductor, magnetic, and optical storage technology also keep advancing and by the time molecular devices are commercially available, they may be on an equal basis. The difference may lie in the cost of the growth path for these radically different technologies. It is estimated that the cost to design and build a 64 Mbit DRAM chip is approximately one billion dollars and the cost could go even higher for higher density chips. Bimolecular systems like bR offer the likelihood that they can be commercially grown, harvested and built using off-the-shelf chemistry and laser diodes which will keep the costs more competitive than the semiconductor technology.
A major issue for the semiconductor technology is the desire to increase speed and decrease size, something semiconductor designers achieve by reducing the width of circuits. Today's fabrication methods produce circuit widths of .35 microns. The current generation of Alpha chips use a circuit geometry of approximately .50 micron ( a human hair is about 75 microns in width). As the circuit width approaches the individual atom level, the quantum effect takes over, causing enormous technical problems for designers. In addition, the build up of heat when faster and denser circuits are built creates a heat dissipation problem for designers. But some of these problems may be overcome by using reversible operations which basically eliminates the second law of thermodynamics. Dr. Edward Fredkin of MIT has proposed a Fredkin gate which provides for reversible operations. The ability and cost of implementing such a technology in silicon may not be feasible but the cost and ability to implement a Fredkin gate using a biological system appears more feasible.
It is estimated that optical, biomolecular memory systems and STM storage systems will become commercially available near the end of the decade. Complete biomolecular computing systems will appears later but the proof of concept and actual experiments have shown that it is possible. The economics of manufacturing and maintaining such systems will probably determine its future.
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* The material for this article was taken from Digital News & Review, July 25, 1994, in an article by Franco Vitaliano entitled "The Next Tide in Computing", pp 15-17.
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