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Quantum computing a step closer

NEW YORK - Australian and US physicists have built a working transistor from a single phosphorus atom embedded in a silicon crystal.

The group of physicists, based at the University of New South Wales and Purdue University, said they had laid the groundwork for a futuristic quantum computer that might one day function in a nanoscale world and would be orders of magnitude smaller and quicker than today’s silicon-based machines.

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In contrast to conventional computers that are based on transistors with distinct “on’’ and “off’’ or “1’’ and “0’’ states, quantum computers are built from devices called qubits that exploit the quirky properties of quantum mechanics. Unlike a transistor, a qubit can represent a multiplicity of values simultaneously.

That might make it possible to factor large numbers more quickly than with conventional machines - thereby undermining modern data-scrambling systems that are the basis of electronic commerce and data privacy. Quantum computers might also make it possible to simulate molecular structures with great speed, an advance that holds promise for designing drugs and other kinds of materials.

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Andreas Heinrich, an IBM physicist, said the research was a significant step toward making a functioning quantum computing system. However, whether quantum computing will ever be harnessed for useful tasks remains uncertain, and the researchers also noted that their work demonstrated the fundamental limits that today’s computers would be able to shrink to.

Demonstrations of single-atom transistors date from 2002, but the researchers from Purdue and New South Wales said they had made advances on two fronts: in the precision with which they were able to place the Lilliputian switch, and in using for the first time industry-standard techniques to build the circuitry, making it possible to read and write information from the tiniest conceivable switch.

The results were reported yesterday in the journal Nature Nanotechnology.

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Until now, single-atom transistors have been created on a hit-or-miss basis, the scientists said.

“But this device is perfect,’’ Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication at the University of New South Wales, said in a statement. “This is the first time anyone has shown control of a single atom in a substrate with this level of precise accuracy.’’

The scientists placed the single phosphorous atom using a device known as a scanning tunneling microscope. They used it to essentially scrape trenches and a small cavity on a surface of silicon covered with a layer of hydrogen atoms. Phosphine gas was then used to deposit a phosphorus atom at a precise location, which was then encased in further layers of silicon atoms.

While offering astounding precision for research, these microscopes are not currently applicable as manufacturing tools to make chips that contain billions or even trillions of transistors. Moreover, the devices now operate at extremely low temperatures characteristic of liquid nitrogen: minus 391 degrees.

Shrinking conventional computer circuitry offers radical increases in the speed at which computers can solve problems, lowers the power they require, and drastically increases the amount of data they can store.

Yet some scientists and engineers believe that even when conventional computers stop improving in performance, quantum computing will offer a way to continue to offer vast improvements, making it possible to solve problems beyond the reach of today’s machines.

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