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| Small calculation is big
victory for quantum computing Carl T. Hall, Chronicle Science Writer |
Thursday, December 20, 2001
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After tinkering for 18 months with an elaborate "quantum computer" prototype, some of the world's most brilliant computer scientists are confirming that 15 equals 5 times 3. "We have proved that beyond any shadow of doubt," said Nabil Amer, who oversees a computer design team at IBM's Almaden Research Center in San Jose. IBM scientists, collaborating with researchers at Stanford University, are reporting results of the research today in the journal Nature. The real point of the experiments, Amer said, went well beyond solving a math problem most people can work out easily in their heads. The work marked one of the first real-world demonstrations of quantum computing, in which computer scientists hope to use the properties of atomic- scale particles to push information technology beyond its current limits. The experiments proved that one of the new field's bedrock formulas worked just as theorists had predicted. The formula is known as "Shor's algorithm," named for Peter Shor, an AT&T mathematician who concocted it in 1994. It boils down to a clever way of harnessing some quantum-mechanical traits of atomic particles to factor numbers. Factoring involves cracking a big number into smaller numbers which, when multiplied together, equal the big number. Once the numbers start getting really big, finding their factors becomes more and more difficult -- and eventually becomes impossible for even the most powerful supercomputers. The difficulty of factoring big numbers underlies a good part of the security apparatus protecting military secrets and electronic commerce. Quantum computers, according to Shor's formula, should have a unique ability to solve such problems, prompting a race to not only prove the theory but also develop a new generation of quantum encryption strategies. Led by former IBM scientist Isaac Chuang, now a professor at MIT, the IBM- Stanford team created special molecules that included five atoms of fluorine and two atoms of carbon. Certain nuclear properties of these seven atoms combined into tiny quantum information-processing units, creating what's known as a "seven-qubit" quantum computer. Conventional computers use dense arrays of switches and wiring etched onto silicon chips. IBM's newfangled quantum setup uses test tubes filled with billions of the designer molecules. Scientists aimed radio waves at the test tubes to generate electric pulses and read out magnetic effects -- essentially the same technology used for NMR (nuclear magnetic resonance) imaging in medical diagnostics and chemical analysis. A quantum property of the atoms known as "electron spin" encoded the information. Such strategies have been used before in cutting-edge computing experiments, but the results reported in Nature were said to be the first time Shor's approach solved a factoring problem. "It's important as proof of principle," said Birgitta Whaley, a theoretical chemist and quantum-computing expert at the University of California at Berkeley. But scientists also emphasized that the IBM-Stanford approach does not necessarily suggest a clear path to any truly useful quantum computing devices. "These are toy quantum computers," said John Watrous, a computer scientist at the University of Calgary. "There's nothing you can do with seven qubits that you can't easily do on your laptop computer right now." David Wineland, head of a quantum-computer research program under way at the National Institute of Standards and Technology in Boulder, Colo., said the NMR approach may soon stumble against some built-in limits. "Signals start to diminish as the number of qubits goes up," he said. And that's a problem, because the number of qubits needed to perform calculations more challenging than factoring a simple number is closer to 20. E-mail Carl T. Hall at chall@sfchronicle.com. |
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