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Computers set for quantum crashes
Chaos could scramble the super-powerful computers of tomorrow, Philip Ball finds. .
20 November 2000
PHILIP BALL
It is a staple of science fiction that as computers become more complex and powerful, they acquire the ability for autonomous thought. But the supercomputers of tomorrow could have a very different fate in store, two physicists now propose. The information-processing circuitry of these machines could fall into chaos, they explain in Physical Review E1.
The devices in question are quantum computers. These exploit the effects of quantum mechanics -- which governs the behaviour of very small objects -- to achieve computing power far in excess of current limits. The idea behind quantum computing is to encode information not in the electronic or magnetic components of today's machines but in components that can exist in quantum-mechanical states.
The advantage of this is that groups of 'quantum objects' can exist in many more states than their 'classical' analogues. A classical switch can only be on or off. A quantum switch, or 'qubit', can be on, off or both at once -- when it is said to be in a 'superposition'. So in theory a large array of such switches has many more states available to it than a classical array, and can therefore store and process more information.
But there are some huge obstacles to actually making a quantum computer. In particular, maintaining many qubits in a superposition all at once is a very delicate business, and so far no one has managed it for more than three or four qubits.
Quantum chaos may be a further hurdle to clear, Bertrand Georgeot and Dima Shepelyansky of the Université Paul Sabatier in Toulouse, France, now report. This affliction is basically caused by excess of choice. The interactions between elements of the array give them so many possible, and virtually equivalent, ways to arrange themselves that they lose the ability to pick one and stay with it. Instead, the system plunges into uncontrollable disorder.
Quantum chaos would mean that no information could get to where it was needed, and nothing would get done. If a quantum computer were to succumb to quantum chaos, it would be useless.
But could it happen? Georgeot and Shepelyansky assess the chances. They simulate a quantum computer on a real computer, and watch its information content melt down into what looks like random static. This happens only if the interactions between the qubits are stronger than a certain threshold.
By identifying what this threshold is, the researchers can tell prospective quantum computer engineers how to avoid chaotic meltdown.
References
Georgeot, B. & Shepelyansky, D. L. Emergence of quantum chaos in the quantum computer core and how to manage it. Physical Review E 62, 6366 - 6375 (2000).
© Nature News Service / Macmillan Magazines Ltd 2001
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