## QUANTUM COMPUTING

### 8/11/2001

Quantum Computing: A somewhat bizarre computer technology expected to come into its own as the size of manufacturable features in semiconductors approaches dimensions where quantum affects are not only possible, but inevitable. Quantum bits are known as 'qubits' and consist of a superposition of all possible values. Operations performed on a register made of a number of qubits are, in affect, performed simultaneously on all the numbers that could be stored in those qubits.
The world of quantum computing follows rules all its own and has unique problems. For example, logical operations such as AND can be performed easily, but arithmetic operations involving bit to bit carries are nowhere near as simple as with conventional binary arithmetic.

Moreover, phenomena such as 'decoherance' have been identified that may cause information to be lost from quantum systems. The correct answer may not be presented at the end of a computation because it "leaked out" during the computation. It is not known how severe an engineering problem decoherance will be in practice.

Difficulties notwithstanding, a number of algorithms have been devised that should work on a quantum computer if and when one is built. It appears that quantum computers when (and if) they are built may be able to reduce many operations that are impractically time consuming on conventional computers to reasonable time spans. For example, it is theoretically possible that quantum computers may be able to crack even very long public crypto keys in the square root of the number of operation required by conventional computers. This means that even very long keys might be breakable in a few seconds of quantum computing.

There are several classes of quantum computers. One class has been shown to be isomorphic with relatively simple optical techniques. A single operation search of 50 elements has been demonstrated using quantum algorithms with an optical device.

It will be a number of years before even the simplest quantum computers are applied to real problems -- assuming they can be built at all. At this time. no one seriously expects quantum computing to replace conventional computing for conventional usages.

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Copyright 1994-2008 by Donald Kenney.