Quantum answer

Computing and trying to ensure that the system is errorless is an endless process, but the solution might be near

Published: Wednesday 30 April 1997

the biggest hindrance in quantum computing - the possibility that it would be irreparably error prone - will soon be overcome, claims Raymond Laflamme of the Los Alamos National Laboratory in New Mexico, us .

In a quantum computer, data would be encoded as quantum states, like a spin of a single electron. Such a quantum system can exist in superposition states in which an electron occupies different spin states, the clockwise or anti-clockwise motion, simultaneously. If different spin states encode bits of information, a quantum computer should be able to perform many calculations at the same time (New Scientist, Vol 153, No 2071).

But quantum systems are fragile and scientists are worried that jostling at the atomic level would cause random changes in data bits, which could lead to fatal errors in quantum computation.

The simplest form of error-checking would be to duplicate each quantum bit or qubit and compare them to spot any errors. But according to Heisenberg's uncertainty principle, one can never know everything about a quantum state. This means that qubits cannot be duplicated exactly. In 1995, Andrew Steen of the University of Oxford, uk, and Peter Shor of at&t 's Bell Laboratories in New Jersey, us , showed that errors could be detected by encoding each qubit in a string of five or seven others. Each possible error in the original qubit alters this coding in a different way, so reading the coded qubits allows the programme to diagnose and correct any error. Although this method could reduce errors greatly, it could still creep into the error-checking procedure itself and sabotage a computation if it ran for a long time.

According to Laflamme and his colleagues, the problem could be solved by looking into each of the error-checking steps. The solution may seem obvious, but researchers expect that successive rounds of error-checking would introduce more errors than they corrected. "In a quantum computer you will spend most of your time doing error correction and very little time computing," says Laflamme. Nevertheless, quantum computers would be so efficient for some problems that they could carry this staggering load and still surpass conventional computers.

But Laflamme's solution may be difficult to implement in the near future, says Neil Gershenfled, an expert on quantum computing at the Massachusetts Institute of Technology in Cambridge, us . However, he hopes to build a 10-qubit quantum computer - a long way from the millions of qubits Laflamme's error-checking routine may require.

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