Quantum theory

Researchers may have to limit the number of operations carried out by quantum computers to prevent a cascade of errors making the technology utterly useless.

Quantum computers could lead to faster signal processing, database searching, cryptography and space research functions than is possible with today’s digital computers, because they can carry out all their calculations simultaneously.

But errors could propagate during this rapid process and make gibberish of any results. Physicists admit that if adequate error correction is not possible the devices may never be useful.

Researchers Drs Tomaz Prosen and Marko Znidaric, of the University of Ljubljana, Slovenia, aim to design a more stable quantum computer to avoid these errors. ‘Problems stem from the fact that there will always be imperfections in the manufacturing of any [quantum] device,’ said Prosen. ‘Our solution is just an abstract idea at the moment. We’re still thinking of some mathematical models, and we need to understand other models.’

To date, quantum computers have been made from solid state microchip-like technology and nuclear magnetic resonance and photonic optical devices.

Whereas conventional computers break information down to a series of ones or zeros, known as bits, the quantum computer manipulates information in the form of qubits. A qubit represents a probability of being both a one and a zero at the same time. A qubit could be 30 per cent a zero and 70 per cent a one, for example.

The problem of errors arises because of the quantum nature of the computers. Individual components within the quantum computer represent each of the qubits. If the qubits are allowed to interact too randomly with each other, they can throw up many erroneous results that overwhelm the system.

Prosen aims to design a quantum computer in which the components are specially arranged to limit their proximity to each other, reducing the ability of the qubits to interact with each other and eliminating, or greatly reducing, the number of errors produced.

Prosen calls this solution ‘the quantum freeze of fidelity’, as fidelity is a measure of error correction in quantum computing.

Quantum computers developed to date have only used four qubits. For any useful scientific work a quantum memory must operate with at least 100 qubits.

But quantum computing researchers admit that they could hit an insurmountable barrier when the computers use 10 qubits, as the error rate could then be so great as to make calculations impossible. This could happen in just three or four years.

Prosen admitted that their ‘freeze’ solution may have limitations, as preventing possible calculations could hinder the computer’s ability to analyse problems.

But even a limited quantum device would be significantly faster than conventional computers.