Quantum theory

Quantum computers using components manufactured from ‘designer’ atoms are a step closer to reality thanks to experiments conducted in the US.

Quantum computers using components manufactured from ‘designer’ atoms are a step closer to reality thanks to experiments conducted in the US.

Transistors are usually made from regular semiconductors. But researchers at Purdue University have built a transistor using designer atoms, known as quantum dots, which could pave the way for computers millions of times more powerful than those in use today.

Researchers build quantum dots by pairing spinning electrons with each other one by one, using the principle that an electron with up-spin will always attract an electron with down-spin.

They then give these dots an extra, unpaired electron – which imparts either up or down-spin to the entire dot. Two quantum dots can then be made to entangle with (or interact with) each other, as the extra electrons in each dot are attracted to each other in the same way.

Quantum physics dictates that once a pair of dots has entangled, they remain linked, regardless of the distance between them, and the spin of one dot will always be opposite to the spin of its mate.

Researchers believe this link could be exploited to createwireless computers and instantaneous communication over vast distances.But quantum dots not only spin up or down, they can also seemingly occupy both spin states at once.

This curious property means that dots in transistors could occupy several binary states at once – not only one or zero, like current semiconductors, but also one-zero, one-one, zero-one and zero-zero.Combined with the microscopic size of the dots – about 200 nanometres wide – this will allow quantum computers to solveproblems in hours that would take today’s computers centuriesDuring their experiment, the researchers built two dots and sandwiched both at different points between two wafers of semiconducting material. By electronically controlling the properties of the dots, they were able to fine-tune entanglement between the two.

It is the first time entanglement has been controlled and monitored while inside a semiconductor – effectively opening the way for their use in transistors.

Physics professor Albert Chang, who conducted the research at Purdue, said the experiment represented a ‘major first step towards really doing quantumcomputing.’

‘We have, for the first time, demonstrated the entanglement of two quantum dots and shown that we can control their properties with great precision,’ he said.

But Chang admitted that his system will need to work at a much faster rate before production of such a transistor can go ahead.