Quantum feat

Physicists plan to use quantum dots to build computers that can complete difficult calculations much faster than today’s machines. Their success will hinge on the ability to create a nanomagnet formed from atomic nuclei.

The quest to produce components that use quantum mechanics to process data is being taken up by a new research group at Bristol University, headed by Dr Ruth Oulton. Researchers have already shown that quantum computing is theoretically possible. Now the team must show that it can be achieved with nanoscale semiconductors.

‘We’ll investigate a new type of architecture for a quantum computer,’ said Oulton. ‘We’ll show it is possible to store information by putting a single electron into a quantum dot.’

The best way to store information in an electron is to encode it in its ‘spin’, which can be changed from ‘up’ to ‘down’.

The quantum dot, a semiconductor crystal lattice, can contain an electron and stop it colliding with others but, so far, it has a significant drawback.

The problem is that the nuclei of the quantum dot lattice have their own spin, generally pointing in all directions.This interferes with the direction of the electron, potentially destroying the information. The challenge is to stop the nuclei of the quantum dot’s lattice from disrupting the electron’s behaviour.

Pulses of polarised light are used to help maintain its state but now it is known that there is a knock-on effect. ‘Quantum dots have been studied for about 10 years, but only recently has it become clear that the spin of the electron also affects the nuclei of the lattice which, in turn, affect the electron,’ said Oulton.

Her team plans to exploit the fact that the lattice nuclei are affected by the electron and to control the effect so that they help in maintaining the electron’s spin and, hence, preserve the information.

There are about 10,000 nuclei in the lattice of a quantum dot and the team will try to use the electron spin to align them all in the same direction, creating a nuclear nanomagnet. If the team can achieve this, then the electron spin may be stored for extremely long times.

Storing electron spin, though, is only half the solution. ‘We need to be able to read out and transport its state,’ said Oulton. This is done by looking for photons emitted by the electron. To boost this process, the team will also design a photonic structure, that will accelerate the speed at which a spinning electron emits the photons.

‘This work will help us to understand not only how to make quantum computers using semiconductors, but will also tell us a great deal about how to make these basic interactions work in other systems,’ said Oulton.

Max Glaskin