Moving a bit nearer to a quantum computer

By the time you`ve had your new computer for six months, much faster processors will already be on the market. But there is a limit to how fast conventional computers can become. This is because computers process information in a step-by-step fashion, carrying out each part of the process in turn. To make things work really fast, we need to build `quantum computers` instead.

Computers today handle information in binary form, representing everything as zeros or ones. In order to process that information, computers contain tiny circuit elements that can either be off (representing the 0 state), or on (representing the 1 state). These circuit elements are called binary digits, or bits for short. In a quantum computer however, the elements representing the data – called quantum bits, or qubits – can be in a combination or `superposition` of both state 0 and state 1 at once. This enables a number of possibilities to be computed simultaneously, vastly speeding up the processing. Physicists are now in a race to try and find a system that can be used as the qubits in a quantum computer. At the 26th International Conference on the Physics of Semiconductors in Edinburgh from 29 July – 2 August some of the progress towards creating such an element will be described.

One potential candidate for a qubit is an electron in a quantum dot. Quantum dots are tiny regions of semiconductor about one billionth of a metre big, that can be made to contain just one electron each. On Tuesday 30 July, Dr Gerhard Ortner from the University of Dortmund in Germany will describe how he and his collaborators at Würzburg University in Germany, the Naval Research Laboratory in the USA, and the Institute of Microstructural Science in Canada are investigating the feasibility of using quantum dots as qubits.

An electron in a quantum dot behaves like a tiny magnet that can only point in two opposite directions. Under specific circumstances, this `electron-magnet` can be regarded as simultaneously pointing in one direction (down or binary 0) and the other (up or binary 1). This means it could be used as a qubit, whose quantum state – the degree to which the magnet is pointing up compared to down – could be controlled by a magnetic field. (The electric charge of the electron can also be used to provide the basis for a qubit). To make a quantum computer, an array of such quantum bits would be needed, that could interact with each other in a controllable way.

Anything that creates an interaction between qubits – such as tiny electrical contacts – is called a gate. “In a quantum gate a controllable interaction is introduced between the quantum bits so that when the bits are addressed to perform a calculation, they are not affected individually like in a classical computer, but all at once” explains Prof Manfred Bayer from the Dortmund team. This enables a quantum computer to perform several operations at the same time.

Over the last few years, scientists had begun to suggest that two quantum dots placed very closely together could interact in such a way that a quantum gate could be produced. To test this idea, the Dortmund/Würzburg researchers created two quantum dots. Each consists of a cluster of atoms formed on top of the semiconductor gallium arsenide. Under certain conditions quantum dots can be made to emit light, and by analysing the spectrum coming from the two quantum dots, the researchers were able to show that there was an interaction between them. “Our results give clear evidence that a quantum gate can indeed be created by placing two quantum dots closely together” says Prof Bayer. However he warns it is still a huge step from these preliminary findings to producing a viable system for quantum information processing.

Learning more about the structure of quantum dots should help scientists make that leap, and is exactly what a team from the COBRA Inter-University Research Institute in the Netherlands, and Universities of Glasgow and Sheffield in the UK, are trying to do. They have been using a powerful microscope known as a scanning tunnelling microscope (STM) to produce pictures of quantum dots so detailed that they reveal the individual atoms from which the quantum dots are made. On Thursday 1 August, Dr Paul Koenraad from COBRA will present their latest work, which includes measurements taken from cross-sectional views of indium arsenide quantum dots just 5 by 15 billionths of a metre across.

As well as possibly finding a use as qubits, quantum dots could also be used to make miniature semiconductor lasers, light detectors or optical switches for transmitting data through the telecommunications networks of the future. And “if we know how quantum dots obtain their size, shape and composition during their formation, we can then design at will the operational properties of laser structures or other optical components” explains Dr Koenraad. Although other groups have used STMs to look at quantum dots, “we were the first to use it on structures that were already characterised by detailed electro-optical experiments. Our experiments were able to quantify and prove the predictions of those previous experiments,” he says.

An alternative contender for a qubit – phosphorus atoms buried inside a silicon computer chip – has also moved nearer to reality thanks to work by researchers at the Centre for Quantum Computer Technology in Australia. On Tuesday 30 July, Dr Fay Stanley from the Centre (which is a collaboration between the Universities of New South Wales, Melbourne, and Queensland and Los Alamos National Laboratory in the USA) will describe how they have managed accurately to insert single phosphorus atoms into a conventional silicon chip.

The development of this new implantation method means they now have all the techniques needed to build a prototype device on a silicon chip based on phosphorus-atom qubits. The team expects to demonstrate the basic operation of this phosphorus-in-silicon device within a year. This would provide the first critical `proof of principle` of the feasibility of making a working quantum computer in silicon. Their implantation technique could also allow existing computer chips to be made with much greater precision.