Features

Quantum computing sounds like the stuff of fantasy. It proposes machines that operate much faster than any current computer because they can carry out thousands of calculations at the same time in parallel universes.

Computing ideas really don't come more bizarre than that, and yet one company claims to have taken a big step towards making this a reality, and hopes to have commercial quantum computers in server rooms within the next couple of years. To find out if that could possibly be true, we'll look at the basic ideas of how to make a computer that uses quantum principles.

Simple Quantum Physics

First, let's throw out the quantum myths. Despite what you've heard, quantum mechanics is not hard to understand, it's not controversial and it doesn't overturn all the previous laws of physics.

Physicist and Nobel laureate Richard Feynman said that nobody understands quantum mechanics - but is this true? The basic principles were worked out in the first half of the last century. By the 1970s, physics students had text books with titles such as Simple Quantum Physics. Now it's on the A-level syllabus. When it comes down to it, quantum behaviour is no more mysterious than any other part of physics.

The fundamentals of quantum mechanics aren't controversial, either. Around 1900, scientists looking at tiny particles saw effects that conventional physics couldn't explain. They developed theories that explained those effects. The theories were surprising, but they fit the facts exactly, and correctly predicted the results of new experiments. The scientific

ADVERTISEMENT

establishment adopted them. They've been tweaked and revised, but they're still in use.

Quantum mechanics improved on existing science, but it didn't consign conventional science to the dustbin. It is a more fundamental explanation of the universe than classical physics. It explains things at the level of atoms and electrons that no other theory can explain.

For instance, every particle can also be described as a wave. It's not one or the other - it's both. Even more freakily, until you observe it, every particle can be in several places at once.

Light, for instance, is a wave, but it's also a particle. The wavelength dictates the colour, and prisms bend it, but physical processes give out light in measurable chunks, called photons.

The well-known double-slit experiment shows quantum mechanics in action. Around 1800, English scientist Thomas Young used it to prove that light was a wave. If you shine a light at a plate with two slits in it, the light spreads out (diffracts) on the far side. On a screen, light and dark interference fringes can be seen; in some places the light adds up, and in others it subtracts because it is out of phase.

In 1961, Claus Jönsson of the University of Tübingen carried out the same experiment with electrons rather than light. These were known to be particles, but the two-slit experiment proved that electrons were also waves.

Pier Giorgio Merli at the University of Milan did a similar experiment in 1974 with sensitive equipment that let him send one electron through the slits at a time. The interaction still happened. Electrons popped up on the screen one by one, but over time they built up into the same interference pattern. In other words, each electron went through both slits, and interfered with itself. It was in two places at once.

Until it interacts with other particles, all the possible states of a particle are 'superposed', and the system is 'coherent'. When it interacts with another particle, it 'decoheres' and settles into one of the possible states. Which one it chooses is a matter of probability, and those probabilities are determined by a mathematical expression called the wave function.