How does a quantum computer work? | Science| In-depth reporting on science and technology | DW | 04.01.2014
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How does a quantum computer work?

When classic bits are transformed into quantum bits it increases a computer's processing power. German researchers are among those who've succeeded in doing this, but so far only under laboratory conditions.

A computer's alphabet for processing data is simple because it contains just two elements: 0 and 1. They describe the so-called bit state of an electrical charge – its voltage level. Andreas Wieck, a physicist at the Ruhr University Bochum, compares the electrons involved with a group of fish sitting in electrical conductors and semiconductors. However, the fish can't move of their own accord. Instead, they have to be stimulated to do so. They start swimming once they come in contact with an electrical current and cluster together at the surface of a piece of metal or of a semiconductor.

When computers process data, the two bit states are encoded, resulting in a series of zeros and ones. But electrons also have other properties that researchers can exploit in order to create so-called quantum bits, Wieck says.

"Expanding bits into quantum bits can dramatically increase computers' processing ability," he explains, adding that the process allows a person to define more than just two possible states.

It was 22 years ago that Wieck predicted this aspect of quantum computing would be possible. In 2012, he put it into practice with the help of colleagues from Grenoble and Tokyo.

From bits to quantum bits

The transformation into a quantum bit works as follows: Normally, the stimulated electrons move through the metal or semiconductor along defined, parallel paths. But researchers interrupted this pattern by sending an additional wave of electricity. In keeping with the metaphor, the wave isolates an electron "fish," which then essentially "surfs" on the wave.

China Supercomputer Tianhe. (Photo: picture-alliance/ dpa)

Not a quantum computer, but still very fast: China's Tianhe-2 supercomputer

Thereafter, the researchers harnessed the electron's surf path into two channels lying very close together. The electron can actually only move itself into one of the two channels, causing it to take on the state coded either as 0 or 1. But by coupling the two channels, the electron exists in both simultaneously. Its overlapping bit states become a quantum bit. That overlapping results in a far broader alphabet for processing data than the binary system. The problem remains, however, that not all of the electrons want to surf along.

"So far, it's only been a couple percent," says Wieck.

The extent to which quantum computing has already been developed remains unknown. The "Washington Post" cited information from the former US intelligence contractor Edward Snowden that the NSA is developing a quantum supercomputer. Thus far, quantum computers have been viewed as a theoretical concept being tested out in laboratories.

Michael Marthaler, a quantum computer developer at the Institute for Theoretical Solid State Physics at the Karlsruhe Institute of Technology, tells DW that all types of research on quantum computers faces a long time horizon of at least 20 years and probably more. He says, "It's very unlikely that a quantum computer with cryptographic capabilities could be built anytime soon."

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