Roland Piquepaille's Technology Trends
How new technologies are modifying our way of life

mardi 4 mai 2004

We might not use quantum computers in our daily jobs before twenty years, but a team of scientists at Purdue and Duke universities said that powerful computers based on quantum mechanics may be a step closer to reality. They used eight tiny converging wires, or "gates," to deposit the electrons in quantum dots one by one. After tuning, each of these interacting quantum dots was able to exist in simultaneous different states, such as down-up and up-down configurations. These up and down configurations will represent the 0s and 1s in quantum bits, or "qubits." Even if this research is promising, these qubits don't change state fast enough to be useful. But it's another step towards real quantum computers. A last remark about the speed of the scientific publishing process: this work was described in May 2003 and is just being published in April 2004.

Here are some excerpts of the Purdue's news release.

By linking a pair of tiny "puddles" of a few dozen electrons sandwiched inside a semiconductor, researchers have enabled these two so-called "quantum dots" to become parts of a transistor -- the vital switching component in computer chips.
"An electron, for example, can behave like a particle or a wave at times, and it has the odd ability to seemingly be in two different states at once," said Albert M. Chang, who is an adjunct professor of physics in Purdue's School of Science. "Physicists need a different set of words and concepts to describe the behavior of objects that can do such counterintuitive things. One concept we use is the 'spin' of an electron, which we loosely imagine as being similar to the way the Earth spins each day on its axis. But it also describes a sort of ordering electrons must obey in one another's presence: When two electrons occupy the same space, they must pair with opposite spins, one electron with 'up' spin, the other 'down.'"
pin is one property that physicists seek to harness for memory storage. After collecting 40 to 60 paired electrons in a puddle within a semiconductor wafer of gallium arsenide and aluminum gallium arsenide, the team then added a single additional unpaired electron to the puddle. This extra electron imparted a net spin of up or down to the entire puddle, which they call a quantum dot. The team also built a second quantum dot nearby with the same net spin.

As usual when talking about quantum computers, the researchers emphasized the fact that quantum bits can exist in several states simultaneously, paving the way to faster computers.

"These computers would have massive parallelism built right in, allowing for the solution of many tough problems," Chang said. "But for us physicists, the possibilities of quantum computers extend beyond any single application. There also exists the potential to explore why there seem to be two kinds of reality in the universe one of which, in everyday language, is said to stop when you cross the border 'into the interior of the atom.'"

Still, lots of research needs to be done before you sit in front of a quantum computer.

For now, though, the team's qubit works too slowly to be used as the basis of a marketable device. Chang said the team would next concentrate on improving the speed at which they can manipulate the spin of the electrons.

The research work has been published in the April 30, 2004 of Physical Review Letters. Here is a link to the abstract of the paper, called "Transition between Quantum States in a Parallel-Coupled Double Quantum Dot."

The paper itself is dated May 13, 2003, and is available here (PDF format, 5 pages, 343 KB).

Sources: Purdue University news release, April 29, 2004; various websites

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