Another step toward quantum computers

One day, we might use super fast computers based on quantum physics. But how these computers will read data? An international team from Germany and the U.S. has just shown that it's possible to read data stored as nuclear 'spins.' This new method allows to read the net spin of thousands of electrons instead of billions with previous ones.
Written by Roland Piquepaille, Inactive

One day, we might use super fast computers based on quantum physics. But how these computers will read data? An international team from Germany and the U.S. has just shown that it's possible to read data stored as nuclear 'spins.' This new way of reading the spin of thousands of electrons is not the ultimate goal: a real quantum computer would need to read the spins of single particles. Still, this new method is far better than previous ones which only allowed to read the net spins of the electrons of billion of atoms combined. But read more...

These experiments have been conducted by Christoph Boehme, an assistant professor of physics at the University of Utah, and his colleagues at the Hahn-Meitner Institute in Berlin and the Technical University of Munich. Here is a quote from Boehme.

"We have resolved a major obstacle for building a particular kind of quantum computer, the phosphorus-and-silicon quantum computer," says Boehme. "For this concept, data readout is the biggest issue, and we have shown a new way to read data."

If you want to refresh your memory, the University of Utah news release explains what is quantum computing and what is nuclear 'spin.' Here are some details.

Spin is difficult to explain. A simplified way to describe spin is to imagine that each particle -- like an electron or proton in an atom -- contains a tiny bar magnet, like a compass needle, that points either up or down to represent the particle's spin. Down and up can represent 0 and 1 in a spin-based quantum computer, in which one qubit could have a value of 0 and 1 simultaneously.

Before going further, below is an illustration showing the sample structure used for the experiments and to read the spins of electrons. "To preferentially locate 31P in the vicinity of c-Si/SiO2 states, the active sample is a 15-nm-thick epitaxial layer of 31P-doped c-Si deposited on an intrinsic Si buffer layer. For the electrical measurement, gold contacts are deposited on top of the Si surface." (Credit: Christoph Boehme and his colleagues)

Reading the spins of electrons

Here are more details about these experiments.

The researchers used a piece of silicon crystal about 300 microns thick -- about three times the width of a human hair -- less than 3 inches long and about one-tenth of an inch wide. The silicon crystal was doped with phosphorus atoms. Phosphorus atoms were embedded in silicon because too many phosphorus atoms too close together would interact with each other so much that they couldn’t store information. The concept is that the nuclear spin from one atom of phosphorus would store one qubit of information.
The scientists used lithography to print two gold electrical contacts onto the doped silicon. Then they placed an extremely thin layer of silicon dioxide -- about two billionths of a meter thick -- onto the silicon between the gold contacts. As a result, the device's surface had tiny spots where the spins of phosphorus atoms could be detected.

And here is the key point of the experiments.

Then the device was chilled with liquid helium to 452 degrees below zero Fahrenheit. That made most of the phosphorus spins point down. Next, the researchers applied a magnetic field and microwave radiation to the sample, which makes the phosphorus spins constantly flop up and down in concert for a few billionths of a second. As a result, the electrical current fluctuated up and down.

With this method, they were able to "read" the net spin of only some thousands of the electrons and nuclei of phosphorus atoms near the surface of the silicon. Of course, more research needs to be done before reading the spin of a single electron.

This research work has been accepted by Nature Physics and is available as an advance online publication under the name "Electrical detection of coherent 31P spin quantum states" (November 19, 2006). Here are two links to the abstract and to the full paper (PDF format, 4 pages, 608 KB). The above figure has been picked from this paper.

So when will we work with quantum computers? Not before a long time. Boehme has a nice formula: "If you want to compare the development of quantum computers with classical computers, we probably would be just before the discovery of the abacus."

Sources: University of Utah news release, November 19, 2006; and various websites

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