Zeroing in on quantum computing

Scientists are a step closer to designing super fast quantum computers with a recent experiment showing how a phosphorus-and-silicon quantum computer might work.

In a study to be published in the December issue of Nature Physics, University of Utah physicist Christoph Boehme shows it's possible to read data stored in the form of the magnetic "spins" of phosphorus atoms.

"Our work represents a breakthrough in the search for a nanoscopic [atomic scale] mechanism that could be used for a data readout device," Boehme says in a news release.

"We have demonstrated experimentally that the nuclear spin orientation of phosphorus atoms embedded in silicon can be measured by very subtle electric currents passing through the phosphorus atoms."

The experiment, conducted with German colleagues at the Hahn-Meitner Institute in Berlin and the Technical University of Munich, relies on the strange principles of quantum mechanics, in which the smallest particles of light and matter can be in different places at the same time.

To understand the significance of Boehme's work, it's necessary to take a step back and look at how today's digital computers work.

Your computer relies on information transmitted by flowing electricity in the form of electrons, which are negatively charged subatomic particles. Transistors in your computer are electrical switches that store data as "bits," in which "off" (no electrical charge) and "on" (charge is present) represent one bit of information: either 0 or 1.

For example, with three bits, there are eight possible combinations of 1 or 0: 1-1-1, 0-1-1, 1-0-1, 1-1-0, 0-0-0, 1-0-0, 0-1-0 and 0-0-1. But three bits in a digital computer can store only one of those eight combinations at a time.

Quantum computers, which don't yet exist, would rely on the fact that the smallest particles can be in different places at the same time.

So in a quantum computer, one quantum bit could be both 0 and 1 at the same time. It follows that with three quantum bits of data, a quantum computer could store all eight combinations of 0 and 1 simultaneously. That means a three-quantum-bit computer could calculate eight times faster than a three-bit digital computer.

Doping silicon, reading spin

Boehme and his colleagues harnessed the unique properties of quantum physics by "doping" silicon the semiconductor used in digital computer chips with atoms of phosphorus. Next they applied electric current to read and process the data stored in the "spins" of those phosphorous atoms' nuclei.

As the scientists themselves admit, 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 (quantum bit) could have a value of 0 and 1 simultaneously," according to their news release.

So in essence, the team's study was about "reading" the net spin of 10,000 of the electrons and nuclei of phosphorus atoms near the surface of the silicon.

Despite the breakthrough, Boehme remains cautious about its application.

"If you want to compare the development of quantum computers with classical computers, we probably would be just before the discovery of the abacus," he says. "We are very early in development."