Researchers pin down quantum particles

Researchers at the Kavli Institute for Nanosciences at Delft University of Technology, have succeeded in getting hold of the environment of a quantum particle. This allows them to exercise greater control over a single electron, and brings the team of researchers, led by Vidi winner and FOM workgroup leader Lieven Vandersypen, a step closer still to the super-fast quantum computer. Their results were published in Nature Physics on 16 August.

One of the unique properties of quantum particles is that they can be in different states at the same time. An atom or electron is then in what is termed a 'superposition' of two conditions. For instance, this means that the 'spin' of an electron can be pointing in two different directions at once. A particle like this can therefore be 0 and 1 at the same time, and not just 0 or 1 as in an ordinary computer connection. This permits super-fast calculations. Until now, however, it has not proved possible to keep a particle in one specific state for any real length of time, because the environment - which also consists of quantum particles - is constantly disrupting the state. Researchers have been unable to get to grips with this until now.

The researchers in Delft tackled the problem by stabilising the environment. They had already shown that it was possible to direct the spin of an electron using a quantum dot - a quantum scale box. The problem, however, is that the nuclei in the material of the box also have their own spins. Because spins operate like miniscule magnets, they pull and push the spin of the electron in the box. But that electron is also pushing and pulling in return.

The interaction between the spin of the electron and the spins of the surrounding nuclei was precisely what allowed the researchers to pin down the nuclear spins. They directed an electrical current through the nano-box and thus influenced the spin direction of the nuclei. The interaction between the spin of the electron and the nuclear spins in the environment finally allowed a situation to be created where the nuclear spins no longer varied at random, but actually became relatively stable. This stable environment now makes it possible to preserve the fragile but important superposition for a longer period.

The article by Ivo Vink, Lieven Vandersypen and colleagues was published as an Advance Online Publication on the website of Nature Physics on 16 August 2009. A detailed theory about the mechanism behind these experimental observations was elaborated by PhD student Jeroen Danon and colleagues, and published a few weeks ago in the journal Physical Review Letters. The research was funded by NWO and the Foundation for Fundamental Research on Matter (FOM).