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Dutch physicists get a grip on the spin of a single electron

Delft University Of Technology : 20 November, 2006  (Technical Article)
Researchers of the Kavli Institute of Nanoscience at Delft University of Technology and the Foundation for Fundamental Research on Matter have succeeded for the first time in the world in controlling the spin of a single electron in a nanostructure. They are able to rotate the spin to every possible direction and to record it accordingly.
This achievement makes it possible to use the electron’s spin as a ‘quantum bit’, the basis of a (still theoretical) future quantum computer. The researchers have published this scientific breakthrough in a Nature article.

Illustration Gemma Plum
An electron does not only have an electrical charge, but it also behaves like an ultrasmall magnet. This is caused by the spinning of the electron around its axis, also called ‘spin’. The spin of a single electron can be used as a quantum bit, an important building block for the (theoretically speaking, superior) future quantum computer. In order to create this type of quantum bit, an electron in a semiconductor material is locked up in a quantum dot, which is a kind of electrical trap for the electron. Already in 2004, the Delft researchers succeeded in locking up a single electron and reading out the direction of its spin. Last year a research team of Harvard succeeded in getting control of the entanglement (the quantum mechanical linkage) of two electrons.

Locked up electrons
However, the final step to produce a real quantum bit, namely the possibility to rotate the spin of a single electron, remained beyond reach for a long time. The rotation of the spin is being executed by switching on and off a magnetic field that oscillates very fast during some billionths of a second. The interfering side effects of a locally generated magnetic field made it hard to rotate the electron spin and yet to keep it locked up at the same time.

Frank Koppens and the other researchers of the Delft team, led by dr. Lieven Vandersypen, were able to get around the side effects. Their approach was to lock up a second electron in another quantum dot alongside the first one and to use it to read out the spin direction of the first electron. A basic principle of quantum mechanics tells us that two electrons that have identically oriented spins cannot stay together, while two electrons that have different spins can. Each time after the spin was rotated, a check was made to see whether two electrons were able to reside close together or not. This then defined to what extend the spin direction was changed.

At the moment further research is being executed by combining the materialized basic ingredients to get a quantum bit. Koppens argues that now the way has been prepared to start executing elementary quantum computations. According to him it may be even more inviting to uncover the peculiar properties of quantum physics with these insights, for example, by revealing the entanglement of the two electrons. Entanglement is also the central theme of the FOM research Focus Group for Solid-State Quantum Information Processing at Delft University and supported by the University of Leiden, which the Vandersypen team is part of.

In addition to F. Koppens and L. Vandersypen, the article was co-authored by C. Buizert, K. Tielrooij, I. Vink, K. Nowack, T. Meunier and L. Kouwenhoven.
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