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Physicists Get a Grip on a Single Electron's Spin
Aug 2006
DELFT, Netherlands, Aug. 28, 2006 -- For the first time, researchers said they have succeeded in controlling the spin of a single electron in a nanostructure. The achievement is a step toward using the electron's spin as a quantum bit, the basis for constructing a still-theoretical quantum computer. SingleElectron.jpg
An artist's impression of two electrons locked up in a nanostructure. (Illustration by Gemma Plum)
Researchers of the Kavli Institute of Nanoscience at Delft University of Technology and the Foundation for Fundamental Research on Matter (FOM) published their breakthrough in the Aug. 17 issue of the journal Nature.

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 quantum computer of the future.

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. 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 at Harvard University succeeded in getting control of the entanglement (the quantum mechanical linkage) of two electrons.SingleElectrons.jpg
An electron microscope image of a similar nanostructure that is used in the Delft experiments. (Image: Frank Koppens)
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 keep it locked up at the same time.

Frank Koppens and the other Delft team researchers, led by 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 says 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 stay close together or not. This then told the scientists to what extent the spin direction was changed.SingleElectrongraphic.jpg
Measurements show directly the rotation of the spin of the electron. (Graphic: Frank Koppens)
At the moment, research is being conducted by combining the materialized basic ingredients to get a quantum bit. Now the way has been paved to start executing elementary quantum computations, Koppens said. 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, Koppens said. 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, of which the Vandersypen team is part.

In addition to Koppens and Vandersypen, the article was co-authored by masters students Christo Buizert and Klaas-Jan Tielrooij, PhD students Ivo Vink and Katja Nowack, postdoctoral candidate Tristan Meunier and faculty member Leo Kouwenhoven. For more information, visit:

A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
quantum mechanics
The science of all complex elements of atomic and molecular spectra, and the interaction of radiation and matter.
Acronym for self-aligned polysilicon interconnect N-channel. A metal-gate process that uses aluminum for the metal-oxide semiconductor (MOS) gate electrode as well as for signal and power supply connectors.
Basic ScienceDelft University of TechnologyelectronentanglementMicroscopynanostructureNews & Featuresquantum bitquantum computerquantum mechanicssemiconductorspinVandersypen

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