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Sunday, November 4, 2007

Single spins controlled by an electric field

Single spins controlled by an electric field

Researchers in the Netherlands have shown that it is possible to control the spin of a single electron by using an electric field rather than a magnetic field, as is usually the case. The breakthrough could have potential applications for spintronics and quantum computing

Spintronics is a growing area of research that exploits the spin as well as the charge of electrons. It is has already been used to increase the amount of data that can be stored on hard-disks and could someday form the basis of practical quantum computers that perform calculations by manipulating the spins of single electrons.

A key element of spintronics is the ability to flip the spin of an electron from a spin-up to a spin-down state. In the new work, a team led by Lieven Vandersypen at the Kavli Institute of Nanoscience at Delft University of Technology deposited metallic gold gates onto a gallium arsenide substrate, creating a small region where only a single electron can sit. The researchers were then able to use these so-called "quantum dots" to manipulate the spin of the electron in a controlled manner.

Although previously researchers have been able to flip the spins of electrons confined in these dots by applying a magnetic field, it is not easy to generate a magnetic field locally on a chip that is strong enough to rotate the spin. "To then manipulate an array of single spins is almost impossible," says Vandersypen.

In their new experiments, the team used two quantum dots each separated by 0.2 ┬Ám. If the spins in the dots are both parallel, neither electron can hop from one dot to the other because of the Pauli exclusion principle. However, applying an electric field causes one of the spins to rotate.

Indeed, if the field is applied for long enough the electron's spin can rotate until it is anti-parallel to the other electron, then it can jump across to the other dot and cause a current flow. Eventually, if the field is applied even longer, the spin goes back to being parallel again. Vandersypen's PhD students Katja Nowack and Frank Koppens, who carried out the experiment, found that the current varies sinusoidally when plotted against the time over which the electric field is applied. Known as Rabi oscillations, this finding proved they were able to control the rotation of the spin.

The driving mechanism for an electric field to control the spin of an electron lies in the spin-orbit interaction. As the electron orbits around a nucleus it produces a magnetic field that changes its own magnetic moment so that, in the electron's rest frame, an electric field appears as a magnetic field. The team calculated that the coupling from the gallium arsenide electric field to the single electron's spin in the quantum dot is strong enough to be able to change the direction of its spin when an electric field is applied.

Having shown that it is possible to control single spins in quantum dots via localized electric fields, the researchers at Delft now plan to produce an array of quantum dots where each electron's spin state can be manipulated. They plan to use these arrays to form controllably coupled spins, which could pave the way for producing entangled states between the electrons.

The spin on a single electron has revealed itself to US researchers thanks to magnetic resonance force microscopy. Their research could lead to a new approach to three-dimensional imaging of biological molecules at atomic resolution and the long-awaited read-out devices for quantum computers

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