[tt] Single spins controlled by an electric field - physicsworld.com

[Brian Atkins]

http://physicsworld.com/cws/article/news/31720

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 (Science DOI: 
10.1126/science.1148092 ).

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 
µm. 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.

-- 
Brian Atkins
Singularity Institute for Artificial Intelligence
http://www.singinst.org/