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Sci Tech
New spin on quantum computing in nanotubes
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The spin of an electron in a carbon nanotube is coupled The cylindrical topology of the tube allows coupling
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In the recent past researchers had hoped to use carbon nano tubes for quantum computing in which the spin of a single electron would represent a bit of data. But now new insights have been gained which indicate that besides spinning in one direction or the other, electrons in a carbon nanotube also orbit clockwise or anticlockwise in the circle of the nanotube.
Electron’s orbit
Cornell physicists have found that the spin of an electron in a carbon nanotube is coupled — that is, interacts with — the electron’s orbit. The finding means researchers will have to change the way they read out or change spin, but offers a new way to manipulate the spin, by manipulating the orbit.
The research is reported in the March 27 issue of the journal Nature by Cornell professors of physics Paul McEthe tubeuen and Daniel Ralph and former Cornell researchers Shahal Ilani, now at the Weizmann Institute of Science in Israel, and Ferdinand Kuemmeth, now at Harvard University.
Tiny cylinders
Carbon nanotubes are tiny cylinders whose walls are made of carbon atoms arranged in connected hexagons, sort of like a rolled up tube of chicken wire.
Rather than orbiting individual atoms, free electrons in a nanotube orbit around the circumference of the tube.
Meanwhile, the electron going around that circle can have its spin oriented in two possible directions according to a Cornell University press release. Until now, physicists believed that the four possible states of an electron — with spin up or down and orbit clockwise or counterclockwise — must be perfectly equivalent.
“It is also interesting from the fundamental physics point of view, because it is the unique cylindrical topology of nanotubes that allows the electrons to have well-defined orbits and therefore to have this coupling,” Ilani said.
Silicon structure
The researchers used the Cornell Nanoscale Science and Technology Facility (CNF) to create a tiny device in which a carbon nanotube about 5 nanometres (nm — a nanometre is a billionth of a meter, about the length of three atoms in a row) in diameter and 500 nm long was mounted between two electrodes above a silicon structure that allows the application of varying electrical charges to the tube.
Quantum dots
The design of the device made it possible to create quantum dots containing a small number of electrons, all the way down to a single electron.
By applying a magnetic field along the axis of the tube and measuring the current flow through the tube, the researchers could determine the energy levels of electrons in the four possible combinations of spin and orbit and found that changing the direction of orbit changes the energy. The orbit of the electron affects its spin and vice versa. — Our Bureau
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