Published online 22 December 2010 | Nature | doi:10.1038/news.2010.694

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Qubit in a nanowire

Quantum bit based on electron spin offers advantages for electronics and optical devices.

qubitsNanowires of indium arsenide give researchers the control they need to turn electrons into quantum bits.Gemma Plum

A type of quantum bit that hinges on the innate link between an electron's spin and its orbit round the nucleus has been developed by physicists in the Netherlands. The system, which should easily integrate with other electronics, is a strong contender for use in future quantum computing or cryptography, according to research published today in Nature1.

Quantum computing relies on the inherent uncertainties of quantum mechanics to process information much faster than any conventional machine. Whereas normal bits of information take only the values zero or one, quantum bits, or 'qubits', exist in a fuzzy superposition of both. This ambiguity allows several qubits to be processed in parallel, so that many calculations can be performed at once.

Researchers are working on different systems to physically realize qubits, from polarized light to superconducting junctions — basically, anything that can take on two distinct states. One promising system is based on electron spin, a magnetic property that can be either 'up' or 'down'.

Electron spins can already be controlled en masse using magnetic fields in, for example, hospital magnetic resonance imaging scans. But extending the control down to single electrons is difficult, because it is tricky to oscillate magnetic fields at the nanoscale.

Spin cycle

Leo Kouwenhoven and his colleagues at the Delft University of Technology, the Netherlands, have circumvented this problem by controlling spins using electric rather than magnetic fields. Although electric fields do not directly influence spin, they can do so indirectly by affecting the orbit of an electron around its parent atom's nucleus.

As an electron orbits, it sees the charge of the nucleus moving, which, according to classical electrodynamics, paints a static magnetic field. So by changing the electron orbit, this magnetic field can be altered, which in turn changes the electron spin — a phenomenon known as the spin–orbit interaction.

In their experiment, Kouwenhoven and his group used a nanowire of indium arsenide (InAs), a semiconductor with heavy, highly-charged atomic nuclei that promote a strong spin–orbit interaction. The researchers applied voltages across five narrow gates surrounding the nanowire to isolate two electrons, which acted as two qubits. By then applying electric field pulses between gates and along the nanowire, they could alter the spin of the qubits from parallel (for example, up and up) to antiparallel (up and down).

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Dane McCamey, an expert in semiconductor spin transport at the University of Sydney, Australia, says that the demonstration of spin–orbit qubits is an "important result", but notes that the measured lifetime of the spin–orbit link is shorter than in previous tests on electrons trapped in the semiconductor gallium arsenide (GaAs)2. Still, the spin–orbit interactions in GaAs were weaker than those in the new InAs nanowires — too weak to demonstrate qubit-level control — and McCamey believes that the InAs spin–orbit lifetimes could improve with more experiments. "The effort should be worth it," he says.

InAs nanowires also have another advantage over electrons in GaAs. Light-emitting diodes based on nanowires have been reported recently, raising the possibility that electron states could be transferred to photons — the backbone of quantum cryptography.

That gives researchers much greater potential for developing "long-lived, controllable qubits that could be readily integrated with other key components such as quantum memory and long range communication via photons", says Karl Petersson, another expert in semiconductor spin transport at Princeton University in New Jersey. 

  • References

    1. Nadj-Perge, S., Frolov, S. M., Bakkers, E. P. A. M. & Kouwenhoven, L. P. Nature 468, 1084-1087 (2010). | Article
    2. Nowack, K. C., Koppens, F. H. L., Nazarov, Yu. V. & Vandersypen, L. M. K. Science 318, 1430-1433 (2007). | Article

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  • #60759

    One of these days maybe one of these widescale applications for graphene will actually be applied on a wide scale. Someday. It's neat that it can do a lot of things in a lab, but I will be a lot more interested when it starts moving toward products and practical use.

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