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December 24, 2010

Better control of building blocks for quantum computer using electrical fields

Scientists from the Kavli Institute of Nanoscience at Delft University of Technology and Eindhoven University of Technology have succeeded in controlling the building blocks of a future super-fast quantum computer. They are now able to manipulate these building blocks (qubits) with electrical rather than magnetic fields, as has been the common practice up till now. They have also been able to embed these qubits into semiconductor nanowires.


Until now, the spin of an electron has been controlled by magnetic fields. However, these field are extremely difficult to generate on a chip. The electron spin in the qubits that are currently being generated by the Dutch scientists can be controlled by a charge or an electric field, rather than by magnetic fields. This form of control has major advantages, as Leo Kouwenhoven, scientist at the Kavli Institute of Nanoscience at TU Delft, points out: "These spin-orbit qubits combine the best of both worlds. They employ the advantages of both electronic control and information storage in the electron spin."

There is another important new development in the Dutch research: the scientists have been able to embed the qubits (two) into nanowires made of a semiconductor material (indium arsenide). These wires are of the order of nanometres in diameter and micrometres in length. Kouwenhoven: "These nanowires are being increasingly used as convenient building blocks in nanoelectronics. Nanowires are an excellent platform for quantum information processing, among other applications."

Nature - Spin–orbit qubit in a semiconductor nanowire
Motion of electrons can influence their spins through a fundamental effect called spin–orbit interaction. This interaction provides a way to control spins electrically and thus lies at the foundation of spintronics. Even at the level of single electrons, the spin–orbit interaction has proven promising for coherent spin rotations. Here we implement a spin–orbit quantum bit (qubit) in an indium arsenide nanowire, where the spin–orbit interaction is so strong that spin and motion can no longer be separated. In this regime, we realize fast qubit rotations and universal single-qubit control using only electric fields; the qubits are hosted in single-electron quantum dots that are individually addressable. We enhance coherence by dynamically decoupling the qubits from the environment. Nanowires offer various advantages for quantum computing: they can serve as one-dimensional templates for scalable qubit registers, and it is possible to vary the material even during wire growth. Such flexibility can be used to design wires with suppressed decoherence and to push semiconductor qubit fidelities towards error correction levels. Furthermore, electrical dots can be integrated with optical dots in p–n junction nanowires. The coherence times achieved here are sufficient for the conversion of an electronic qubit into a photon, which can serve as a flying qubit for long-distance quantum communication.

Arxiv - Spin-orbit qubit in a semiconductor nanowire (11 pages)


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