Spin doctors operate on a quantum dot

A quantum dot is a nanostructure that confines a single conduction-band electron in three spatial dimensions. Researchers have long sought to use the spin on that electron as a quantum bit—or qubit—to store binary information for a quantum computer. First, however, they must show that they have precise and rapid control over the quantum-dot qubit. Unfortunately, flipping the spins with oscillating magnetic fields requires high-frequency fields and very low temperatures, posing a challenge to experimenters. An alternative is to use purely electric fields and exploit the spin–orbit coupling of electrons: The orbital motion relative to the background of the semiconductor’s nuclear charges causes the electron to see a magnetic field, which couples to its spin. In traditional gallium arsenide quantum dots, the spin manipulation times obtained with spin–orbit coupling are too slow. Recently, Leo Kouwenhoven and his colleagues at Delft University of Technology and at Eindhoven University of Technology, both in the Netherlands, have turned to indium arsenide, which is known to have much larger spin–orbit coupling. Furthermore, the team formed the qubits in an InAs nanowire, which offers interesting possibilities for combining with other semiconductors. For example, one might make an optoelectronic device that converts the spin state to a photon for long-distance transportation. A more exotic prediction is that InAs nanowires might be useful for topological quantum computing. (S. Nadj-Perge, S. M. Frolov, E. P. A. M. Bakkers, L. P. Kouwenhoven, Nature 468, 1084, 2010 .)—Barbara Goss Levi