Electronic Acrobats: Tidily tweaking electrons’ twirls

Controlling a quantum trait of electrons that could be vital for future computers may just have gotten easier. Instead of manipulating electrons’ spins with microscale magnetic fields, which tend to be weak and sluggish, researchers in California and Pennsylvania have devised a simpler, electric means of controlling the spins.

The scientists did their experiments at a temperature near absolute zero. However, if the new tactic can work at room temperature, it would remove a major obstacle to the development of so-called spintronics–circuitry that exploits electronic spin in addition to electronic charge, says David D. Awschalom of the University of California, Santa Barbara. He and Jeremy Levy of the University of Pittsburgh led a collaboration working toward this goal. Spintronic circuits would be faster, denser, and more energy efficient than conventional ones, the scientists predict.

If the technique can also be extended to single electrons, it might lead to so-called quantum computers, which are expected to decipher codes and search databases millions of times faster than conventional computing machines can (SN: 2/1/03, p. 77: Quantum computers to keep an eye on; also see “Knotty Calculations,” in this week’s issue: Knotty Calculations).

The new work, reported in the Feb. 21 Science, is “an extremely important contribution to both spintronics and quantum computation,” comments Michael E. Flatté of the University of Iowa in Iowa City.

The spin of an electron generates a tiny magnetic field along the particle’s spin axis. Spintronics schemes generally encode a stream of information as variations in the three-dimensional orientation of electrons’ spin axes.

Knowing that electrons’ spins are pushed and pulled by magnetic fields as if the electrons were tiny bar magnets, spintronics investigators have struggled for years to incorporate compact magnetic fields onto semiconductor chips to control spin orientation.

“It’s very hard to produce tiny magnetic fields that are localized,” notes Levy. He, Awschalom, and their colleagues skirted that challenge. They painstakingly fabricated a transistor-size microstructure atomic layer by atomic layer from the semiconductors gallium arsenide and aluminum gallium arsenide. They manipulated the composition and interatomic spacings of the structure’s crystal lattice so that the lattice would influence electrons’ spins–without requiring microscale magnets.

With the help of a laser, the team next produced within the microstructure many electrons with the same spin orientation. By applying voltages, the researchers pushed the electrons along specific trajectories through the structure. By the interplay between the electrons and the crystal, the spins were forced en masse to take on new orientations and rates. Since even minute amounts of heat would disrupt the spins, maintaining the temperature near absolute zero was necessary, Awschalom notes.

Is electric control of spin at room temperature possible? That’s hard to say, says Awschalom, quickly adding that the just-reported experiment “seemed impossible a year ago.”

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