"Long before it's in the papers"
June 21, 2011

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Step toward “spin” computing could save energy

June 21, 2011
Courtesy of the University of Arizona
and World Science staff

Phys­i­cists have pro­posed a way to get easily meas­ur­a­ble sig­nals out of the spin of elec­tri­cal par­t­i­cles with­in atoms. That should re­move a key road­block to the de­vel­op­ment of a new genera­t­ion of pow­er-saving com­put­ers, as well as even­tu­ally to even more ad­vanced “quan­tum” com­put­ing, they say.

The first of these goals is known as spin­tron­ics com­put­ing, which does­n’t rely the elec­tri­cal charge to dig­it­ize in­forma­t­ion.

Just like a mag­net with a north and a south pole (left), elec­trons are sur­rounded by a mag­netic field (right). This mag­netic mo­men­tum, or spin, could be used to store in­for­ma­tion in more ef­fi­cient ways. (Cred­it: Philippe Jac­quod)

Physicists have proposed a way to get easily measurable signals out of the spin of electrical particles within atoms. That should remove a key roadblock to the development of a new generation of power-saving computers, as well as eventually to even more advanced “quantum” computing, they say. The first of these goals is known as spintronics computing, which doesn’t rely the electrical charge to digitize information. Unlike normal computers, which require electric charges to flow on a circuit, spintronics processes and stores information using the magnetic properties of electrons, the electric-charge carrying particles in atoms. Regular computers use this charge rather than the magnetism to handle information. Spintronics could “overcome several shortcomings of conventional, charge-based computing,” said University of Arizona physicist Philippe Jacquod, who published the new research with his postdoctoral assistant in the journal Physical Review Letters. “Microprocessors store information only as long as they are powered up, which is the reason computers take time to boot up and lose any data in their working memory” if power goes out, he said. Charge-based microprocessors, he added, are “have to run an electric current all the time just to keep the data in their working memory at their right value… that’s one reason why laptops get hot.” “Spintronics avoids this because it treats the electrons as tiny magnets that retain the information they store even when the device is powered down. That might save a lot of energy.” To understand spintronics, picture electrons as tiny magnets, Jacquod said. “Every electron has a certain mass [weight], a certain charge and a certain magnetic moment, or as we physicists call it, a spin,” he said. “The electron is not physically spinning around, but it has a magnetic north pole and a magnetic south pole. Its spin depends on which pole is pointing up.” Current microprocessors digitize information into bits, or “zeroes” and “ones,” reflecting the absence or presence of electronic charges. “Zero” means very few charges are there; “one” means many of them are. In spintronics, only the orientation of an electron’s “spin” determines whether it counts as a zero or a one. “You want as many magnetic units as possible, but you also want to be able to manipulate them to generate, transfer and exchange information, while making them as small as possible,” Jacquod said. Exploiting electrons’ magnetic moment means converting their spin into an electric signal, he went on. This is commonly done using contacts consisting of common iron magnets or with large magnetic fields. But iron magnets are too crude to work at the tiny scale of tomorrow’s microprocessors, while large magnetic fields disturb the very currents they’re supposed to measure. “Controlling the spin of the electrons is very difficult because it responds very weakly to external magnetic fields,” Jacquod explained. “In addition, it is very hard to localize magnetic fields. Both make it hard to miniaturize this technology.” “It would be much better if you could read out the spin by making an electric measurement instead of a magnetic measurement, because miniaturized electric circuits are already widely available.” In their report, based on theoretical calculations controlled by computer simulations, Jacquod and Stano propose a protocol using existing technology and requiring only small magnetic fields to measure electron spin. They take advantage of an atomic-scale structure “known as a quantum point contact, which one can think of as the ultimate bottleneck for electrons,” Jacquod explained. “As the electrons are flowing through the circuit, their motion through that bottleneck is constrained by quantum mechanics,” the physics of fundamental particles, he added. “Placing a small magnetic field around that constriction allows us to measure the spin of the electrons.” “Our experience tells us that our protocol has a very good chance to work in practice because we have done similar calculations of other phenomena,” Jacquod said. “That gives us the confidence in the reliability of these results.” “We are hopeful that a fundamental stumbling block will very soon be removed from the spintronics roadmap,” added Peter Stano, the postdoctoral assistant. Spintronics could be also a stepping stone for quantum computing, he added, in which an electron not only encodes zero or one, but many intermediate states simultaneously.