"Long before it's in the papers"
January 27, 2015


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)

Nor­mal com­put­ers re­quire elec­trons, the elec­tric-charge car­ry­ing par­t­i­cles in atoms, to flow on a cir­cuit, and han­dle in­forma­t­ion using these charges. Spin­tron­ics pro­cesses and stores in­forma­t­ion us­ing the mag­net­ic prop­er­ties of elec­trons in­stead. That 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 dies, he said. Charge-based mi­cro­pro­ces­sors, he added, must “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,” or di­rect them into a small space. “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 at once.

* * *

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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.