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August 03, 2010

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Stride toward quantum computer reported

Feb. 6, 2010
Courtesy Princeton University
and World Science staff

Re­search­ers are re­port­ing that they have passed a ma­jor hur­dle in the quest to cre­ate a radic­ally new kind of com­put­er, the quan­tum com­put­er.

Quan­tum com­put­ers would ex­ploit the some­times ap­par­ently non­sen­si­cal laws of quan­tum phys­ics, or na­ture at the sub­a­tom­ic scale, to achieve un­prec­e­dent­ed pow­er and speed.

A ma­jor chal­lenge been find­ing a way to ma­ni­pu­late in­di­vid­ual elec­trons, elec­tric­ally charged com­po­nents of atoms. Elec­trons are seen as the most likely can­di­dates to con­sti­tute the new machi­nes’ pro­cess­ing com­po­nents, or “qu­bits.”

Prince­ton phys­i­cist Ja­son Pet­ta said he and some col­leagues have dem­on­strat­ed a meth­od that al­ters the prop­er­ties of a lone elec­tron with­out dis­turb­ing the tril­lions of elec­trons in its im­me­di­ate sur­round­ings. The feat is con­sid­ered es­sen­tial to the de­vel­op­ment of quan­tum com­put­ers.

Petta has fash­ioned a new meth­od of trap­ping one or two elec­trons in mi­cro­scop­ic cor­rals cre­ated by ap­ply­ing to mi­nus­cule elec­trodes volt­ages, or elec­tric fields that move elec­trons. Writ­ing in the Feb. 5 edi­tion of the re­search jour­nal Sci­ence, Pet­ta and col­leagues de­scribe how elec­trons trapped in these cor­rals form “spin qu­bits,” quan­tum ver­sions of clas­sic com­put­er in­forma­t­ion un­its known as bits. 

Pre­vi­ous ex­pe­ri­ments used a tech­nique in which elec­trons were ex­posed to mi­cro­wave radia­t­ion. How­ev­er, be­cause it af­fect­ed all the elec­trons un­iformly, the tech­nique could not be used to ma­ni­pu­late sin­gle elec­trons in spin qu­bits. It al­so was slow. Pet­ta’s meth­od not only achieves con­trol of sin­gle elec­trons, but it does so ex­tremely rap­id­ly, he said—in a bil­lionth of a sec­ond.

Sub­a­tom­ic par­t­i­cles are found to fol­low the laws of quan­tum phys­ics—in which, for ex­am­ple, they can be in two places at once—as long as these par­t­i­cles stay alone or in very small groups. When they come into con­tact with a great­er mass, the quan­tum ef­fects norm­ally ap­pear to van­ish.

“If you can take a small enough ob­ject like a sin­gle elec­tron and iso­late it well enough from ex­ter­nal per­turba­t­ions, then it will be­have quan­tum me­chan­ic­ally for a long pe­ri­od of time,” said Pet­ta. “All we want is for the elec­tron to just sit there and do what we tell it to do. But the out­side world is sort of pok­ing at it, and that pro­cess of the out­side world pok­ing at it causes it to lose its quan­tum me­chan­ical na­ture.”

When the elec­trons in Pet­ta’s ex­pe­ri­ment are in what he calls their quan­tum state, they are “co­her­en­t,” fol­lowing rules that are radic­ally dif­fer­ent from the world seen by the na­ked eye. Liv­ing for frac­tions of a sec­ond in the realm of quan­tum phys­ics be­fore they are rat­tled by ex­ter­nal forc­es, the elec­trons obey a un­ique set of phys­i­cal laws that gov­ern the be­hav­ior of ultra-small ob­jects. Quan­tum com­put­ers would be de­signed to take ad­van­tage of these char­ac­ter­is­tics.

In ad­di­tion to elec­trical charge, elec­trons pos­sess some­thing akin to rota­t­ion. In the quan­tum world, ob­jects can turn in ways that are at odds with com­mon ex­perience. The Aus­tri­an the­o­ret­i­cal phys­i­cist Wolf­gang Pau­li, who won the No­bel Prize in Phys­ics in 1945, pro­posed that an elec­tron in a quan­tum state can as­sume one of two states, “spin-up” or “spin-down.” It can be im­ag­ined as be­hav­ing like a ti­ny ba­r mag­net with spin-up cor­re­spond­ing to the north pole point­ing up and spin-down cor­re­spond­ing to the north pole point­ing down.

An elec­tron in a quan­tum state can sim­ul­ta­ne­ous­ly be par­tially in the spin-up state and par­tially in the spin-down state or any­where in be­tween, a quan­tum me­chan­ical prop­er­ty called “su­per­po­si­tion of states.” A qu­bit based on the spin of an elec­tron could have nearly lim­it­less po­ten­tial be­cause it can be nei­ther strictly on nor strictly off.

New de­signs could take ad­van­tage of a rich set of pos­si­bil­i­ties of­fered by har­ness­ing this prop­er­ty to en­hance com­put­ing pow­er. In the past dec­ade, the­o­rists and math­e­mati­cians have de­signed for­mu­las that ex­ploit this mys­te­ri­ous su­per­po­si­tion to per­form in­tri­cate cal­cula­t­ions at speeds un­matched by supercom­put­ers to­day.

Pet­ta’s work is aimed at ex­ploiting elec­tron spin.

“In the quest to build a quan­tum com­put­er with elec­tron spin qu­bits, nu­clear spins are typ­ic­ally a nui­sance,” said Gui­do Burk­ard, a the­o­ret­i­cal phys­i­cist at the Uni­vers­ity of Kon­stanz in Germany. “Petta and cowork­ers dem­on­strate a new meth­od that uti­lizes the nu­clear spins for per­forming fast quan­tum opera­t­ions. For sol­id-state quan­tum com­put­ing, their re­sult is a big step for­ward.”

Pet­ta’s spin qubits, which he en­vi­sions as the co­re of fu­ture quan­tum log­ic el­e­ments, are cooled to ultra-cold tem­per­a­tures and trapped in two ti­ny cor­rals known as quan­tum wells on the sur­face of a chip made of high-pur­ity gal­li­um ar­se­nide. The depth of each well is con­trolled by var­y­ing the volt­age on ti­ny elec­trodes or gates. Like a jug­gler toss­ing two balls be­tween his hands, Petta can move the elec­trons from one well to the oth­er by se­lec­tively switch­ing the gate volt­ages.

Be­fore this ex­pe­ri­ment, it was­n’t clear how ex­pe­ri­menters could ma­ni­pu­late the spin of one elec­tron with­out dis­turb­ing the spin of anoth­er in a closely packed space, ac­cord­ing to phys­i­cist Phuan Ong, di­rec­tor of the Prince­ton Cen­ter for Com­plex Ma­te­ri­als.

Pet­ta’s re­search al­so is part of the fledg­ling field of “spin­tron­ics” in which sci­en­tists are stu­dy­ing how to use an elec­tron’s spin to cre­ate new types of elec­tronic de­vices. Most elec­trical de­vices to­day op­er­ate on the ba­sis of anoth­er key prop­er­ty of the elec­tron, its charge.

There are many more chal­lenges to face, Pet­ta said. “Our ap­proach is really to look at the build­ing blocks of the sys­tem, to think deeply about what the lim­ita­t­ions are and what we can do to over­come them,” he added. “But we are still at the lev­el of just ma­ni­pu­lat­ing one or two quan­tum bits, and you really need hun­dreds to do some­thing use­ful.” As ex­cit­ed as he is about pre­s­ent prog­ress, long-term ap­plica­t­ions are still years away, he added; “it’s a one-day-at-a-time ap­proach.”

* * *

Researchers are reporting that they have passed a major hurdle in the quest to create a radically new kind of computer, the quantum computer. Quantum computers would exploit the sometimes apparently nonsensical laws of quantum physics, or nature at the subatomic scale, to achieve unprecedented power and speed. A major challenge been finding a way to manipulate individual electrons, electrically charged components of atoms. Electrons are seen as the most likely candidates for constitute the new machines’ processing components, or “qubits.” Princeton physicist Jason Petta said he and some colleagues have demonstrated a method that alters the properties of a lone electron without disturbing the trillions of electrons in its immediate surroundings. The feat is considered essential to the development of future varieties of superfast computers with near-limitless capacities for data. Petta has fashioned a new method of trapping one or two electrons in microscopic corrals created by applying voltages, or electric fields that would attract electrons, to minuscule electrodes. Writing in the Feb. 5 edition of the research journal Science, Petta and colleagues describe how electrons trapped in these corrals form “spin qubits,” quantum versions of classic computer information units known as bits. Previous experiments used a technique in which electrons in a sample were exposed to microwave radiation. However, because it affected all the electrons uniformly, the technique could not be used to manipulate single electrons in spin qubits. It also was slow. Petta’s method not only achieves control of single electrons, but it does so extremely rapidly, he said—in a billionth of a second. Subatomic particles follow the laws of quantum physics—in which, for example, they can be in two places at once—as long as these particles stay alone or in very small groups. When they are part of a greater mass of particles, the quantum effects are no longer evident. “If you can take a small enough object like a single electron and isolate it well enough from external perturbations, then it will behave quantum mechanically for a long period of time,” said Petta. “All we want is for the electron to just sit there and do what we tell it to do. But the outside world is sort of poking at it, and that process of the outside world poking at it causes it to lose its quantum mechanical nature.” When the electrons in Petta’s experiment are in what he calls their quantum state, they are “coherent,” following rules that are radically different from the world seen by the naked eye. Living for fractions of a second in the realm of quantum physics before they are rattled by external forces, the electrons obey a unique set of physical laws that govern the behavior of ultra-small objects. Scientists like Petta are working in a field known as quantum control where they are learning how to manipulate materials under the influence of quantum mechanics so they can exploit those properties to power advanced technologies like quantum computing. Quantum computers would be designed to take advantage of these characteristics to enrich their capacities in many ways. In addition to electrical charge, electrons possess something akin to rotation. In the quantum world, objects can turn in ways that are at odds with common experience. The Austrian theoretical physicist Wolfgang Pauli, who won the Nobel Prize in Physics in 1945, proposed that an electron in a quantum state can assume one of two states, “spin-up” or “spin-down.” It can be imagined as behaving like a tiny bar magnet with spin-up corresponding to the north pole pointing up and spin-down corresponding to the north pole pointing down. An electron in a quantum state can simultaneously be partially in the spin-up state and partially in the spin-down state or anywhere in between, a quantum mechanical property called “superposition of states.” A qubit based on the spin of an electron could have nearly limitless potential because it can be neither strictly on nor strictly off. New designs could take advantage of a rich set of possibilities offered by harnessing this property to enhance computing power. In the past decade, theorists and mathematicians have designed formulas that exploit this mysterious superposition to perform intricate calculations at speeds unmatched by supercomputers today. Petta’s work is aimed at exploiting electron spin. “In the quest to build a quantum computer with electron spin qubits, nuclear spins are typically a nuisance,” said Guido Burkard, a theoretical physicist at the University of Konstanz in Germany. “Petta and coworkers demonstrate a new method that utilizes the nuclear spins for performing fast quantum operations. For solid-state quantum computing, their result is a big step forward.” Petta’s spin qubits, which he envisions as the core of future quantum logic elements, are cooled to ultra-cold temperatures and trapped in two tiny corrals known as quantum wells on the surface of a chip made of high-purity gallium arsenide. The depth of each well is controlled by varying the voltage on tiny electrodes or gates. Like a juggler tossing two balls between his hands, Petta can move the electrons from one well to the other by selectively switching the gate voltages. Before this experiment, it wasn’t clear how experimenters could manipulate the spin of one electron without disturbing the spin of another in a closely packed space, according to physicist Phuan Ong, director of the Princeton Center for Complex Materials. Petta’s research also is part of the fledgling field of “spintronics” in which scientists are studying how to use an electron’s spin to create new types of electronic devices. Most electrical devices today operate on the basis of another key property of the electron, its charge. There are many more challenges to face, Petta said. “Our approach is really to look at the building blocks of the system, to think deeply about what the limitations are and what we can do to overcome them,” Petta said. “But we are still at the level of just manipulating one or two quantum bits, and you really need hundreds to do something useful.” As excited as he is about present progress, long-term applications are still years away, he added. “It’s a one-day-at-a-time approach,” Petta said.