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A Step Toward Quantum Computers

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A step toward quantum computers


Nov. 22, 2006

Courtesy University of Utah

and World Science staff


Physi­cists say they’ve tak­en a step to­ward build­ing com­put­ers that work at blind­ing speeds thanks to the weird real­i­ties of quan­tum phys­ics, the sci­ence of sub­a­tom­ic par­t­i­cles.




Phys­i­cist Chris­toph Boeh­me works with equip­ment that he uses to show the fea­si­bil­i­ty of a quan­tum com­put­er's read­ing da­ta stored in the form of atom­ic "spins." (Cour­te­sy John Lup­ton, U. of Utah)




In a study to ap­pear in the De­cem­ber is­sue of the re­search jour­nal Na­ture Phys­ics, they claim to show the pos­si­bil­i­ty of read­ing da­ta stored in the form of the “spins” of at­oms.


These spins “can be meas­ured by very sub­tle elec­tric cur­rents pass­ing through,” said the Uni­ver­si­ty of Utah’s Chris­toph Boeh­me, one of the re­search­ers.


This re­solves “a ma­jor ob­sta­cle for build­ing a par­tic­u­lar kind of quan­t­um com­put­er,” called the phos­pho­rus-and-sil­i­con type, he added. The prob­lem in­volves how to get the com­put­er to read data.


Many road­blocks re­main, he cau­tioned. “If you want to com­pare the de­vel­op­ment of quan­tum com­put­ers with clas­si­cal com­put­ers, we prob­a­bly would be just be­fore the dis­co­very of the aba­cus.”


Modern com­put­ers con­tain tran­sis­tors, elec­trical switches that store da­ta in pieces called bits. A bit is a chunk of in­for­ma­tion con­sist­ing of ei­ther a 0 or a 1, rep­re­sent­ing ei­ther no elec­trical charge, or some charge, re­spec­tive­ly. A com­put­er with three bits thus con­tains eight pos­si­ble com­bi­na­tions of the two digits: 111, 011, 101, 110, 000, 100, 010 and 001. Three bits in an or­di­na­ry, dig­it­al com­put­er can store on­ly one of those eight groups at a time.


Quan­tum com­put­ers would be based on the strange prin­ci­ples of quan­tum me­chan­ics, in which the small­est par­t­i­cles can be in dif­fer­ent places at the same time.


In a quan­tum com­put­er, one “qu­bit,” or quan­tum bit, could be 0 and 1 si­mul­ta­ne­ous­ly. So with three qu­bits, the de­vice could store all eight com­bi­na­tions at once, and cal­cu­late eight times faster than a three-bit di­g­it­al com­put­er. With more bits, the quan­tum com­put­er’s ad­van­tage grows ex­po­nen­tial­ly. A quan­tum com­put­er with 64 qu­bits would be fas­ter by 2 to the 64th pow­er, or about 18 bil­lion bil­lion times, than a typ­i­cal per­son­al com­put­er.


A ques­tion is how to phys­i­cal­ly rep­re­sent the 0s and 1s in a quan­tum com­put­er. One ap­proach is to en­code this as the “spins” of the nu­cle­i, or cores, of at­oms.


Sub­a­tom­ic par­t­i­cles have a prop­er­ty known as spin, which is akin, though not iden­ti­cal, to ac­tu­al spin­ning: in short, they can act some­what as though they were spin­ning. Sci­en­tists in­fer this from the fact that they act as ti­ny mag­nets, and al­so are elec­trically charged. A mov­ing charge cre­ates a mag­net­ic field ac­cord­ing to cer­tain rules. For sub­a­tom­ic par­t­i­cles, a spin­ning mo­tion can ac­count for the meas­ured mag­net­ic fields. The cal­cu­la­tions show that par­t­i­cles can spin in two op­po­site di­rec­tions, termed “up” and “down.”


The rea­son ac­tu­al spin­ning is­n’t be­lieved to oc­cur is that if it did, at the speed re­quired, parts of the par­t­i­cle’s sur­face would move faster than light. That would vi­o­late Ein­stein’s well-es­tab­lished The­o­ry of Rel­a­tiv­i­ty.


In a spin-based quan­tum com­put­er, down and up spins would rep­re­sent 0 and 1. One qu­bit could have a both val­ues si­mul­ta­ne­ous­ly.


Boeh­me’s study follows a quan­tum com­put­ing stra­te­gy pro­posed in 1998 by Aus­tral­ian phys­i­cist Bruce Kane. In such a com­put­er, phos­pho­rus at­oms would be sprin­k­led in­to a stick of sil­i­con, the sem­i­con­duc­tor used in dig­it­al com­put­er chips. The goal is to keep phos­pho­rus at­oms from be­ing too close to­geth­er, which would let them in­ter­act in a way that dis­rupts the in­for­ma­tion.


Da­ta would be en­coded in the spins of those at­oms’ nu­clei. Ex­ter­nal­ly ap­plied elec­tric fields could serve to read the spins. In this way, Boeh­me and col­leagues wrote that they were able to read the com­bined spin of 10,000 of the nu­clei and elec­trons—charge-car­ry­ing par­t­i­cles—of phos­pho­rus at­oms near the sil­i­con’s sur­face.


A real com­put­er would need to read the spins of sin­gle par­t­i­cles, not thou­sands. But past ef­forts, based on a tech­nique called mag­net­ic res­o­nance, were able to read on­ly the com­bined spins of the elec­trons of 10 bil­lion phos­pho­rus at­oms, Boeh­me said. So the new work rep­re­sents a million-fold im­prove­ment, and shows sin­gle spins are reada­ble in prin­ci­ple—though it would take an­oth­er 10,000-fold im­prove­ment, Boeh­me ar­gues.


But the stu­dy’s point, he added, is that it shows one can elec­tric­al­ly “read” da­ta stored as not on­ly elec­tron spins but as the more sta­ble spins of nu­clei.


The re­search­ers used a sliver of sil­i­con crys­tal about three times the width of a hu­man hair. The nu­cle­ar spin of one phos­pho­rus at­om would store one qu­bit. The sci­en­tists then al­lowed a ti­ny elec­trical cur­rent to run through the de­vice. The cur­rent’s ex­act size would de­pend on the spin di­rec­tion of the phos­pho­rus elec­trons. That gives “a read­out of phos­pho­rus elec­tron spins,” which in turn al­so re­veals the spins of the nu­clei, since the two have a known re­la­tion­ship, Boeh­me said.

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  • 3 weeks later...

The trouble with such super computers is that they are too small. A stray cosmic ray, an emmission from a nearby atom or similar and you get a hugely wrong answer.


Though the Voyager computers were antiques by today's standards, they did the job whereas today's far smaller computers probably would not have as cosmic radiation would have made endless "ghosts in the machines", ruining their efficiency and accuracy.

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