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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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Physicists say they’ve taken a step toward developing computers that work at blinding speeds by exploiting the weird realities of quantum physics, the science of subatomic particles. In a study to appear in the December issue of the 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 quantum computer, he added. This problem involves how to get the computer to read data. But many roadblacks 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.” A Bit about Quantum Computing Computers contain transistors, electrical switches that store data as “bits.” A bit is a piece 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 1 or 0: 111, 011, 101, 110, 000, 100, 010 and 001. But three bits in a digital computer can store only one of those eight combinations at a time. Quantum computers, which have not been built yet, would be based on the strange principles of quantum mechanics, in which the smallest particles of light and matter can be in different places at the same time. In a quantum computer, one “qubit” – quantum bit – could be both 0 and 1 at the same time. So with three qubits of data, a quantum computer could store all eight combinations of 0 and 1 simultaneously. That means a three-qubit quantum computer could calculate eight times faster than a three-bit digital computer. With larger number of bits, the advantage to quantum computer rises exponentially. Typical personal computers calculate 64 bits of data at a time. A quantum computer with 64 qubits would be 2 to the 64th power faster, or about 18 billion billion times faster. 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 centers of atoms. Subatomic particles have a property known as spin, which is akin, though not identical, to actual spinning. They behave 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 electrical charge creates a magnetic field according to certain rules. For subatomic particles, a spinning motion can precisely 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, though, is that if it did occur 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. A New Spin on Quantum Computers Down and up spins would represent 0 and 1 in a spin-based quantum computer, in which one qubit could have a value of 0 and 1 simultaneously. Boehme’s study deals with an approach to a quantum computer proposed in 1998 by Australian physicist Bruce Kane. In such a computer, phosphorus atoms would be inserted into a stick of silicon, the semiconductor used in digital computer chips. This is intended to prevent phosphorus atoms from being too close together, in which case they would interact in such a way that disrupts the information storage. Data would be encoded in the spins of those atoms’ nuclei. Externally applied electric fields could serve read the spins. In the new study, Boehme and colleagues used silicon doped with phosphorus atoms. By applying an external electrical current, they were able to “read” the net spin of 10,000 of the electrons and nuclei of phosphorus atoms near the surface of the silicon. A real quantum computer would need to read the spins of single particles, not thousands of them. But previous efforts, which used a technique called magnetic resonance, were able to read only the combined spins of the electrons of 10 billion phosphorus atoms combined. So the new study 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 point of the study, he adds, is that it demonstrates it is possible to use electrical methods to detect or “read” data stored as not only electron spins but as the more stable spins of atomic nuclei. Details of the Experiment The researchers used a piece of silicon crystal about 300 microns thick—about three times the width of a human hair—less than three inches (8 cm) long and about 0.1 inch (¼ cm) wide. Within this slab, the nuclear spin of one phosphorus atom would store one qubit of information. The scientists then allowed a tiny electrical current to run through the device The exact size of the current would depend on the spin orientation of the electrons in the phosphorus. “That is basically a readout of phosphorus electron spins,” which, in turn, also can be used to determine the spins of the phosphorus atoms’ nuclei based on a previously known relationship between electron spins and nuclear spins, Boehme said.