6787578 Quantam Computers

8/18/2019 6787578 Quantam Computers

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QUANTUM COMPUTERS

1. INTRODUCTION

Behold your computer. Your computer represents the culmination of years of

technological advancements beginning with the early ideas of Charles Babbage (1791-171!

and eventual creation of the first computer by "erman engineer #onrad $use in 19%1.

&urprisingly however' the high speed modern computer sitting in front of you is

fundamentally no different from its gargantuan ) ton ancestors' which were e*uipped with

some 1))) vacuum tubes and +)) miles of wiring, lthough computers have become more

compact and considerably faster in performing their tas' the tas remains the same/ to

manipulate and interpret an encoding of binary bits into a useful computational result. bit

is a fundamental unit of information' classically represented as a ) or 1 in your digital

computer. 0ach classical bit is physically realied through a macroscopic physical system'

such as the magnetiation on a hard dis or the charge on a capacitor. document' for

e2ample' comprised of n-characters stored on the hard drive of a typical computer is

accordingly described by a string of n eros and ones. 3erein lies a ey difference between

your classical computer and a *uantum computer. 4here a classical computer obeys the well

understood laws of classical physics' a *uantum computer is a device that harnesses physical

phenomenon uni*ue to *uantum mechanics (especially *uantum interference! to realie a

fundamentally new mode of information processing.

*uantum computer is one which e2ploits *uantum-mechanical interactions in order

to function5 this behavior' found in nature' possesses incredible potential to manipulate data

in ways unattainable by machines today. 6he harnessing and organiation of this power'

however' poses no small difficulty to those who *uest after it.

&ubse*uently' the concept of *uantum computing' birthed in the early )s by

physicist 8ichard eynman' has e2isted largely in the realm of theory. :iraculous algorithms

which potentially would tae a billionth of the time re*uired for classical computers to

perform certain mathematical feats' and are implementable only on *uantum computers' as

such have not yet been realied. two-bit *uantum system' recently developed by a coalition

of researchers' constitutes the sole concrete manifestation of the idea.

;n a *uantum computer' the fundamental unit of information (called a *uantum bit or

*ubit!' is not binary but rather more *uaternary in nature. 6his *ubit property arises as a

direct conse*uence of its adherence to the laws of *uantum mechanics which differ radically

from the laws of classical physics. *ubit can e2ist not only in a state corresponding to the


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QUANTUM COMPUTERS

logical state ) or 1 as in a classical bit' but also in states corresponding to a blend or

superposition of these classical states. ;n other words' a *ubit can e2ist as a ero' a one' or

simultaneously as both ) and 1' with a numerical coefficient representing the probability for

each state.

2.HISTORY OF QUANTUM COMPUTATION


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4idespread interest in *uantum computation is generally accepted to stem from

eynmans observation that classical systems cannot effectively model *uantum mechanical

systems. 3e proposed that the only way to effectively model a *uantum mechanical system

would be by using another *uantum mechanical system (eynman' 19=!. eynmans

observation suggests that computers based on the laws of *uantum mechanics instead of

classical physics could be used to model *uantum mechanical systems.


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(3ogg' 199D! and "rovers algorithm for database search ("rover' 1997! all contribute to the

relatively small number of nown *uantum algorithms.

#nown *uantum algorithms can be partitioned into three groups depending on the

methods they use. 6he first group contains algorithms which are based on determining acommon property of all the output values such as the period of a function' e.g. &hors

algorithm' the second contains those which transform the state to increase the lielihood that

the output of interest will be read (amplification!' e.g. "rovers algorithm and the third

contains algorithms which are based on a combination of methods from the previous two

groups' e.g. the appro2imate counting algorithm (Brassard' 3oyer and 6app' 199!. ;t is not

nown whether additional types of *uantum algorithm e2ist or whether every *uantum

algorithm can be classified as a member of one of a finite number of groups.t the same time as the number of nown *uantum algorithms was e2panding'

significant progress was being made in developing the techni*ues necessary to produce

*uantum hardware. ;on trap technology and nuclear mass resonance (E:8! technology are

two technologies which have successfully been used to develop = and *ubit systems. 6hese

tiny *uantum computers have been used to implement :osca'

199! and "rovers algorithm (?ones' :osca > 3ansen' 199! and show that they can run on

*uantum hardware. 3owever' both ion trap and E:8 technologies appear to have limitations'it seems probable that it will be possible to utilise them to produce systems of up to

appro2imately %) *ubits. or larger numbers of *ubits an alternative technology will be

needed' at present it seems liely that this technology will be solid state.

Fuantum computation simulation languages and systems have been developed which

attempt to allow simulations of *uantum algorithms' these include FCG ( Hmer' 199!' F-gol

(Baer' 1997!' Fubiter (6ucci' 199! and the simulation system currently being developed by

the penFubit group (penFubit' 199!. F-gol was an attempt to write a high level programming language to allow researchers to describe algorithms designed to run on

*uantum computers. Fubiter taes as input an arbitrary unitary matri2 and returns as output

an e*uivalent se*uence of elementary operations (e.g. controlled-nots and *ubit rotations!.

6ogether with simulations produced within mathematical toolits e.g. :athematica and

implementation of algorithms using actual *ubits' this has allowed verification that the

nown *uantum algorithms wor and enabled investigation into how they function.


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s development wor has progressed' additional uses have been proposed for *uantum

computation' from modelling *uantum mechanical systems' breaing public ey encryption'

searching databases' generating true random numbers to providing secure communication

using *uantum ey distribution. ;t has also been suggested that *uantum mechanics may be

playing a role in consciousness' if a *uantum mechanical model of mind and consciousness

was developed this would have significant impact on computational and artificial

intelligence. ;f the brain handles *uantum type transformations somewhere in its neural

networ this could lead to future *uantum computers being biologicalIbiochemical in nature.

t present' the maJority of the research effort in *uantum computation is devoted to the

physics orientated aspects of *uantum computation' in particular the development of

hardware. 4ithin this area researchers are mainly focussing on E:8 technology. ;on traptechnology is beginning to catch up with what has been achieved using E:8 but solid state

technology is still very much in its infancy. 6he computer scienceIinformation science

research aspects are being pursued' but less emphasis is placed on these at present. Krinciple

centres of *uantum computation research include those at Gos lamos' &tanford' ;B:'

@CG' the 2ford Centre for Fuantum Computation' the @niversity of :ontreal' ;nnsbruc

and Caltech' :;6 and @&C.


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3.WORKING

Fuantum computers are basically wors on *uantum phenomenon. 6his may seem

counterintuitive because everyday phenomenon are governed by classical physics' not

*uantum mechanics which taes over at the atomic level. 6his rather difficult concept is

perhaps best e2plained through an e2periment. Consider figure a below/

3ere a light source emits a photon along a path towards a half-silvered mirror. 6hismirror splits the light' reflecting half vertically toward detector and transmiting half toward

detector B. photon' however' is a single *uantied pacet of light and cannot be split' so it

is detected with e*ual probability at either or B. ;ntuition would say that the photon

randomly leaves the mirror in either the vertical or horiontal direction. 3owever' *uantum

mechanics predicts that the photon actually travels both paths simultaneously, 6his is more

clearly demonstrated in figure b.

;n an e2periment lie that in figure a' where a photon is fired at a half-silvered mirror'it can be shown that the photon does not actually split by verifying that if one detector

registers a signal' then no other detector does. 4ith this piece of information' one might

thin that any given photon travels either vertically or horiontally' randomly choosing

between the two paths. 3owever' *uantum mechanics predicts that the photon actually

travels both paths simultaneously' collapsing down to one path only upon measurement. 6his

effect' nown as single-particle interference' can be better illustrated in a slightly more

elaborate e2periment' outlined in figure b below/


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;n this e2periment' the photon first encounters a half-silvered mirror' then a fully

silvered mirror' and finally another half-silvered mirror before reaching a detector' where

each half-silvered mirror introduces the probability of the photon traveling down one path or

the other. nce a photon stries the mirror along either of the two paths after the first beam

splitter' the arrangement is identical to that in figure a' and so one might hypothesie that the

photon will reach either detector or detector B with e*ual probability. 3owever'

e2periment shows that in reality this arrangement causes detector to register 1))L of the

time' and never at detector B, 3ow can this beM

igure b depicts an interesting e2periment that demonstrates the phenomenon of

single-particle interference. ;n this case' e2periment shows that the photon always reaches

detector ' never detector B, ;f a single photon travels vertically and stries the mirror' then'

by comparison to the e2periment in figure a' there should be an e*ual probability that the

photon will strie either detector or detector B. 6he same goes for a photon traveling down

the horiontal path. 3owever' the actual result is drastically different. 6he only conceivable

conclusion is therefore that the photon somehow traveled both paths simultaneously' creating

an interference at the point of intersection that destroyed the possibility of the signal reaching

B. 6his is nown as *uantum interference and results from the superposition of the possible

photon states' or potential paths. &o although only a single photon is emitted' it appears as

though an identical photon e2ists and travels the path not taen' only detectable by the

interference it causes with the original photon when their paths come together again. ;f' for


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e2ample' either of the paths are bloced with an absorbing screen' then detector B begins

registering hits again Just as in the first e2periment, 6his uni*ue characteristic' among others'

maes the current research in *uantum computing not merely a continuation of todays idea

of a computer' but rather an entirely new branch of thought. nd it is because *uantum

computers harness these special characteristics that give them the potential to be incredibly

powerful computational devices.


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4. THE POTENTIAL AND POWER OF QUANTUM COMPUTING

;n a traditional computer' information is encoded in a series of bits' and these bits are

manipulated via Boolean logic gates arranged in succession to produce an end result.

&imilarly' a *uantum computer manipulates *ubits by e2ecuting a series of *uantum gates'

each a unitary transformation acting on a single *ubit or pair of *ubits. ;n applying these

gates in succession' a *uantum computer can perform a complicated unitary transformation to

a set of *ubits in some initial state. 6he *ubits can then be measured' with this measurement

serving as the final computational result. 6his similarity in calculation between a classical

and *uantum computer affords that in theory' a classical computer can accurately simulate a

*uantum computer. ;n other words' a classical computer would be able to do anything a

*uantum computer can. &o why bother with *uantum computersM lthough a classicalcomputer can theoretically simulate a *uantum computer' it is incredibly inefficient' so much

so that a classical computer is effectively incapable of performing many tass that a *uantum

computer could perform with ease. 6he simulation of a *uantum computer on a classical one

is a computationally hard problem because the correlations among *uantum bits are

*ualitatively different from correlations among classical bits' as first e2plained by ?ohn Bell.

6ae for e2ample a system of only a few hundred *ubits' this e2ists in a 3ilbert space of

dimension N1)

9)

that in simulation would re*uire a classical computer to wor withe2ponentially large matrices (to perform calculations on each individual state' which is also

represented as a matri2!' meaning it would tae an e2ponentially longer time than even a

primitive *uantum computer.

8ichard eynman was among the first to recognie the potential in *uantum

superposition for solving such problems much much faster. or e2ample' a system of +))

*ubits' which is impossible to simulate classically' represents a *uantum superposition of as

many as =+))

states. 0ach state would be classically e*uivalent to a single list of +)) 1s and)s. ny *uantum operation on that system --a particular pulse of radio waves' for instance'

whose action might be to e2ecute a controlled-E6 operation on the 1))th and 1)1st *ubits--

would simultaneously operate on all =+)) states. 3ence with one fell swoop' one tic of the

computer cloc' a *uantum operation could compute not Just on one machine state' as serial

computers do' but on =+)) machine states at once, 0ventually' however' observing the system

would cause it to collapse into a single *uantum state corresponding to a single answer' a

single list of +)) 1s and )s' as dictated by the measurement a2iom of *uantum mechanics.6he reason this is an e2citing result is because this answer' derived from the massive


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*uantum parallelism achieved through superposition' is the e*uivalent of performing the

same operation on a classical super computer with N1)1+) separate processors (which is of

course impossible!

0arly investigators in this field were naturally e2cited by the potential of suchimmense computing power' and soon after realiing its potential' the hunt was on to find

something interesting for a *uantum computer to do. Keter &hor ' a research and computer

scientist at 6>6s Bell Gaboratories in Eew ?ersey' provided such an application by devising

the first *uantum computer algorithm. &hors algorithm harnesses the power of *uantum

superposition to rapidly factor very large numbers (on the order N1)=)) digits and greater! in a

matter of seconds. 6he premier application of a *uantum computer capable of implementing

this algorithm lies in the field of encryption' where one common (and best! encryption code'nown as 8&' relies heavily on the difficulty of factoring very large composite numbers into

their primes. computer which can do this easily is naturally of great interest to numerous

government agencies that use 8& -- previously considered to be AuncracableA -- and

anyone interested in electronic and financial privacy.

0ncryption' however' is only one application of a *uantum computer. ;n addition'

&hor has put together a toolbo2 of mathematical operations that can only be performed on a

*uantum computer' many of which he used in his factoriation algorithm. urthermore'eynman asserted that a *uantum computer could function as a ind of simulator for *uantum

physics' potentially opening the doors to many discoveries in the field. Currently the power

and capability of a *uantum computer is primarily theoretical speculation5 the advent of the

first fully functional *uantum computer will undoubtedly bring many new and e2citing

applications.

6he power of computers has increased by si2 orders of magnitude in the last D years

and it will increase by a further si2 orders of magnitude in the ne2t D yearsA' claimed Eic


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3e has reasons for his convictions and' in a fascinating address' he referred to various areas

of research that ;B: was involved in which led him to conclude that :oores Gaw will

remain on the ;6 statute boos. 3ere they are/

• Nanot!" t"#$no%o&'. Eanotubes are microscopic tubes constructed from carbonrings which can be used to build logic circuits. Currently this technology is between

+) to 1)) times denser and therefore faster than current silicon. &o in its current infant

state' it offers about two orders of magnitude improvement and is e2pected to offer

more in time.

• Nano()*+ . ;B: has built nano-machines that can store data on and erase data from a

surface by puncturing a hole in it (or removing it by heating the surface up!' using an

array of minute cantilevered arms. 6his is effectively a nanodis which is =+ to +)

times smaller than current diss and can probably be made even smaller.

T$" Mo%"#%a, Ca*#a(". ;B: has been building molecules using an electron tunneling

microscope. ne of the things it has built is a big molecule that can act rather lie Babbages

computer as originally conceived with balls rolling down paths and passing through gates'

e2cept of course that the balls in this instance are atoms. ;t is thus possible to build a

molecular computer' the smallest nano-machine yet

NEWS ANALYSIS -- ;B: scientists recently furthered the cause of *uantum computing'

announcing they had solved an order-finding mathematical problem in a single cycle using

fluorine atoms -- instead of the usual silicon gates -- as the computing elements. 6he

achievement may be the best evidence to date that an architecture based on atoms and nuclei

can solve multi-step problems that overwhelm even the most powerful of traditional

computers.

lthough the researchers advise that the e2periment was modest' these and other

advances suggest to some that smaller and abler architectures may arise in the future to help

the computer industry push on to new levels of processing power.

or years' concerns have grown about the limits of semiconductor electronics as the

limits of "ordon :oores Gaw have been approached.

;ntel founder :oore said that the number of transistors the industry could place on a

silicon chip would double every year or two. 6he conventional means of pushing hordes of


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electrons around in order to do calculations has wored' of course' and smaller and smaller

chip dice have meant larger memory and faster processing.

6he most immediate obstacle to further miniaturiation is that current chip lithography

techni*ues are nearing their ultimate resolution. 6o go smaller' chip maers will have toemploy new 2-ray fabrication techni*ues' which will be *uite e2pensive to implement.

But even with better chip fab technology' e2perts see an eventual breadown in that trend as

the sie of silicon logic gates shrins down to the sie of atoms.

Q!)t -o",/

4hile pursuing molecular computing research' ;B: and other researchers decided to

e2plore a somewhat non-Boolean approach that is based on the comple2 states of *uantum

matter.

6he team included scientists from ;B:s lmaden 8esearch Center' &tanford

@niversity' and the @niversity of Calgary. 6he group fashioned fluorine atoms as *ubits'

computing units that e2ploit the *uantum physical properties of matter to represent a

spectrum of states' not Just Boolean )s and 1s as in conventional digital computing.

;saac A;eA Chuang' the ;B: research staff member who led the team' said the first

applications for *uantum computing will probably be on coprocessors for specific functionssuch as database looup. 3e also sees the technology addressing mathematical problems such

as the 6raveling &alesman problem that tries to compute the best route between many

locations. 6hose problems can overcome conventional computers.

A4ith *uantum computing' you can do the problem in linear time. 6hat could mean a

difference in OcomputingP time between millions of years and a few wees'A he said.

6he era of *uantum computing will begin in about =)=)' when Acircuit features are

predicted to be the sie of atoms and molecules'A Chuang proJected.

3e noted that accelerating word processing or 4eb surfing would not be well suited

to a *uantum computers capabilities.


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0.OSTACLES AND RESEARCH

OSTACLES SEEN

6he comple2 lab e2periment' not reproducible in the usual corporate environment'

entailed the building of five fluorine atoms within a molecule so that the fluorine nucleis

spins could interact to effect calculations. 6he atoms were programmed by radio fre*uency

pulses' and results were detected by nuclear magnetic resonance instruments' which

according to Chuang' are Asimilar to those commonly found in hospitals and chemistry labs.A

Chuang said the obstacles to commercialiation are Ahuge.A t the present time' *uantum

computing re*uires Aa lot of e2pensive e*uipment'A he said. A6he apparatus fills half a lab.A:oreover' only five *ubits were operative in the e2periment. :any more are re*uired to

really tacle tough tass.

6here are important potential benefits to *uantum computing' said Ginley "wennap' a

microprocessor industry analyst with the Ginley "roup of :ountain iew' Calif.

A&ilicon-based technology is going to taper off and not be able to continue increasing in

performance'A he said. A6heoretically' these *uantum computers should be able to operate

much faster than conventional transistor-based computers.A

&caling to mass production scale is liely to be the biggest hurdle for commercial *uantum

computing. A8ight now they are literally dragging atoms around to create *uantum structures'

then using very e2pensive *uantum microscopes to observe the behavior and e2tract the

information'A "wennap said.

lso' researchers are uncertain about how to independently address molecules or atoms' as

increasing numbers of molecules become part of the molecular computer.

;n other molecular computing news of late' @CG chemists reported the first demonstration

of a reconfigurable molecular switch. :olecule-level switches to this point could switch only

once5 the @CG crew said it was able to switch states hundreds of times. 6hey used synthetic

molecules nown as catenanenes.

6he field of *uantum information processing has made numerous promising

advancements since its conception' including the building of two- and three-*ubit *uantum

computers capable of some simple arithmetic and data sorting. 3owever' a few potentially


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large obstacles still remain that prevent us from AJust building one'A or more precisely'

building a *uantum computer that can rival todays modern digital computer. mong these

difficulties' error correction' decoherence' and hardware architecture are probably the most

formidable. 0rror correction is rather self e2planatory' but what errors need correctionM 6he

answer is primarily those errors that arise as a direct result of decoherence' or the tendency of

a *uantum computer to decay from a given *uantum state into an incoherent state as it

interacts' or entangles' with the state of the environment. 6hese interactions between the

environment and *ubits are unavoidable' and induce the breadown of information stored in

the *uantum computer' and thus errors in computation. Before any *uantum computer will

be capable of solving hard problems' research must devise a way to maintain decoherence

and other potential sources of error at an acceptable level. 6hans to the theory (and now

reality! of *uantum error correction' first proposed in 199+ and continually developed since'

small scale *uantum computers have been built and the prospects of large *uantum computers

are looing up. Krobably the most important idea in this field is the application of error

correction in phase coherence as a means to e2tract information and reduce error in a

*uantum system without actually measuring that system. ;n 199' researches at Gos lamos

Eational Gaboratory and :;6 led by 8aymond Gaflamme managed to spread a single bit of

*uantum information (*ubit! across three nuclear spins in each molecule of a li*uid solution

of alanine or trichloroethylene molecules. 6hey accomplished this using the techni*ues of

nuclear magnetic resonance (E:8!. 6his e2periment is significant because spreading out the

information actually made it harder to corrupt. Fuantum mechanics tells us that directly

measuring the state of a *ubit invariably destroys the superposition of states in which it

e2ists' forcing it to become either a ) or 1. 6he techni*ue of spreading out the information

allows researchers to utilie the property of entanglement to study the interactions between

states as an indirect method for analying the *uantum information. 8ather than a direct

measurement' the group compared the spins to see if any new differences arose between them

without learning the information itself. 6his techni*ue gave them the ability to detect and fi2

errors in a *ubits phase coherence' and thus maintain a higher level of coherence in the

*uantum system. 6his milestone has provided argument against septics' and hope for

believers. Currently' research in *uantum error correction continues with groups at Caltech

(Kresill' #imble!' :icrosoft' Gos lamos' and elsewhere.

t this point' only a few of the benefits of *uantum computation and *uantum computers

are readily obvious' but before more possibilities are uncovered theory must be put to thetest. ;n order to do this' devices capable of *uantum computation must be constructed.


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Fuantum computing hardware is' however' still in its infancy. s a result of several

significant e2periments' nuclear magnetic resonance (E:8! has become the most popular

component in *uantum hardware architecture. nly within the past year' a group from Gos

lamos Eational Gaboratory and :;6 constructed the first e2perimental demonstrations of a

*uantum computer using nuclear magnetic resonance (E:8! technology. Currently' research

is underway to discover methods for battling the destructive effects of decoherence' to

develop an optimal hardware architecture for designing and building a *uantum computer'

and to further uncover *uantum algorithms to utilie the immense computing power available

in these devices. Eaturally this pursuit is intimately related to *uantum error correction codes

and *uantum algorithms' so a number of groups are doing simultaneous research in a number

of these fields. 6o date' designs have involved ion traps' cavity *uantum electrodynamics

(F0

in speed by the vibration fre*uency of the modes in the trap. E:8 devices have an

e2ponential attenuation of signal to noise as the number of *ubits in a system increases.

Cavity F0< is slightly more promising5 however' it still has only been demonstrated with a

few *ubits. &eth Gloyd of :;6 is currently a prominent researcher in *uantum hardware.

6he future of *uantum computer hardware architecture is liely to be very different from

what we now today5 however' the current research has helped to provide insight as to what

obstacles the future will hold for these devices.

6he main difficulty that the research-and-development engineers have encountered is the fact

that it is e2tremely difficult to get particles to behave in the proper way for a significant

length of time. 6he slightest disturbance will cause the machine to cease woring in *uantum

fashion and revert to Asingle-thoughtA mode lie a conventional computer. &tray

electromagnetic fields' physical movement' or a tiny electrical discharge can disrupt the

process.


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.APPLICATIONS WITH QUANTUM COMPUTERS

6o date an operating system F& has been created. ;t has been designed to coordinate

the timing and configuration of the computer as well as processing various signals of output.

@sing F& simple algorithms have been devised such as the following algorithm nown

as "rovers algorithm with runs a search on AEA unsorted items.

Fuantum computers' due to their large scale processing abilities are ideal for such

tass as large number crunching. 02amples of this is factoriation. actoriation is a ey idea

in encryption technology. 6he higher and more comple2 the number the harder it is to crac.

4ith *uantum technology' computers can decrypt much faster than before. lso along the

same lines of decryption is cracing passwords and things lie that. @sing its superposition

states it can run through many different inputs' evaluate the outcome and move to the ne2t

input much much faster than a regular computer.

Fuantum computers might prove especially useful in the following applications/

• Breaing ciphers

• &tatistical analysis

• actoring large numbers

• &olving problems in theoretical physics

• &olving optimiation problems in many variables

S)%)#on #o-t",/

*uantum computer - a new ind of computer far more powerful than any that

currently e2ist - could be made today' say 6haddeus Gadd of &tanford @niversity ' #ohei ;toh

of #eio @niversity in ?apan' and their co-worers. 6hey have setched a blueprint for a

silicon *uantum computer that could be built using current fabrication and measurement

techni*ues.

6he microelectronics industry has decades of e2perience of controlling and fine-

tuning the structure and properties of silicon. 6hese sills would give a silicon-based

*uantum computer a head start over other schemes for putting one together.


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Fuantum and conventional computers encode' store and manipulate information as

se*uences of binary digits' or bits' denoted as 1s and )s. ;n a normal computer' each bit is a

switch' which can be either on or off.

;n a *uantum computer' switches can be on' off or in a superposition of states - on and off atthe same time. 6hese e2tra configurations mean that *uantum bits' or *ubits' can encode more

information than classical switches.

6hat increase in capacity would' in theory' mae *uantum computers faster and more

powerful. ;n practice it is e2tremely difficult to maintain a superposition of more than a few

*uantum states for any length of time. &o far' *uantum computing has been demonstrated

with only four *ubits' compared with the billions of bits that conventional silicon

microprocessors handle.

&everal *uantum-computing demonstrations have used nuclear magnetic resonance

(E:8! to control and detect the *uantum states of atoms floating in solution. But this beaer-

of-li*uid approach is unliely to remain viable beyond ten or so *ubits.

:any researchers suspect that maing a *uantum computer with as many *ubits as a

Kentium chip has transistors will tae the same ind of technology' recording the information

in solid-state devices.

;n 199' Bruce #ane of the @niversity of Eew &outh 4ales in ustralia showed that

solid-state *uantum computing was conceivable' but not practical. 3e suggested that atoms of

phosphorus in crystalline films of silicon could store *ubits that could be read and

manipulated using E:8 sensitive enough to detect single atoms.

6he device proposed by Gadd and his colleagues is similar' but more within the reach

of current technical capabilities. 6hey suggest that *ubits could be encoded in an isotope of

silicon called silicon-=9' or=9

&i.

;tohs group in ?apan believes it has the capability to grow grid-lie arrays of =9&i

chains atom by atom on the surface of the most abundant silicon isotope' =9&i. tiny magnet

and radio waves would then be used to control the magnetic *uantum states of =&i.

Crucially' each *ubit would be stored not Just in a single =9&i atom but in many

thousand copies' one in each =9&i chain. 6his would avoid the problem of maing

measurements on single atoms. 6he readout could be performed using magnetic resonance


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REFERENCES

1. www.computer.howstuffwors.com

=. www.cas.org

. www.apt.net.au

%. www.*ubit.org

6787578 Quantam Computers

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