第1期第4期.docx

第1期第4期.docx

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第1期第4期

目录

2006第1期第4期

纳米前沿

Newlookfornanodetection

4

DNApyramidsmaketheirdebut

5

Nanotubesbeamoutbrightlight

9

Nanotubesbreaksuperconductingrecord

11

Nanotubefoamsarestrongandflexible

12

通信快讯

Searchenginesarenotunfair

12

DCSAnnouncesOptiWatchIntelligentFiberSystem

28

RicohLaunchesEncryptEaseCD-RHybridSecurityDisc

28

物理新知

Breakthroughforquantummeasurement

1

FasterPlasticCircuits

6

Doingphysicswithbacteria

9

QuantumMemoryAdvancement

15

Quantumchaosdemonstratedduringatomionisationforthefirsttime

21

Photonscreateprimitivequantumnetwork

23

Quantumgravityforreal

24

科界动态

CalculatingwithBosecondensates

2

Fluidsmixinreverse

3

Ionstrappedonachip

8

Opticaldevicesgetfishy

10

Electronslosetheirmassincarbonsheets

17

Cosmicmagnetismrevealed

18

Pseudogappuzzleforsuperconductors

19

Entanglementreachesnewlevels

20

Theoristsclaimdarkenergydoesnotexist

25

Theimportanceofstayingclean

26

Siliconchipputsthebrakesonlight

26

Fluidlensesfeelthepressure

27

主办单位：

电子科技大学图书馆

编员会：

张晓东黄思芬汪育健侯壮曹学艳李世兰刘静

张宇娥张毓晗黄崇成陈茂兴

本期责编：

张毓晗编辑部电话：

（028）832023402006年3月10日出版

Breakthroughforquantummeasurement

TwoteamsofphysicistshavemeasuredthecapacitanceofaJosephsonjunctionforthefirsttime.Themethodsdevelopedbythetwoteamscouldbeusedtomeasurethestateofquantumbitsinaquantumcomputerwithoutdisturbingthestate.

AJosephsonjunctionconsistsoftwosuperconductinglayersseparatedbyathininsulatinglayer.BrianJosephsonofCambridgeUniversitywontheNobelprizein1973forpredicting,whilehewasstillaPhDstudent,thattheCooperpairsinthesuperconductinglayerswouldbeabletotunnelthroughtheinsulatinglayerwithoutlosingtheirsuperconductingproperties.Josephsonjunctionsarewidelyusedinmanyelectronicdevices,includinglogiccircuits,memorycellsandamplifiers.Superconductingquantuminterferencedevices（SQUIDs）,alsorelyonthejunctionstomeasureextremelysmallmagneticfields.

Figure1

Intheclassicalregime,thejunctionbehaveslikeaninductance.Inthe1980s,however,theoristspredictedthataJosephsonjunctionwouldbehavelikeacapacitorifitwassmallenough.Now,PerDelsingandcolleaguesatChalmersUniversityofTechnologyinSweden,andindependently,PerttiHakonenandco-workersatHelsinkiUniversityofTechnologyandtheLandauInstituteofTheoreticalPhysicsinMoscowhaveobservedthiseffectinexperimentsforthefirsttime.

Figure2

TheSwedenteammeasuredtheeffectinaCooper-pairtransistor,adevicethatcontainstwoJosephsonjunctionsinseries（Phys.Rev.Lett.95206806）.TheHelsinki-MoscowgroupsawtheeffectinaCooper-pairbox,whichcontainsonejunction（Phys.Rev.Lett.95206807）.

DelsingandcolleaguesatChalmersUniversitybeganbyembeddingtheirCooper-pairtransistorinaresonantcircuit.Next,theycooledthedevicedowntomillikelvintemperaturesandmeasuredhowthephaseofaradio-frequencysignalchangedwhenitwasreflectedfromthecircuit.Basedonthesemeasurements,theteamwasabletoshowthatthedevicebehavedlikeaquantumcapacitor.Hakonenandco-workersinHelsinkiandMoscowgroupemployedasimilartechnique.Bothteamsfoundthatthedevicesbehavedaspredictedbytheory.

Theeffectcouldbeusedtoreadoutquantumbits（qubits）inareliablewaybecausethequantumcapacitanceoftheexcitedstateofthequbithastheoppositesigntothegroundstate.Thesestatescouldbeusedasthe"1s"and"0s"inaquantumcomputer.IndeedHakonenandcolleagueshavealreadyusedthisapproachtoreadthevalueofaqubitwithoutchangingitsvalue--whichisalmostalwaysaproblemwhenmeasuringthequantumstateofanysystem.

"Inthefuture,theJosephsoncapacitancecouldbeusedforoperationsinalarge-scalequantumcomputer,"saysMikaSillanpaaofHelsinkiUniversity."TheJosephsoninductanceandJosephsoncapacitancetogetherwouldalsoallowustobuildnewtypesofquantum'bandengineered'electronicdevices,suchaslow-noiseparametricamplifiers."

http:

//physicsweb.org/articles/news/9/11/13/1

CalculatingwithBosecondensates

CouldBoseEinsteincondensatesbeusedascalculators?

TheanswertothisquestioncouldbeyesaccordingtophysicistsinFranceandItaly.YvanCastinandcolleaguesoftheKastlerBrosselLaboratoryinParisandSandroStringariattheUniversityofTrentosaythatcondensatescouldbeusedtoexperimentallyobservetherootsofmathematicalexpressionscalled"randompolynomials"forthefirsttime（Phys.Rev.Lett.96040405）.Theresultsmighthelpimproveourunderstandinginmanyareasofphysics--suchaschaos,forexample--inwhichrandompolynomialsormatricesareoftenapplied

Polynomialsaremathematicalexpressionsinvolvingasumofpowersinoneormorevariablesmultipliedbycoefficients.RandompolynomialsaresimplypolynomialswithrandomcoefficientsthathaveaGaussianorbell-shapedprobabilitydistribution.Solvingtherootsofrandompolynomialsisanimportantfieldintheoreticalphysicsandalthoughsuchpolynomialshavebeenextensivelystudied,noonehaseverseenwhattheseexpressionsmightactuallylooklike.Now,Castinandcolleagueshaveshownthatthelocationof"vortices"inarotating2DBose-Einsteingascouldbeusedtophysicallyrepresenttherootsofapolynomial.

ABose-Einsteincondensate（BEC）isanultra-coldcloudofgasatomsthatareallinthesamequantumstate,andcanthereforebedescribedbythesamewavefunction.Quantumvorticescanforminthesecondensatesiftheyarerapidlyrotated.AccordingtoCastinandco-workers,thewavefunctionofarotatingcondensateformedwithnon-interactingatomscanbedescribedbyarandompolynomial.

Figure1

Avortex"sits"ateachlocationwherethewavefunction"vanishes",anditcanbeassociatedwitharootofthepolynomial.Eachrootisacomplexnumberwitharealandimaginarypart,whichcanbeviewedasthetwospatialcoordinatesofthevortex.Therootsinteractwitheachotherandthisisrepresentedbythevorticesrepellingeachother.

Figure2

TheteamsaysthatthefictitiousBECformedbytherootsisinterestinginthiscontextbecauseitisaveryrareexampleofanexactlysolvablemany-bodyprobleminphysics."Themathematicaltheoryofrandompolynomialsandrandommatriceshasalreadyfoundmanyapplicationsinphysics,"saysteammemberJeanDalibard."IndeedtheHamiltonianofacomplexorchaoticsystemcanoftenbeviewedassuchamatrix."

SohowdidtheFrance-ItalyteamfindtheunlikelyconnectionbetweenrandompolynomialtheoryandBECs?

"Wewereinterestedinthecaseofinteractingatomsinwhichtherootsofthepolynomials--thatisthelocationofthequantizedvortices--formaregulararray,"explainsDalibard."Tooursurprisewenoticedthatevenforstrictlynon-interactingatoms,alocalorderofthevortexdistributionremained."Thisisexactlywhathappensinmathematicalrandompolynomials--althoughthecoefficientsofthepolynomialareindependent,therootdistributionstillexhibitssomecorrelations.

Fluidsmixinreverse

Whenyoustircreamintoyourcupofcoffee,youwouldamazedtoseethetwofluidsreturntowheretheystartedsimplybyreversingthedirectionofstirring.However,ateamofphysicistsintheUSandIsraelhasnowdiscoveredthatsuchmixingcanindeedbereversibleundercertainconditions.Theworkcouldbeimportantformixingprocessesinindustryandbiology（Nature438997）.

DavidPineofNewYorkUniversityandcolleaguesattheHaverfordCollege,theCaliforniaInstituteofTechnologyandtheIsraelInstituteofTechnologystudiedthemotionoftinypolymerbeadssuspendedinaviscousfluidtrappedbetweentwoconcentriccylindersheld2.5millimetresapart.Whentheteamrotatedtheinnercylinderinonedirectionandthenbackagain,theyfoundthatthebeadsreturnedtotheirstartingpositions.Butthebehaviourisonlyseenifthesolutionisrelativelydiluteandthebeadsarestirredforashorttime.Athigherconcentrationandlongertimes,mixingbecomesirreversible.

Accordingtotheresearchers,theobservedbehaviourcanbeexplainedbycollisionsbetweenindividualbeads.Mixingcanbereversediftheparticlesdonotcollidewitheachother,whichisthecaseatlowconcentrations.Butasthesolutionbecomesmoreconcentrated--andmorecollisionsoccur--theprocessbecomesirreversible.

"Theirreversibilityoftheseparticlesmaybeexplainedbytheextremesensitivityoftheirtrajectoriestoimperceptiblysmallchangesoftheparticlepositions,"explainsPine.Suchperturbationsmightarisefromalmostanything–fromsmallimperfectionsintheparticlesorbysmallexternalforces-andaremagnifiedexponentiallybecauseofthemotionofotherparticlessuspendedintheliquid,hesays.Physicalsystemsthatexhibitsuchextremesensitivitytosmallperturbationsaresaidtobe'chaotic',whichmeansthattheirbehaviourcannotbedeterminedinadvance.

TheUS-Israelteamsaysthatanirreversibleflowcouldbetransformedintoareversibleoneatapredictablepointbyreducingthenumberofparticlessincethismakescollisionsbetweentheparticleslesslikely.Thiscouldbeimportantforscalinguplaboratoryexperimentstoindustriallevels,whichisdifficultsimplybecauseoftheunpredictablebehaviouroftheparticlesinvolved.Possibleapplicationsincludemixingofpharmaceuticalsuspensionsandthecatalysisofpetrochemicalsinfluidbeds.Theworkcouldalsohelpinunderstandingparticlemigrationduringceramicprocessingandinthecultureofblood-makingcells.

http:

//physicsweb.org/arti