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电子科技大学图书馆

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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

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（028）832023402006年3月10日出版

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

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.

Avortex"

sits"

ateachlocationwherethewavefunction"

vanishes"

anditcanbeassociatedwitharootofthepolynomial.Eachrootisacomplexnumberwitharealandimaginarypart,whichcanbeviewedasthetwospatialcoordinatesofthevortex.Therootsinteractwitheachotherandthisisrepresentedbythevorticesrepellingeachother.

TheteamsaysthatthefictitiousBECformedbytherootsisinterestinginthiscontextbecauseitisaveryrareexampleofanexactlysolvablemany-bodyprobleminphysics."

Themathematicaltheoryofrandompolynomialsandrandommatriceshasalreadyfoundmanyapplicationsinphysics,"

saysteammemberJeanDalibard."

IndeedtheHamiltonianofacomplexorchaoticsystemcanoftenbeviewedassuchamatrix."

SohowdidtheFrance-ItalyteamfindtheunlikelyconnectionbetweenrandompolynomialtheoryandBECs?

"

Wewereinterestedinthecaseofinteractingatomsinwhichtherootsofthepolynomials--thatisthelocationofthequantizedvortices--formaregulararray,"

explainsDalibard."

Tooursurprisewenoticedthatevenforstrictlynon-interactingatoms,alocalorderofthevortexdistributionremained."

Thisisexactlywhathappensinmathematicalrandompolynomials--althoughthecoefficientsofthepolynomialareindependent,therootdistributionstillexhibitssomecorrelations.

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.

//physicsweb.org/arti