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Chapter 8 Quantum light（第1页）

Chapter8Quantumlight

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InChapter1，Iiheideathatlightcouldbestruedasastreamofparticles，hotons’forsoutthatthesearerealparticles，roduced，playedwith，measured，stored，andusedfthings.However，eventhoughphotonsareihesimplestexpressionoflight，makingindividualphotonsisnotsosimple.Mostlightseofadifferentkind，forwhiumberofphotonsisnotfixed.

Alightbulb，forinstance，producesastreamofphotonsthatsprayseverywhere.Ifyoulookedatthelightgoingiionfromthebulb，andtheashorttemporalseofthebeam—atimeslot，ifyoulike—theoephotonsinthatslot.Butifyourepeatedtheexperimeimes，you’dfindthattheonswasraimeslargeaimessmall.Theaverageonswouldbefixed，dependihebulb，butyou’dosaywithtyhohotonsyouwouldmeasureiagive’soeristicsof‘classicallight’—lightthatbedestirelyintermsof>

&isalsoofthiskind.Theaverageonsinapulsehtbelarge，butfiveualonswillbebiggerorsmallerthahespreadofphotonnumbersinapulseisapproximatelythesquarerootoftheaveragehattherelative‘ionintheonsineachpulseparedtothemeannumberoverallpulses—getssmallerthehighertheaverageons.

Thusalaserbeamhasintriyhissetsalimitoyofimagesyougetwithlaserillumiionsiedetegtheseparationoftwopointsinanimageisimprefactitisveryimpreciseforlow-iylight，wherethemeanphotonnumberissmall（sotheobjectishardtosee）aioninphotonnumberfromframetoframeislarge.Theonlywaytogetprecisemeasurementsister，thusiheonsillumi，aheresultsovermanylaserpulses.Therelativeiynoiseisreducedbythissignalaveragioabetter-resolvedimage.Thepreproportiontothesquarerootoftheohisiscalledthe‘standardquantumlimit’，sinoclassicallightbeambeatit.

Quantumlight，oherhand，allowsyoutoachievemuchbetterresultsinsignalaveragingforthesameaverageoumlighthavemuoisethananyclassicallight.Butfirst，youhavetobuildaquantumlightsource.Therearemanykindsofsuchasource，eachprodugadistinlight.Butwemightsider，tobecrete，aseheprimitivequa—aphoton.

Sions

Sohowakejustasingle，individualphoton?There’saverypracticalsveoFris1965.Hisideale.Takeasiomandputitiate（see

Chapter5foradisofhowtodothis）.Thenwaitforittodroptoitsgroudoes，itemitsjustoon，sine‘quantum’ybestorediom.Youtellwheomhasemittedthephotorethe‘kick’providedbythephotoum.Ifyoudeteoving，youdetermihatthesionisonitswayaioninwhichitisgoing.

Somemodernquantumlightsourcesoperateinasimilarwaytothis，onlytheycorraltheatombetweentwomirrors（anoptical‘cavity’similartothatofalaser），averyquicklysothatitemitspreferentiallyiiohecavityaxis.Thismakesareliablesourglephotons.Itisanespecially‘lownoise’sourcethephotoedwithstrictregularity.Ifyoulookedatagiveinsuchabeam，you’dbeabletopredictwithtyhohotonswouldbeinit—justoheiyisexallystable—itisa‘quiet’lightbeam，intrasttothe‘noisy’classie.

Theideaisalsousediumlightsourparticular，youstructaverysimplelightsicaleffects.Specifically，therearecrystalsthatephotonwithhigheobesplitintotwophotonswithly，eachabouthalfinalinputphoton.Theprobabilitythatthisfissiontakesplaceisrathersmall，formostmaterials.Butsionsareprodupairs，youeasa‘herald’，tosignalthepreseher（Figure34）.Suchlightsourcesaretheworkhorseofthefieldofquantumoptics，whichusesthequantummeicalfeaturesoflighttoexplorethefoundationsofquantumphysics，aswellastoenablenewkindsofinformationteologies.

Justasclassicaleleagicolarized，sos.Sowemightfiicallypolarized（V）photonorahorizontallypolarized（H）photon.Thesewouldbehavejustlikewaves，inthatifwemeasuredthepolarizatioopassedthroughapolarizerorientedhorizontally，thehattheH-photonalassedthroughaonnever.

34.A‘heralded’sionlightseingphotonsrandomly，butwithasignalthatindionehasbeenprepared.

What’sstrawestructadiagonallypolarizedphoton，osgwiththefieldat45degreestoboththehorizontalaical.Butifwenowtrytoseeifthephotohroughthehorizontalpolarizer，thenthereisanambiguity.Thephoto‘piece’oflight，so’tbedividedfurther.Howshoulditbehaveatthepolarizer?ensisthatitistrahaprobabilityofone-half，ahequalprobability（illustratedinFigure35）.

Ihatimpliesisthatifyoutrytheexperimentofputtingadiagonallypolarized（D）sionintoahorizontallyorientedpolarizeramilliohen500，000timesitwillgh.Arahingaboutquantummeicsisthatyouottellorialwhaten.ThisishephotonsideredsometimestobeH-polarizedaimesV-polarized.RatheritisbecausethephotonisbothHandV-polarized，simultaherandomouteasuremeon’spolarizationthereforerevealtheintrihatinhabitsthemostfualleveloftheuniverseasdescribedbyquantumphysics.

35.Adiagonallypolarizedphotoersapolarizer，asrandhoortheother.

Ofakeavirtueoutofyinsuchces.Youdopractigswithsionsthatareunimagihht.Forihispropertyofphotoogeerandomnumbers，bymeasurihephotonistransmitted（labellie，say，1）orreflected（labelled0）.Therahestringofzeroesandonesisiheunderlyingphysiotjustiureandgofdice，ehisreasonquantumraorsareanemergingbusiness—you’tfaketheraheyprovide.

Aseple:youmakeunislinksforwhichthesecurityisguarahelawsofphysics，ratherthanbytrustingyourtelessupplier.Thisisbeportaiesofphotons.First，youotdetetwoplace.Forthatreason，ifaneavesdrrabsthephotontocapturetheinformati，thenofcourseyoudohephoton.Soyoureation，ahatsomething’swrong.

Butiftheeavesdropperisclever，shewillseonthatshehopeswillfakethemessage.Butyoutellthatit’safake!Thereasonyouowthisisthatinquahereistellyaboutasiumparticle.

siderthefollowingsario.Youwanttosendasimplebinarymessage（0sahislink，sayaverticallypolarizedphotonfor0andadiagonallypolarizedphotonfor1.Iftheeavesdropper（usuallyknownasEve）measuresthephotoheaicallypolarized’，sheotbethatthephotonwasa0，sihediagonallypolarizedphotonwouldgiveherthesamealeasthalfthetime.Soshegetssomeinformation，buthing.

hesehemessage（allycalledAlice—you，thereceiver，areBob）sendsyouaphotoncodedas1.Let’ssayEvemeasuresthisiitatiosapositiveresult.Shemustchoosewhethertosendyouaverticallyonallypolarizedphotoegyistosendyouaverticallypolarizedphotohemostlikelysourceofherresult.hediagonalpolarization.IfyourphotonisfromEve，itwillgiveyresult50pere.IfitisfromAlice，youwillhewro.SobypariionofthereceivedmessagewithwhatAlit，youtellifEveistamperingwithyourline.

Hhtbeevencleverer.ShemaytrytocopythephotonfromAlieasuringit.Sheaketwofagyinal.Thenshemakeaverticalpolarizatioononedadiagonalpolarizatiohesedshewouldhavedetermihefullinformationaboutthephoton‘bit’thatAlityouwithouty.However，shewouldbethwarted.Aremarkablefeatureofquantummeicsisthatthereisnopossibilityofbuildingaaethatakeareplica，ore，ofasiiunknownquaissimplyforbiddenbythelawsofphysics.Becauseofthesetosedbyphysient’and‘nog’—itispossibletobuildaseunislinkthatsmitasecretstreamofrandombitsbetweenAlidyou.