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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’ybestoredioYoutellwheomhasemittedthephotorethe‘kick’providedbythephotouIfyoudeteoving，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.
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