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1、Physics7,6(2014)ViewpointPurePhotonsforQuantumCommunicationsMonikaSchleier-SmithDepartmentofPhysics,StanfordUniversity,Stanford,CA94305,USAHarukaTanji-SuzukiPhotonScienceCenter,TheUniversityofTokyo,Tokyo113-8656,JapanPublishedJanuary21,2014Researcherscharacteri
2、zeasingle-photonsource,provingitemitsnonclassicalstatesoflightthatmaybeusefulforquantumcommunicationschemes.SubjectAreas:AtomicandMolecularPhysics,QuantumInformation,QuantumPhysicsAViewpointon:HomodyneTomographyofaSinglePhotonRetrievedonDemandfromaCavity-Enhanc
3、edColdAtomMemoryErwanBimbard,RajivBoddeda,NicolasVitrant,AndreyGrankin,ValentinaParigi,JovicaStanojevic,AlexeiOur-joumtsev,andPhilippeGrangierPhys.Rev.Lett.112,2014–PublishedJanuary21,2014Thevisionariesofquantuminformationforeseeaneraarbitraryprecision.Conseque
4、ntly,itisnotalwayspossi-whereilliciteavesdroppingonfiber-opticcommunica-bletowritedownajointprobabilitydistributionP(x,p)tionswillbeprohibitedbythefundamentallawsofquan-formeasurementsofxandp.Theclosestanalogal-tummechanics.Ifeachqubitofinformationisconveyedlowe
5、dbyquantummechanicsistheso-called“Wignerbyasingle,indivisiblephoton,thenitcannotbeinter-function”W(x,p),definedsothatintegratingoverallceptedwithoutattractingthenoticeoftheintendedre-possiblevaluesofpyieldstheprobabilitydistributionforcipient.Yetinpractice,photo
6、nsdonotordinarilytravelmeasurementoutcomesofxandviceversa.ThisWigneralone:alaserpulsecontainingonaverageonephoton“quasiprobability,”unlikeatrueprobabilitydistribution,sometimescontainstwoandoftencontainsnone.Hencecantakeonnegativevalues—astrikingsignatureofnon-
7、considerableefforthasbeendirectedtowardsdevelop-classicality.Bimbardetal.showthattheoutputoftheiringdevicescapableofproducingpreciselyonephotonsingle-photonsourceexhibitsnegativityoftheWigneratthepushofabutton.Typically,theperformanceoffunctionandthereforedefiesd
8、escriptionbyanyclassicalsingle-photonsourcesisbenchmarkedbythesuppressionmodel.oftwo-photonevents.Butamorecompletecharacter-Whysubjectasingle-photonsource—adevicede-izationo
