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1、PHYSICALREVIEWLETTERSweekendingVOLUME92,NUMBER1914MAY2004MeasuringaPhotonicQubitwithoutDestroyingItG.J.Pryde,J.L.OBrien,A.G.White,S.D.Bartlett,andT.C.RalphCentreforQuantumComputerTechnology,DepartmentofPhysics,UniversityofQueensland,Brisbane4072,Australia(Received5December2003;published12May2004)Me
2、asuringthepolarizationofasinglephotontypicallyresultsinitsdestruction.Wepropose,demonstrate,andcompletelycharacterizeaquantumnondemolition(QND)schemeforrealizingsuchameasurementnondestructively.Thisschemeusesonlylinearopticsandphotodetectionofancillarymodestoinduceastrongnonlinearityatthesingle-pho
3、tonlevel,nondeterministically.WevarythisQNDmeasurementcontinuouslyintotheweakregimeanduseittoperformanondestructivetestofcomplementarityinquantummechanics.Ourschemerealizesthemostadvancedgeneralmeasurementofaqubittodate:itisnondestructive,canbemadeinanybasis,andwitharbitrarystrength.DOI:10.1103/Phy
4、sRevLett.92.190402PACSnumbers:03.65.Ta,03.67.a,42.50.XaAttheheartofquantummechanicsistheprinciplethatHerewepropose,demonstrate,andcompletelycharac-theveryactofmeasuringasystemdisturbsit.AquantumterizeaschemefortheQNDmeasurementofthepolar-nondemolition(QND)schemeseekstomakeameasure-izationofafree-pr
5、opagatingsingle-photonqubitamentsuchthatthisinherentbackactionfeedsonlyintoflyingqubit.Thisisachievednondeterministicallybyunwantedobservables[1,2].Suchameasurementshouldusingameasurementinducednonlinearity.Theschemesatisfythefollowingcriteria[3]:(i)Themeasurementisheraldedbutrequiresphotonnumberres
6、olvingdetec-outcomeiscorrelatedwiththeinput,(ii)themeasurementtors.Ourdemonstrationusesstandarddetectorsandthere-doesnotalterthevalueofthemeasuredobservable,andforerequiressimultaneousdetectionoftwophotons.The(iii)repeatedmeasurementyieldsthesameresultmeasurementcanbeperformedonallpossibleinputquan
7、tumstatepreparation(QSP).Originallyproposedstates.Eigenstateinputsresultinstrongcorrelationwithforgravitywavedetectors,mostprogressinQNDhasthemeasurementoutcome;coherentsuperpositionsex-beeninthecontinuousv
