ExperimentalDemonstrationofCounterfactualQuantumCommunicationUniversityofScienceandTechnologyofChina第十五届全国量子光学会议,2012.07.15KaiChenIn collaboration with Yang Liu, Lei Ju, Xiao-Lei Liang, Shi-Biao Tang, Guo-Liang Shen Tu, Lei Zhou, Cheng-Zhi Peng, Teng-Yun Chen, Zeng-Bing Chen, and Jian-Wei Pan1Motivations and BackgroundCQC SchemeExperimental CQCCounterfactualQuantumCommunicationOutlook2SecureCommunicationAliceBobEve3ClassicalCryptographyModernCryptographyRSA,Rabin,Diffie-Hellman,ElGamaletc.AES,MD5etc.QuantumKeyDistribution(QuantumCryptography)BB84E91ContinuousVariableDPSbased(DifferentialPhaseShiftQuantumKeyDistribution)ClassicalandQuantumCryptography4ApplicationsofCounterfactualQuantumPhenomenaThepresenceofanon-transmittingobjectisascertainedseeminglywithoutinteractingwithit,i.e.,withnophoton absorbed or scatteredbytheobject.Theory:A.C.Elitzur,L.Vaidman,Found.Phys.23,987(1993).Experiment:P.G.Kwiatet al.,Phys.Rev.Lett.83,4725(1999).“Interaction-free”QuantumInterrogationMeasurements5ApplicationsofCounterfactualQuantumPhenomenaCounterfactualquantumcomputationthroughquantuminterrogationTheory:G.Mitchison,R.Jozsa,Proc.R.Soc.Lond.A457,1175(2001).Experiment:O.Hosten,M.T.Rakher,J.T.Barreiro,N.A.Peters,P.G.Kwiat,Nature439,949(2006).Counterfactualcomputationisaccomplishedbyputtingthecomputerinasuperpositionofrunningandnotrunningstates,andtheninterferingthetwohistories.Conditionalontheas-yet-unknownoutcomeofthecomputation,itissometimespossibletocounterfactuallyinferinformationaboutthesolution.6CQC SchemeExperimental CQCCounterfactualQuantumCommunicationOutlookMotivations and Background7Anovelnewscheme!BasedonorthogonalquantumstateInnormalscheme,thesecurityisbasedonnon-orthogonalquantumstates,e.g.,BB84protocolEnabletogeneratekeyswhen“nophoton”travelsInnormalparadigmforcommunication,keyscanonlybegeneratedwhenphotonstravelacrosstwostationsCounterfactualQuantumCommunicationT.G.Noh,Phys.Rev.Lett.103,230501(2009)8T.G.Noh,Phys.Rev.Lett.103,230501(2009)CQCScheme9T.G.Noh,Phys.Rev.Lett.103,230501(2009)CQCSchemeProtocol:1. A triggering single-photon source S, which emits a short optical pulsecontainingasinglephoton.ThepolarizationischosenatrandomtohaveeitherhorizontalpolarizationHrepresentingthebitvalue0,orverticalpolarizationVrepresenting1.2.Bobalsorandomlychoosesoneofthetwopolarizationsrepresentinghisbitvalue.Bobblocksthe opticalpath b ofthe single-photonpulseifthepolarization of the pulse is identical to his polarization. The blocking ofoptical path b in such a polarization-selective way can be suitablyaccomplished,forinstance,usingthesetupdepictedinBobssite.3.Ontheotherhand,ifthesingle-photonpulsehasapolarizationorthogonaltoBobs,itsopticalpathbisnotaffectedbytheSW.Hence,asplitpulsetravellingthroughpathbmaybereflectedbytheFMinBobssiteandisreturnedbacktotheBS.4.Theinterferometercanbestabilizedusingfeedbackcontrol.10CQCSchemeAnalysis:1.IfAlicesandBobsbitvaluesdiffer,thephotonleavestheinterferometergoingtowarddetectorD2withcertaintyowingtotheinterferenceeffect.2.If,however,AlicesandBobsbitvaluesareequal,thesplitpulseinpathbisblockedbydetectorD3andtheinterferenceisdestroyed.Inthiscase,therearethreepossibilitiesforasinglephoton:(i)thephotontravelsthroughpathaandisdetectedatdetectorD1withprobabilityRT;(ii)thephotontravelsthroughpathaandisdetectedatdetectorD2withprobabilityR2;(iii)thephotongoestoBobthroughpathbandisdetectedatdetectorD3withprobabilityT.3.After the detection of a photon is completed, Alice and Bob tell each otherwhetherornoteachofthedetectorsclicked.IfD1clicksalone,Alicecomparesthe detected polarization state to her initial polarization state: if they areconsistent,shedoesnotrevealanyinformationaboutthepolarizationstates;otherwise,shealsoannounceshermeasurementresults.AliceandBobcanthenestablishanidenticalbitstring(asiftedkey)byselectingonlytheeventsforwhich D1 alone detects a photon with a correct final polarization state. Theydisregardallotherevents.4.Monitoring:IfD2orD3clicks,theyalsoannounceboththedetectedpolarizationstate and the initial polarization states that were chosen. This is intended todetect Eves intervention by monitoring the correct operation of theinterferometer.11No-cloning principle of orthogonal states in acomposite system which consists of twosubsystems.Ifreduceddensitymatricesofanavailablesystemarenonorthogonalandiftheothersubsystem is not allowed access, it isimpossible to distinguish two orthogonalquantumstateswithoutdisturbingthem.CQC-whyisitsecure?T.G.Noh,Phys.Rev.Lett.103,230501(2009)12Accordingtothechosenbitvalue,theinitialquantumstateaftertheBSisgivenbyoneofthetwoorthogonalstatesinourCQCschemeQuantumstatesutilized13AsiftedkeyiscreatedbyselectingonlytheeventsduringwhichasinglephotonisdetectedatD1.Inidealcases,thephotonsusedtocreateasiftedkeyhavenottravelledthroughpathbbutonlythroughpatha(ifthephotonshavetraveledthroughthepathb,theymusthavebeendetectedatD3).The task of a secret key distribution, therefore, can beaccomplishedwithoutanyphotoncarryingsecretinformationbeingsentthroughthequantumchannel(pathb). A photon that carries secret information has beenconfinedfromitsbirthtodeathwithinAlicessecurestation,andEvecanneveraccessthephoton.Bob in fact extracts a secret key from the non-detectionevents.CQC-whycounterfactual?14Experimental CQCCQC SchemeCounterfactualQuantumCommunicationOutlookMotivations and Background15Fullimplementationwithtableand1kmfiberspoolExperimentalSetup16Singlemodefiberupto1kmlengthUsingon-shelfcomponentsSimilarconditionwithrealworldenvironmentPolarizationadjustmentiscapableOriginalexperimentsetupandimplementationMonitoralldetecteventsImplementationofexperiment17RandomdatagenerationModulationsequentiallogicMassiveDataandLogicalControlSpecialdesignedFPGA18Fastanalogsignal(fromFPGA)Highspeedswitch(100ns)PropertiesequencesFastPulseSwitchBobneedstochoosefastly“whichpulsetodetect”KeytoenhancesystemworkingfrequencyBasisforsuccessfulrealization19Fiberlengthestimation&adjusting(10cm)Opticaldelayline(0.1mm)Fiberstretcher(0.01m)AccurateOpticalDelayAdjustmentAccurateopticaldelayadjustment(98%)Opticalmisalignmentofabout1%Contributingabout3.8%errorrateDetectoreventsforkeysaremuchsmallerthanthatofothersExtinctionratioofopticalswitch(17dB)Contributingabout2%errorrateDetectordarkcountrate(1e-5)Contributingtoabout0.5%errorrate5.8%total3.8%+2%+0.5%Erroranalysis26OutlookCQC SchemeCounterfactualQuantumCommunicationExperimental CQCMotivations and Background27UnconditionalsecuritywithsinglephotonsourceZ.Q.Yinet al.,PRA82,042335(2010)MethodtoenhanceefficiencyY.Sunet al.,PRA82,052318(2010)AnexperimenttomodifiedschemeM.Renet al.,LaserPhysics21,755(2011)AsimpledesktopexperimentG.Bridaet al.,LaserPhysicsLetters9,247(2012)DirectcounterfactualquantumcommunicationH.Salihet al.,(2012)CurrentStatusofCQC28nNovel scheme of secure key distribution: Counterfactual quantum communication realizationLiu et al., to appear in PRL (2012)RDemonstrations implemented with a desktop setup, and with 1 km fiber cable, respectively.RDevelopments of high-precision active feed-back control technology for maintaining real-time stabilization of Michelson-type interferometer with high visibilities of greater than 98%, which ensures high-speed and steady generation of keysIfAlicesandBobschoicesforbitvaluesdoesnotdestroyinterference,onlydetectorD2willclickduetointerferenceeffect.IfdetectorD1clicks,AlicesandBobschoicesforpolarizationswilldestroyinterference.Particularly,thephotondoesnottravelalongpathb(counterfactual).SummaryandOutlook29Wehaveachievedthefirstfaithfullyproof-in-principledemonstrationsrealizationofcounterfactualquantumcommunication,inwhichprocessinformationcarriersareseeminglynottraveledinthequantumchannel.Fromadesktoptesttoasetupwith1kmfibercable,wehavegivenaconfirminganswerforfeasibilityofCQCOnecaninferthatthemerepossibilityforsignalparticlestobetransmittedissufficienttocreateasecretkey.Weremarkthat,toensuresuchpossibility,partialsignalparticlesstillneedtorandomlytravelalongquantumchannelfordetectionofpossibleeavesdropping.SummaryandOutlook30Suchaimplementationbyexploitingcounterfactualeffecthasrevealednewsurprisingphysicsbehindquantummechanics,inadditiontoexistingexperimentaldemonstrationsof“interaction-free”measurements,andcounterfactualquantumcomputation.Activefeed-backcontroltechnologyformaintainingreal-timestabilizationofMichelson-typeinterferometer.Tailoredoptical,controllingelectronicsdesigns,andspecialopticalswitcharemadeandchosenforattaininghighvisibilities.Long-distancerealizationareaheadwithbyutilizingstate-of-the-arttechnology.OtherCQCschemesdemonstrationsSummaryandOutlook31Thank you!陈凯中国科学技术大学Mobile:Tel.32AcompleteanalysisoftheQKDsecurity,includingvariousexperimentalimperfections,isverychallengingandopenforfuturestudyWeakcoherentpulsesrealizations:Evecannotdeterminethenumberofphotonsineachpulsebecausesheisnotallowedtoaccesspatha.ItisimpossibleforEvetomeasureeventhenumberofphotonstravellingthroughthequantumchannel(pathb),providedthatshedoesnotdisturbthestates.Eveobtainswhich-pathinformationthroughthephotonnumbermeasurementinpathb,andshedestroystheinterference.Hence,Evemaycausedetectionerrors,andshemaybedetectedduetothephotonnumbermeasurementitself.CQCScheme-SecurityAdvantagesT.G.Noh,Phys.Rev.Lett.103,230501(2009)33AcompleteanalysisoftheQKDsecurity,includingvariousexperimentalimperfections,isverychallengingandopenforfuturestudyWeakcoherentpulsesrealizations:Thus,thepresentprotocolisinherentlyrobustagainsttheso-calledphotonnumber-splittingattack.Evecannotsplitaphotonwhenallofthephotonsinthepulsetravelthroughpatha.Thatis,ifallofthephotonsaredetectedatD1aftertravelingthroughpatha,thebitinformationisnotrevealedtoEve,evenwhenamultiphotonpulseisused.Evecannotobtainacopyoftheinitialquantumstateevenwhenshesucceedsinsplittingaphoton.Thatis,Eveonlyobtainsacollapsedstatewheneversheknowsthatshehasaphoton,andsheremainslimitedbytheno-cloningtheorem.CQCScheme-SecurityAdvantagesT.G.Noh,Phys.Rev.Lett.103,230501(2009)34