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1、Physics6,133(2013)ViewpointWhatCanweSayaboutaPhoton’sPast?JeffLundeenDepartmentofPhysics,UniversityofOttawa,MacDonaldHall,150LouisPasteurRoad,Ottawa,OntarioK1N6N5,CanadaPublishedDecember9,2013Anexperimentdemonstratesthatevenwhenphysiciststhinkaquantumparticl
2、ehasfollowedasinglepathitmightnothave.SubjectAreas:Optics,QuantumPhysicsAViewpointon:AskingPhotonsWhereTheyHaveBeenA.Danan,D.Farfurnik,S.Bar-Ad,andL.VaidmanPhys.Rev.Lett.111,2013–PublishedDecember9,2013Quantumtheoryprovidesthefoundationformuchoftheroutethro
3、ughtheupperslit.Thatis,itappearsthatthetechnologyofmodernsociety.Andyet,physiciststhereisnoconfusioninthiscase:thephotonhadasinglearestillnotsurehowtothinkaboutsomeofitsunder-trajectoryasitshistory.lyingconcepts.OneoftheseisthehistoryofaquantumTheTel-Avivex
4、perimentcallseventhisseeminglysafeparticle:whenwemeasureanelectronorphotoninaassertionintoquestion.Dananetal.useaMach-Zehnderdetector,wecan’tbesureofwhattrajectoryittooktointerferometer(MZI),inwhichthetwooutputportsofagettherefromthesource.Whereasaclassical
5、particlebeamsplitter—thereflectionpathandthetransmissiontravelingbetweentwopointsalwayshasasingletrajec-path—taketheplaceofthetwoslits(Fig.1,bottom).toryasitshistory(thinkofacannonball),thehistoryThesetwooutputsleadtoanupperandlowerarm,whichofaquantumparticl
6、earrivingatadetectorismadeuprecombinethephotonsatasecondbeamsplitter.Theyofeverypathitcouldhavetakenfromwhereitstarted.setupthepathlengthsofthetwoarmssothattheirout-WritinginPhysicalReviewLetters,ArielDananandcol-putsinterfereconstructively,ensuringthatallp
7、hotonsleaguesatTel-AvivUniversity,Israel,presentamodernexitthesecondbeamsplitterinthesamedirection.In-versionofthefamousdouble-slitexperimentwithlightsteadofasking“whichslit?”thephotontravelsthrough,thatunderscoreshowdifficultitistoknowaquantumtheauthorsask“w
8、hicharm?”particle’spast[1].Theyshowthatevenwhentheirex-perimentblockslightfromtakingcertainpathstoade-Theauthorsprobewherethephotonswereintwosit-tector,theseforbiddenpathsstillappeartobepartofuations:w
