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1、Entanglement swapping is a unique feature of quantum physics.By entangling twoindependentparties,whichhaveneverinteractedbefore,entanglementswappinghasbeenusedinstudiesofphysicsfoundationssuchasnonlocalityandwaveparticleduality.Itisalsoacentralelementinquantumnetworks,appearingintheformofquantumrela
2、yandquantumrepeater.Theintegrityofanexperimentalrealizationofentanglementswappingisensured only by satisfying the following criteria:proper causal disconnection betweenrelevantevents,andindependentquantumsourceswithoutacommonpast.Driven by the application of future quantum networks,there has been si
3、gnificantprogressinexperimentalentanglementswappingsinceitsfirstexperimentaldemonstration.Quantum interference with independent sources was addressed in a number ofexperimental settings.A quantum relay was simulated with coiled optical fibers in alaboratoryenvironment.Recently,entanglementswappingan
4、dquantumteleportationwererealizedinbothfree-spaceandopticalfiberlinksoveradistanceofabout100km,inwhichthequantumsourcessharedacommonpast,theBellstatemeasurements(BSMs)wereperformed locally,and the swapped(teleported)photonic qubits were sent over a longdistanceafterwardsforanalysis.Morerecently,seve
5、ralteamssucceededinentanglementswappingandteleportationwithhighintegrityoverafibernetwork(afewtensofkilometers)intherealworld,inwhichtheyovercamethechallengeinremovingthedistinguishabilitybetweenphotonsfromseparatequantumsourcesbydefeatingthenoiseintherealworld.Todate,thereisnoreportonentanglementsw
6、appingwithindependentsourcesoveropticalfiber of 100 km nor with suspended optical fiber,which is more susceptible to theenvironmentbutunavoidableforapplicationsinopticalfibernetworks.Here,wepresentanimplementationofentanglementswappinginanintercityquantumnetwork,whichiscomposedofabout77kmofopticalfi
7、berinsidethelab,25kmofopticalfiber outside the lab but kept underground,and 1 km of optical fiber suspended in airoutsidethelabtoaccountforvarioustypesofnoisemechanismsintherealworld.2.ExperimentalImplementation4.ConclusionsEntanglementswappingover100kmopticalfiberwithindependententangledphoton-pair
8、sourcesQI-CHAOSUN1,2,YANG-FANJIANG1,2,YA-LIMAO1,2,.&QIANGZHANG1,2,ANDJIAN-WEIPAN1,2(1.NationalLaboratoryforPhysicalSciencesatMicroscaleandDepartmentofModernPhysics,ShanghaiBranch,UniversityofScienceandTechnologyofChina,Hefei,Anhui2300262.CASCenterforExcellenceandSynergeticInnovationCenterinQuantumIn
9、formationandQuantumPhysics,ShanghaiBranch,UniversityofScienceandTechnologyofChina,Hefei,Anhui230026,Chinae-mail:&;Theseauthorscontributedequallytothiswork)Aschematicofthesequentialtime-binentangledphoton-pairsourceisdepictedinFig.1(a).Wecarvethecontinuous-wave(CW)outputofadistributedfeedbackperiodic
10、allyintopulses atarateof1GHzwithanelectro-optical modulator(EOM).Thesequential laserpulsesdifferby1ns.Afteramplificationwithanerbium-dopedfiberamplifierandspectralFig.1Schemeoftheentanglementswappingexperiment.filteringwithdensewavelength-divisionmultiplexingfilters(DWDMs),thelaserpulses(withaspectr
11、umbandwidthofabout8GHzandpulsedurationofabout(60ps)arefedintoa300mdispersionshiftedfiberimmersedinliquidnitrogentoproducephotonpairsviaspontaneousfour-wavemixing.Thequantumstateofaphotonpairproducedisgivenbyrepresentstimebinwherekandiandsareidler(1555.73nm)andsignal(1549.36nm)photons,respectively.As
12、chematicoftheentanglementswappingexperimentisshowninFig.1(b),with(out)the gray shaded areas indicating the indoor(outdoor)environment.The two quantumsourcesareplaced12.5kmapartatnodesAliceandBob,andBSMisperformedatthethirdnode(Charlie)betweenthem,asshowninFig.1(e).Tosynchronizetheoperationofeachnode
13、inthequantumnetwork,Charliekeepsamasterclock,whichsends75pslaserpulsesto Alice and Bob at 1GHz,which is used to drive the EOM to synchronize independentquantumsources,seeFigs.1(c)and1(d),respectively.Fig.2Typicaldelaycompensation(blueline)andrelativedelaybetweenthearrivaltimeofphotonsfromAliceandBob
14、(redline)underdifferentweatherconditions.MeasuredbyaTDCwithtimeresolutionof4ps,thestandarddeviationsoftherelativedelayin(a),(b),and(c)are6.7ps,6.0ps,and6.5ps,respectively.Figure3showsthatthemeasuredtwo-foldcoincidencecountsbetweenAlice(Bob)andCharlie change sinusoidally as functions of phase(which i
15、s dialed by sweeping thetemperatureoftheMZI),withvisibilityof(89.8-0.5)%forAlicessourceand(82.91.2)%forBobssource.Notethatthedevicetemperatureismaintainedwithstabilityofabout0.005Cand no accidental coincidence counts are subtracted.Two-fold coincidence counts of thesequentialtime-binentangledphotonp
16、airsdistributedby(a)Aliceand(b)BobasfunctionsofthetemperatureoftheMZIs.Fig.3Two-foldcoincidencecountsofthesequentialtime-binentangledphotonpairsdistributedby(a)Aliceand(b)BobasfunctionsofthetemperatureoftheMZIs.To verify the entanglement swapping,both Alice and Bob use a MZI with a path differenceof
17、 1 ns followed by SNSPDs and implement a conditioned Franson-type measurement for a time-binentangledstate.TheexperimentalresultwithoutsubtractingaccidentalcoincidencecountsisshowninFig.4.Fig.4Four-foldcoincidencecountprobabilitiesasfunctionsofthetemperatureofAlicesMZI.Theerrorbarsindicateonestandar
18、ddeviationcalculatedfromthemeasuredcountsassumingPoissoniandetectionstatistics.Eachdatapointiscalculatedbyusing100four-foldcoincidencecounts.Thevisibilityofthe fitted curve is(74.88.7)%and(71.7 7.2)%for the measured results with 20.175 and 20,respectively.In summary,we have demonstrated entanglement
19、 swapping with two independentsources12.5kmapartusingopticalfiberof103km.Comparedwithpreviousexperimentswithindependentsources,wehaveincreasedthelengthoftheopticalfiberfrommetropolitandistancetointercitydistance.Specifically,suspendedopticalfiberisusedinourexperiment.Thetransmissionlossandstabilityo
20、ftheopticalfiberchannelinourexperimentareenoughto match those of typical underground deployed optical fiber of more than 100 km.Toincreasetheeventrate,weupdatedthesourcesto1GHzsequential time-binentangledphoton-pair sources.In addition,we improve the polarization and delay compensation system.Ourres
21、ultsshowthatrealizingentanglementswappingbetweentwocitiesistechnicallyfeasible,even if more suspended fiber is used(see Supplement 1 for more discussion).Moreover,theconfigurationinourexperimentallowsaspace-likeseparationbetweenanytwomeasurementsofthoseperformedinthethreenodes,andvarioustime-spacere
22、lationscanbeachievedbycombiningbothacoiledopticalfiberandadeployedopticalfiber.Thisdistinguishingfeaturetogetherwiththeindependentsourcesmakesoursetupapromisingplatformformanyinterestingfundamentaltests.1.Introduction3.ExperimentalResultsIntheBSM,itiscriticalthatthesignalphotonssentbyAliceandBobarri
23、veata5050beam splitter(BS)simultaneously.However,the arrival time changes drastically due tofluctuationoftheeffectivelengthoftheopticalfiberlink.AsshowninFig.2,thetypicalchangesinpeak-to-peakrelativeoftherealworld.AsshowninFig.2,thetypicalchangesinpeak-to-peakrelativedelaysbetweenthearrivaltimesofthephotonsfromAliceandBobare200ps,500ps,and1000psonrainydays,cloudydays,andsunnydays,respectively,whicharemuchlargerthanthecoherenttimeofthesignalphotons(110ps).Optica4.10(2017):1214-1218