We demonstrate a new technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time resolved single photon detection by femtosecond upconversion. We measure for the first time the joint temporal density o

FIG.1:(Coloronline)(a)Synchronizedupconversionanddownconversionexperimentdrivenbythesameultrafastpump.(b)Noncollinearphase-matchinggeometryforsingle-photonupconversion.IF:interference lter;DM:dichroicmirror;FPBS: berpolarizingbeamsplitter.

photon

upconversiontechniquewithatemporalresolu-tionof～150fs[11].Anultrafastupconvertingpumppulsewasusedtotime-stampthesignalandidlerar-rivaltimes,andwemappedtheirrelativearrivaltimesbyvaryingtheinputdelaylinesindependentlyandrecordingthecoincidencesbetweenthetwoupconversionchannels.Thecoincidencestatisticsyieldedthetemporalstructureofthetwo-photonstate.

Ourexperimentalsetupforultrafasttype-IIphase-matchedSPDCandsubsequentJTDmeasurementwithtime-resolvedupconversionisshowninFig.1(a).BothSPDCandupconversionwerepumpedsynchronouslywiththesameultrafastsourceat790nmwitha6-nmbandwidthand80MHzrepetitionrate,therebyelimi-natingthepumptimingjitterfortheJTDmeasurement.WeoperatedthePPKTPSPDCcrystalunderextendedphase-matchingconditionstogenerateacoincident-frequencyentangledtwo-photonstate[3,4].ByFourierduality,thispositivefrequencycorrelationcorrespondedtoanti-correlationinthetimedomainwherethesignalandidlerphotonswith～350-fssingle-photoncoherencetimesweresymmetricallylocatedaboutthecenterofa～1.4-pstwo-photoncoherencetimewindow,asmeasuredbyHOMinterference[4].Thesignalandidlerphotonswerecoupledintoapolarization-maintainingsingle-mode berandseparatedata berpolarizingbeamsplitter.Thesignalandidlerdelaylineswereindividuallyad-justedsothattheyarriveattheupconversioncrystalinthesametimeslotasthepumppulse.Finetuningoftherelativetimingcanbeachievedwithtranslationstages.WeusedthesamesetupasinRef.[11]fortime-resolvedsingle-photonupconversion,brie ydescribedhere.AssketchedinFig.1(b),a1-mmlongperiodicallypoledMgO-dopedstoichiometriclithiumtantalate(PP-MgSLT)crystalwitha8.5µmgratingperiodwasusedfornoncollineartype-0phase-matchedsum-frequencygen-eration(1580nm+790nm→526.7nm).Weusedthenoncollineargeometrytoimplementtwoindependentup-

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converterswithasinglecrystal.Thesingle-photonbeamswerealignedparalleltothepumpbeamwith～3mmlat-eraland～1.5mmverticalseparationfromthepumpaxis,andtheywerefocusedintothePPMgSLTcrystal.Thenon-planarfocusingcon gurationallowedustoavoidthesimultaneousdetectionofthenon-phase-matchedpara-metricphotonpairsthatwerebothgeneratedandupcon-vertedbythepumpatthePPMgSLTcrystal.Therefore,evenwitha nitebackgroundforsingles,thecoincidencepro leshowsnegligibleaccidentals[11].Theupconvertedoutputswere lteredbydichroicmirrorsand10-nmpass-bandinterference lters,coupledintosingle-mode bersanddetectedwith ber-coupledSiAPDs.Werecordedthesinglescountsandalsothecoincidencecountsbe-tweenthetwoSiAPDswithina1.8nscoincidencewin-dow.

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tnuoC dezialmroN-2000-1000010002000

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FIG.2:(Coloronline)Normalizedsingles(a)andcoincidence

(b)histogramsbytime-resolvedupconversion.Thepumppulsewasscannedthroughcollocatedsignalandidlerarrivalwindows.SolidlinesareGaussian tstothedata.

Wemeasuredthesinglesandcoincidencesbyscan-ningtheupconversionpumppulsedelayrelativetothesignalandidlerarrivalwindows,andeachdatapointwasaveragedfor60seconds.ThenormalizedhistogramsareplottedinFig.2withoutanybackgroundsubtrac-tion.Fortheoptimalpumppowerratio(～360mWfordownconversion,～580mWforupconversion)themaxi-mumsingles(coincidence)rateatthecenterofthedistri-butionwas～5300/s(～17/s),includingthebackground.Thebackgroundlevelinsinglescountswere～1900/sfortheoptimalpumppower-ratio,correspondingtoaback-groundprobabilityperpulseof～2.4×10 5.Thetem-poralwidthforsinglesdistributionwas～1.3ps,consis-tentwiththetwo-photoncoherencetimeof～1.4ps[4].Duetothetimeanti-correlatedgenerationofsignalandidler,thecoincidencepro leexhibiteda～165fsFWHMwidth,whichissigni cantlynarrowerthanthesingles