PicoQuant MultiHarp 150 - Page 43

PicoQuantGmbH MultiHarpSoftwareV.3.1.0.0
lierpairinCH1(i–nogreenloop)wouldfalselybealignedwithaphotonfromalaterpairinCH2(ii).Counting
thiseventasacoincidencewouldnotallowtorevealthetruecorrelations.Thisexamplesillustratestheadvan-
tageofmodernTCSPCsystems:whenusingahardwarecoincidencelogicanalyzerthewholedatasetofthe
measurementwouldbelostinthecaseofawrongdelayalignment.WithmoderntimetaggingTCSPCsystems
thecorrectcorrelationscanbeeasilyrecoveredbyadjustingΔtduringdataanalysis.
However,coincidencelogicanalyzersalsohavetheiruniqueadvantage.Inthesketchedquantumopticsappli-
cationscenario,thecorrelationsofinterestonlyexistbetweenpairsorhighernumbersofphotons.Oftenlower-
orderevents,e.g.,singleeventsinthecaseoftwo-foldcoincidencescontainlittleinformationfortheseexperi-
ments.Coincidencelogicanalyzersalreadydiscardtheselower-ordereventsandhencereducetherecorded
datasize,oftendrastically.Thisdoesnotonlymakedataanalysislesscumbersomeandfilesizesmaller,but
alsoallowsforhigherdataratesforthesignalsofinterest,asthebusinterfacetothehostPCisnotburdened
withunnecessarydata.Hence,datacanbeprocessedanddisplayedonahostPCinrealtime.
TheeventfilterprovidedbythemostrecentMultiHarpgatewarecombinestheadvantagesofTCPSCdevices
withtheadvantagesoftraditionalcoincidencelogicanalyzers.Inordertomakethefilterbeneficialforawide
rangeofquantumopticalapplicationscenariositisdesignedtooperateonallinputchannelsoftheMultiHarpin
useandhencethefullsetoftheirpossiblecombinations.Traditionalcoincidencelogicanalyzersoftenpredefine
logicalcombinationsbetweenchannels(coincidencepatterns)toberecorded,whileallothereventsaredis-
carded.Thismaylimitapplicationsasthesetofpredefinedpatternsisusuallyhardwarelimited.TheMultiHarp
eventfilterstrivestoresolvethisbydefiningdiscardableordersofevents(e.g.singleevents)andkeepingwhat
maybeofinterest.Thisreducesthedataloadsufficientlyforlosslessbustransferandreasonablefilesize.
Computinglogicalcombinationsbetweenchannelsintheremainingdatacanthenbeeasilydoneonthehost
PCatvirtuallyunlimitedcomplexity.
Inthesimplestsetting,thefilterdiscardsalleventsthatdonothavepartners(inanychannel)withinthecoinci-
dencewindowofprogrammablewidth.Thisalreadypermitsasignificantdatareductionwithoutlossofgeneral-
ityforanysubsequentsoftwareanalysis.Forfurtherreductiontheorderoffiltering(thenumberofpartners)can
besettohighernumberbywayofaparameter“matchcount”,sothatonly2-fold,3-fold,…,upto6-foldcoinci-
dencesarerecorded.
Incontrasttomanytraditionalcoincidencelogicanalyzersandmostofthecurrenttimetaggingsystems,allraw
eventsofthecontributingchannelspassingthefilterarefullyrecordedwiththeirtimetag.Theircompleteinfor-
mationcontentisthereforeavailableforrefinedanalyisinsubsequentsoftwareprocessing.Settingthetimewin-
dow(setbythefilterparameter‘range’)canthereforenotonlyactasanimmediatecoincidencetimewindow,
butinitiallycanalsobesetbroaderforafirstmilddatareduction.Asalleventsarepreciselytimetagged,anar-
rowercoincidencetimewindow(ormultiplesuchwindows)canalwaysbeappliedinpost-processing.The
“range”settinguptoacertaindegreetuneshowclosethesystemwillbehavetoaplainTCSPCortimetagger
withouteventfilterand,intheopposite,ageneralizedcoincidencelogicanalyzer.Note,thatforthehardware
eventfiltertoworkproperlythechannelsneedtobetemporallyaligned.Thisiseasilyachievedbymeansofthe
programmableoffsetineachoftheMultiHarp‘sinputchannels.
Inadditiontothefilterfunctionalityoutlinedsofar,alleventtimingchannelscanoptionallybemarkedas‘pass
channels’thatwillbepassedthroughthefilterunconditionally.Thiscanbeveryusefultoprovidereference
channels,e.g.,ifthechannelcountscontaininformationtotrackdriftofthesetupinthesamemeasurement.
Asafurthermeansofflexibilityandfortestpurposesthefilterprovidesanoptional‘inverted’setting.Inthatcase
thefilterdiscardsalleventsthatdohavepartnersaccordingto“matchcount”,e.g.,eventsthathave2,3,..,6
partnersarethendiscarded.
Tosummarize:Inmanyquantumopticalexperimentsitistypicallyoflittleinteresttocollectphotoneventsthat
occurwithoutoneormorephotonsinclosetemporalcontext.Recordingsuchsuperfluouseventsdoesnoharm
totheexperimentalobjectivesastheycanbefilteredoutinsoftwarepost-processing.However,itunnecessarily
consumesbusbandwidthandbloatstherecordeddatafiles.TheMultiHarpinparticularhasanextremelyhigh
inputbandwidth:eachchannelcaninprinciplecaptureeventsatupto1.5GHz.DespiteasophisticatedFIFO
bufferingscheme(seesection5.3.2)combinedovertheinstrument’smanychannelsthishighratecannotbe
sustainedforlongininternalprocessingandUSBtransfer.Itisthereforeofinteresttoreducethedatarateatan
earlystagedirectlyinthehardware.ThisobjectiveisachievedbytheMultiHarp‘seventfilteringmechanism
availableasofsoftwareversion3.1.0.0andassociatedgateware.Thelattercanbefield-upgradedfortheMulti-
Harp150PandMultiHarp160models.PleasecontactPicoQuantSalesorSupport.Notethatthebudgetmod-
elsMultiHarp150Ncannotbeupgraded.
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