Dickinson College Single Photon Quantum Mechanics Cheat Sheet

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1 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet The following document attempts to highlight the essential points for (a) using a blue laser to achieve spontaneous parametric down conversion (SPDC), (b) detect SPDC in a Grangier type experiment, and (c) perform single photon interference in a Mach Zehnder interferometer. This documentisnotintendedasadetailedstep by stepguidetotheentireprocess,butinsteadtoserveas areminderoftheimportantstepswhenaligningtheexperimentsaswedoatdickinsoncollege. Moredetailscanbefoundintheupper levellaboratorymanualsofmarkbeck(whitmancollege) andkikogalvez(colgateuniversity).notethatourapproachismoresimilartowhitman sfortheinitial experiments(single photonproductionanddetection),butclosertocolgate sforthelaterexperiments (single photoninterferenceandthequantumeraser). WhitmanSiteandLabManual: ColgateSiteandLabManual: %20Manual2010.pdf BlueBeamAlignment 1. Ensurethebluelaserbeamisleveltothetableandalignedalongtherowsofholesinthetableby adjustingthetwoblueturningmirrors. a. Useasingleiristodefinethebeamheight.Theiriscanbepushedflushwithtwopairsof screwsatdifferentlocationsontabletosetalignmenttoholes.(seegalvezp.13fortheir descriptionofthisprocess.)donotadjustheightofirisaftercompletingthisstep. b. Insertand dog down anotheririsfardownstreamandcenteredonbluebeam.thisiris definesthebluepathincaseyouneedtostartover. 2. Insertthebluehalf wave plate(hwp)inthebluebeamand retro reflect thebackreflection. 3. Insert the down conversion crystal in the blue beam after the HWP and retro reflect the back reflection. 4. Blockbluebeamforsafety. 1

2 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 A DetectorAlignment 1. Usingaruleronthetable,measurea3 o coneangleandmounttheaandbfiber coupleddetectors at the appropriate locations. Detectors should be mounted such that they can slide along a rail. (SeeBeckp.1 5fordetails.) 2. Useafiber coupledalignmentlaser(eitherhe Neorintegrateddiode)toshinelightbackoutofthe Adetectorinorderto back align theadetectortothecrystal.(seebeckp.1 5fordetails.) a. Alignmentbeammustbeleveltothetableandhitcrystalatsamespotasbluebeam does.usetheoriginalblueiristodefineheightofbeam.youwillneedtoadjustthe verticalpositionofthefiber coupledmountitself,aswellastheverticalknob,tomake the beam level to the table at the appropriate height. The A detector is now roughly aligned. 3. Turn off alignment laser and attach fiber from A detector to the single photon counting module (SPCM). Open LabVIEW software, turn off room lights, and unblock blue beam. You should (hopefully!)seecountsintheachannelabovethebackgroundlevel.ourgoalistomaximizethese countswiththefollowingadjustments: a. Tweak horizontal (H) and vertical (V) knobs on the A detector mount to increase A counts.remembertoavoidswitchingaxesunlesscountsareatamaximum. b. TweakblueHWPtoincreaseAcounts.NotefinallocationofHWP. c. AdjustH positionofamount(physicallymovethemount)alongrail,iteratingwiththe H knobtoincreaseacounts.(thisisthehardpart!) d. Tweakphase matchingangleofcrystaltoincreaseacounts. e. Insert 10 nm bandpass filter before A detector and repeat process until minimal improvementseen. f. AlignmentofAdetectoriscomplete!PutinapairofirisestodefineAbeampathas follows: i. BlockbluebeamandturnoffSPCM. ii. Use fiber coupled alignment laser to shine light from A detector back toward crystal(do not adjust the A mount). Insert and center two irises(at different locations)onthisbeam.makesureupstreamirisdoesnotinterferewiththeb beampath(itcanblocktheblueifnecessary). 2

3 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 B DetectorAlignment 1. AlignmentofBdetectorproceedssimilarlytoAdetector(Step(3)above).Thetwokeydifferences are: a. Youshouldbeoptimizing the AB coincidence countsduringeachstep(asopposedto the individual B counts). Although these typically move together, the phase matching bandwidthofthecrystalcanbequitebroad,anditisimportantthatthebdetectoris seeingthesamephotonpairsthatgototheadetector.(seebeckp.1 8fordetails.) b. DonotadjusttheblueHWPorcrystaluntilyouhaveiteratedthroughalltheBdetector adjustmentsacoupleoftimes. 2. OncedetectorBisoptimized,cantweakthebluebeamsteering(esp.vertical)slightlyinorderto optimizeabcoincidencecounts. 4. AlignmentoftheBdetectoriscomplete!AswiththeAdetector,putinapairofirisestodefineB beampathasdidforaarm(step(3f)above).alsomakesurethatthedownstreamirisisfarenough awayfrombdetectortoallowinsertionofwaveplateandpolarizingbeamsplitterinbarm. 5. You are now ready to observe the anticorrelation parameter between detectors A and B (see Experiments section).thisshouldbequitehigh,sinceintheorythephotonsarrivingatdetectors AandBareperfectlycorrelated. 3

4 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 Beamsplitterand B prime DetectorAlignment Therearedifferentapproachestothisstep,andweoutlineoursbelow.SeeBeckp.2 5through2 7 foranalternativeapproach. 1. Usingtwoseparatemirrors,alignthebeamfromapolarizedHe Nealignmentlasertotheirisesthat definethebbeampath,notingthefollowingdetails: a. MakesuretheBfiberisdisconnectedfromtheSPCMbeforeproceeding. b. TheHe Neisnotfiber coupledforthisstep. c. Sincethesecondmirror needstositdirectlyinthebbeampath,themountmustbe removable. There are various flip or magnetic mount options that can maintain their alignmentwhenremoved. 2. OncetheHe NeisalignedtotheirisesdefiningtheBbeampath,insertthepolarizingbeamsplitter (PBS)intotheBarmnearBdetector.ThePBSshouldbecenteredontheHe Nebeamandshould beadjustedinordertoretro reflectthebackreflection.alsomakesurethereflectedlightfromthe PBSgoestowardtheB location. 3. InsertredHWPupstreamfromthePBS.Again,centerandretro reflectthebackreflectionofthe alignmentlaser.youmayhavetoadjustthehwprotationtosendlighttowardb. 4. WiththeB fiberdisconnectedfromthespcm,insertb detectortolookathe Nelightreflecting offofthepbs. a. AdjustVknobonB detectortohavedetectorlensroughlyparalleltothetable. b. MoveVpositionofB mountsothathe Nelighthitsatthecenterofdetectorlens. c. AdjustHpositionandrotationofB mountuntilyouseehe Nelightcomingoutthefree endofthefiber(thiscanbetricky). d. Onceyoufindthelight,carefullydog downbaseandtightenpostscrewonb mount. e. TweakHandVknobsonB detectortooptimizehe Nethroughput. 5. TurnoffHe NeandremoveitsmirrorintheBarm.ConnectB fibertospcm,turnoffroomlights, unblockbluelaser,andlookforcounts.aswiththebdetector,youarelookingtooptimizetheab coincidence counts. The optimization process is similar to the B detector, with the following two notes: a. BeginbyadjustingtheredHWPtosendallthelighttotheB detector. b. You can tweak the PBS during the process as well. (For small adjustments, the transmittedlightthroughpbsundergoesnegligiblechanges.) 6. Alignment of the B detector is complete! You are now ready to measure the anticorrelation parameterforavarietyofinputstates(see Experiments section).besuretorotatetheredhwp toroughlyequalizethenumberofabandab coincidences. 4

5 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 InterferometerAlignment WeuseastandardMach Zehnder(MZ)interferometer.Althoughthisgeometryisnotoptimalfor mirror translation, we find that the obvious beam path separation and relatively easy alignment have pedagogicaladvantages.thetwoprimarygoalswhenaligningtheinterferometerare: 1. Afterrecombinationofthetwobeampaths,theoutputbeamsmustbecollineartoaverygood approximation. As noted below, this is achieved by adjusting for a single interference fringe usingavisiblelaser. 2. Sincethebandwidthofthedowncovertedlightisquitebroad,thepathlengthsmustbeequalto averygoodapproximation.asnotedbelow,thisisachievedbyfirstaligningthetwoarmsto theholesintheopticaltableandthenusingawhite lightsourcetofindzerorelativetimedelay ( t=0 ). We outline the essential steps of our procedure below. See Galvez p. 11 through 16 for a more detaileddescription. 1. WeusethesameHe NealignmentlaserpaththatweusedfortheB detector.first,withtheband B fibers removed from the SPCM, ensure that the He Ne is aligned to the irises in the B arm. Second,insertanotherremovablemirrorthatwillsendthebeamtotheinterferometer. 2. Usethismirrortomakesurethebeamisbothlevel(paralleltothesurfaceoftheopticaltable)and aligned along the rows of holes in the optical table. Do this using the original blue iris at two separatedlocationsonthetable.besuretopushtheirisbaseflushagainstscrewsinthesamerow ofholesalongthetable.thegreatertheseparationbetweenirispositions,thebettertheprecision. (SeeGalvezp.13fortheirdescriptionofthisprocess.) 3. Insertaturningmirrorbeforethefirstbeamsplitterthatchangesthebeamdirectionby90 o.once again,usetheirisalignmentproceduretoensurethebeamcomingoffthismirrorisbothlevelto thetableandparalleltotheholes. 4. Insertthefirstbeamsplitter(BS1).ThebeamshouldbecenteredonBS1.Usingtheirisalignment procedure,adjustbs1sothatthereflectedbeamstayslevelandtravelsparalleltotheholesinthe table. Note: depending on your implementation of the iris alignment procedure, this may involve iteratingbetweenmovingthebaseofbs1andadjustingtheknobsonthebs1mount. 5. Insert the two mirrors of the MZ interferometer. Once again, use the iris to have the reflected beamscomeoffleveltothetableandparalleltotherowsofholes.atleastoneofthemirrorswill beonatranslationstage alignthestagesothatitsaxisoftranslationmakesa45 o anglewiththe holesonthetable.notethatsincewearealwaysmaking90 o turns,itisnotnecessaryforthetwo mirrorstobeexactlythesamedistancefrombs1. 6. Insertthesecondbeamsplitter(BS2).BS2needstobepositionedsuchthatthetwobeamsintersect atthebeam splittingsurfaceandexitbs2collinearly.usingtheirisalignmentprocedure,tweakthe angleadjustmentsofbs2toensurethatthereflectedbeamisleveltothetableandalignedalong 5

6 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 theholes.thismeansthatthereflectedbeamisnowparalleltothetransmittedbeam,whichwas alreadyalignedtothetableinthismanner.nowusethetranslationstagetochangetheoverlapof thetwobeamsatthebeam splittingsurface.thebeamsshouldnowbecollinearexitingbs2. 7. Use a diverging lens to expand the output beams on a screen. You should see an interference patternatthispoint.adjusttheknobsofbs2toachievea singlefringe intheinterferencepattern. Theinterferometerisnowalignedwithroughlyequalpathlengths. 6

7 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 InterferometerDetectorAlignmentandFindingt=0 The procedure above produces a MZ interferometer with roughly equal path lengths. Since interferencecanonlybeseenforapathlengthdifferencelessthanthecoherencelengthofthelight,we need to ensure that the two paths in the interferometer are exactly the same (t=0). In addition, we need to align the fiber detectors to the interferometer output beams. We begin with the detector alignment: 1. WiththefiberdisconnectedfromtheSPCM,insertoneofthedetectorstolookattheHe Nelight fromoneoutputportoftheinterferometer.alignthisdetectortotheoutputbeamasdetailedin Step (4) of the B detector alignment procedure above. The detector should ideally be mounted alongarailsothatitcanslidetransversetothebeamdirection(likedetectorsaandbpreviously). 2. Repeatthisprocessforthedetectorattheotheroutputport(ifdesired). 3. TurnofftheHe Neandmountawhite lightsource(incandescentbulb)neartheentrancemirrorto interferometer.connectthefiberfromoneoftheoutputdetectorstoavisiblewavelengthrange spectrometer(oroffofagratingontoacard). 4. By adjusting only the position/orientation of the white light source, look for a signal on the spectrometer.donotadjustanyportionoftheinterferometer. 5. Iftheinterferometerisproperlyaligned,youshouldseemodulationsinthespectrumofwhitelight, since some colors are interfering constructively and others destructively, depending on the exact path length difference. Only when the path length difference is exactly zero will all colors interference constructively (or destructively). By adjusting the translation stage position small amounts,youshouldseethemodulationpatterninthespectrumchange.movethestageuntilall thecolorsinterfereinthesamemanner( singlewhite lightfringe ).Thepathlengthdifferenceis nowessentiallyzero.(seegalvezp.15forafigure.) 6. Finallyweneedtooptimizethedown conversioncoincidencecountsbetweendetectoraandthe twodetectorsattheoutputofthemzinterferometer(whichweagaincallbandb ).Removethe white lightsource,connectthefiberstothespcm,turnofftheroomlights,unblockthebluelaser, and look for counts. As before, you are looking to optimize the AB and AB coincidence counts. Theoptimizationprocessissimilartotheearlierdetectors,withthefollowingtwonotes: a. It is very helpful to block one arm of the interferometer with a card so that you are not sensitivetothenoiseduetoopticalinterference. b. AdjustmentsshouldbemadeprimarilytothenewBandB detectors(onlyadjustaandthe bluehwp/crystalattheend). c. Remembertousethe10nmbandpassfiltersforfinaloptimization. 7. Theinterferometerisnowaligned.Youarereadytoperformthesingle photoninterferenceand quantumeraserexperiments. 7

8 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 TheExperiments ThedescriptionsbelowhighlightthemainexperimentswedoatDickinsonCollege. 1. AnticorrelationParameterforIncandescentLightBulb LightSource:Incandescentbulbplacedjustafterdown conversioncrystal. Filters:NDfiltersimmediatelyafterbulb. Detectors:BandB. Software:LabVIEWDownConversionCoincidence,g(2)2 det.onbb. Result:Two detectorg(2)=1. 2. AnticorrelationParameterforLaserLight LightSource:He NelaseraimedintoB B arm. Filters:NDfiltersimmediatelyafterlaser. Detectors:BandB. Software:LabVIEWDownConversionCoincidence,g(2)2 det.onbb. Result:Two detectorg(2)=1. 3. Two DetectorAnticorrelationParameterforAandBArmsofDown ConvertedLight LightSource:SPDCintoAandBarms. Filters:10nmbandpassfiltersbeforeredHWPinbothAandBarms. Detectors:AandB.(NoteneedtoswitchcablesforAandB forsoftware.) Software:LabVIEWDownConversionCoincidence,g(2)2 det.onbb (whichisreallyba). Result:Two detectorg(2)inthehundreds. 4. Two DetectorAnticorrelationParameterforSingle ArmofDown ConvertedLight LightSource:SPDCintoB B arm. Filters:10nmbandpassfilterbeforeredHWPinB B arm. Detectors:BandB. Software:LabVIEWDownConversionCoincidence,g(2)2 det.onbb. 8

9 DickinsonCollegeSingle PhotonQuantumMechanics CheatSheet Version1:Spring2010 Result:Two detectorg(2)=1. 5. Three DetectorAnticorrelationParameterforDown ConvertedLight(Grangier) LightSource:SPDCintoAandB B arms. Filters:10nmbandpassfiltersbeforeredHWPinbothAandB B arms. Detectors:A,B,andB. Software:LabVIEWDownConversionCoincidence,g(2)3 det.onabb. Result:Three detectorg(2)approximatelyequalto0. 6. White LightInterference LightSource:IncandescentbulbplacedjustbeforeentrancemirrortoMZinterferometer. Filters:None. Detectors:BafterMZinterferometergoingtoRedTideSpectrometer(donotletintoSPCM). Software:LoggerPro whitelight_fringes.cmbl template(spectrometerviewingsoftware). Result:Singlewhite lightfringewhenatt=0. 7. Single PhotonInterferenceandQuantumEraser LightSource:SPDCintoAarmandBdetectoratoutputofMZinterferometer. Filters:10nmbandpassfiltersbeforeredHWPinAarmandbeforeMZinterferometer. Detectors:AandBafterMZinterferometer. Software: APT User (control for stage in MZ interferometer). LabVIEW Down Conversion Coincidence.MonitorABcoincidencerateinLabVIEWastranslatestage. Result:InterferenceinABcoincidencerateasstagemoveswhenpolarizationsparallel( untagged ). No interference when polarizations perpendicular( tagged ). Interference returns when polarizer placed after MZ interferometer( erased ). Also note can get rid of interference by moving stage pastcoherencelengthoflightwithallfiltersremoved.interferencewillreturnwhennarrow band filterplacedinaarmonly. 9