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High-visibilty two-photon interference at a telecom
wavelength using picosecond regime separated sources
Pierre Aboussouan, Olivier Alibart, Daniel Barry Ostrowsky, Pascal Baldi,
Sébastien Tanzilli
To cite this version:
Pierre Aboussouan, Olivier Alibart, Daniel Barry Ostrowsky, Pascal Baldi, Sébastien Tanzilli.
High-visibilty two-photon interference at a telecom wavelength using picosecond regime separated
sources. Physical Review A, American Physical Society, 2010, 81 (2), pp.021801(R).
�10.1103/Phys-RevA.81.021801�. �hal-00511975�
arXiv:0912.5312v1 [quant-ph] 29 Dec 2009
Pierre Aboussouan,OlivierAlibart, Daniel B.Ostrowsky,Pas alBaldi, and SébastienTanzilli
∗
Laboratoire de Physique de la Matière Condensée, CNRS UMR 6622, Université de Ni e Sophia Antipolis, Par Valrose, 06108 Ni e Cedex 2, Fran e.
(Dated: De ember31,2009)
Wereportonatwo-photoninterferen eexperiment inaquantumrelay on guration usingtwo pi ose ondregimePPLNwaveguidebasedsour esemittingpairedphotonsat1550nm. Theresults show that the pi ose ond regime asso iated with a guided-wave s heme should have important reper ussionsforquantumrelayimplementationsinreal onditions,essentialforimprovingboththe workingdistan eandthee ien yofquantum ryptographyandnetworkingsystems. In ontrast toalreadyreportedregimes, namelyfemtose ond andCW,it allowsa hievinga99%netvisibility two-photon interferen e while maintaining a high ee tive photon pair rate using only standard tele om omponentsanddete tors.
PACSnumbers:03.67.-a,03.67.Bg,03.67.Dd,42.50.Dv,42.50.Ex,42.65.Lm,42.65.Wi Keywords: QuantumCommuni ation,Two-photoninterferen e,Guided-waveOpti s
Forthe realization ofquantum networks,interferen e betweenphotonsprodu edbyindependentsour esis ne -essary. Photon oales en e(or two-photoninterferen e) lies at the heart of quantum operations and is seen as a rststeptowardsa hievingbothteleportation[1℄ and entanglementswapping[2,3, 4,5,6,7℄. Thisee t has been extensively studied theoreti ally [8, 9℄ and exper-imentally, initially based on two photons oming from asingledown onversionsour eand thereforesharinga ommonpast[10,11,12℄. However,experiments involv-ing truly independent photons represent an important hallengefora hievinglongerquantumlinksbymeansof quantum relays[13℄. Inthis frame, it hasbeen demon-strated theoreti ally that atwo-photon interferen e net visibility ofat least 95%isrequiredfor pra ti al imple-mentationsusing urrentlyavailablephotonpairsour es and multimodequantum memories [14℄. Rea hing su h ahighvisibilitythereforeappearstobeahardtasksin e a perfe t syn hronization between independent sour es isne essarytopreventanykindofdistinguishability be-tween the interfering photons. Several papers fo using onthesyn hronizationissuehavedemonstratedthat en-tanglementswappingwithfullyindependentsour esisin prin iplefeasibleinthefemtose ond[3,4℄andCW[5,7℄ regimes. Unfortunately, beyond the fundamental inter-est,thereportedinterferen evisibilitiesremaineitherfar from95%orshowverylowoverallphotonpairrate. For instan e, the best visibility reported so far in the fem-tose ondregimehasbeenobtainedby ompensatingthe syn hronization-indu ed temporal distinguishability by dramati ally in reasing the photons oheren e time up toafewpi ose ondsattheexpenseoftheoverall bright-ness [4℄. Sin e laser avities an easily be syn hronized to subpi ose onda ura y[15, 16℄, thestudy of the pi- ose ondregime [6,17℄,itsasso iated lterbandwidths,
∗
Ele troni address: sebastien.tanzilliuni e.fr
andthetypeofphotonpairgenerators,be omesofprime interesttoensuresimultaneouslyahighdegreeof indis-tinguishabilityandahighoverallbrightness.
Here, we demonstrate that to a hieve this, the pi- ose ondregimeshould provideane ienttrade-o en-ablingnear-perfe ttwo-photoninterferen eandhigh ef-fe tive photon pair rates, when asso iated with stan-dard omponentsavailablefromthetele ommuni ations industry. More pre isely, we realized an experiment basedon a single pi ose ond pump laser and two peri-odi allypoledlithiumniobate(PPLN)waveguides emit-tingpairedphotonsaround1550nmtoapproximate in-dependent sour es. We report the highest two-photon interferen enetvisibility,i.e. of99%,everdemonstrated ina on gurationextendable toquantum relays. Su ha proof-of-prin ipleemphasizes why guided-wave te hnol-ogy, in the pi ose ond regime, should lead to realisti quantum relays hemes,namely byoeringan redu ed- onstraint solution for the syn hronization issue when two ompletelyindependentlasers areemployed. Inthe following, we brie y introdu e the entanglement swap-ping based quantum relay s heme and dis uss why the pi ose ondregimeisavaluabletrade-o. Wethenfo us onourexperimental demonstration. Finally, wedetaila omparativestudy ofperforman e withsimilarreported experiments.
A basi s heme of a quantum relay based on entan-glementswapping is givenin FIG.1 (see aption for de-tails). Forlongdistan equantum ommuni ationlinks, the preferred qubit arriers are photonsat 1550nm al-lowing the users, namely Ali e and Bob, to take ad-vantage of standard opti al bers for distribution pur-poses. Theoreti ally, the two inner photons an ome from any type of sour e, provided theyare identi al at thebeam-splitter(BS)(pre-sele tion)oratthedete tors (post-sele tion). Fromtheexperimentalside,wehaveto ompare their oheren e time,
τ
oh
, to the time un er-tainty,
t
un ert
FIG. 1: S hemati sof a quantum relay involving twopairs of entangled photons emitted by two syn hronized sour es (EPR).Thetwoinnerphotonsare senttoa50/50BSwhere aBellstatemeasurement(BSM),basedontwo-photon inter-feren e,isperformed. Usingadedi ateddete tion, entangle-ment anbe swapped fromthese photonsto theouterones makingAli eandBob onne tedbyentanglement,asifthey hadea hre eivedonephotonfromanentangledpairdire tly. The BSM serves as a triggerfor Ali e and Bob's dete tors. Itthereforeenablesredu ingtheSNRoftheoverallquantum linkandfurtherin reasingthemaximuma hievabledistan e.
te tor'stiming jitters),whi h anbewrittenas:
τ
oh
≥ t
un ert(1) Suitable bandpass lters are therefore employed to a hieveoptimalinterferen evisibilities. Upto now,this issuehasbeenaddressedusingdierentapproa hesbased onpulsedorCWlasers(seealsoTableIfor omparison). Ontheonehand,the rstexperimentsreportedtwo in-dependent rystalspumpedbyfemtose ondlasers,sin e this regime allows working with broadband lters, i.e. ontheorderofafew nm[2℄. However,thelargephoton bandwidths makes them more prone to hromati and polarizationdispersionsinopti al bersandleadsto re-ported visibilities below 85% [18℄. Moreover when two independent lasers are used, the two related laser avi-tieshavetobeproperlysyn hronizedsoastobe identi- alwithinthepumppulseduration(
∼
100fs)[3,4℄. This anbea hievedusingphase-lo kedloopsoratomi lo ks and dedi ated ele troni s[19, 20℄. Anyremaining jitter between the two lasers has to be ompensated by nar-rowing the lters whi h implies a substantialredu tion ofthepairprodu tionrateforequivalentpowers[4℄.On the other hand, the development of narrow-bandwidth berBragggrating(FBG) ltersallowsusing twoindependentCWlasersstabilizedagainstanatomi transition [5, 6℄. Sin e this regime does not provide anyreferen e lo k,thetimingfun tion istransferredto the oin iden edete tionafter theBS.Toensureahigh quality interferen e, the photons oheren e time hasto be longer than the jitter of the dete tors. Experimen-tally, thisis madepossibleusing asuitable ombination oflow-jitterdete tors (basedonsuper ondu tingor up- onversionte hnologies),ontheorderof70ps,asso iated withultra-narrowFBG lters,ontheorderof10pm[5℄. In su h asituation, onegets rid of thesyn hronization
u tuationsofthe lters entralwavelengths,low exper-imental rates due to the narrow lters oupled to the randomnessof entanglingphotonsby oin iden e dete -tion, and high statisti al u tuations due to low ount rates. All theseissuespreventrea hinghigh visibilities.
Betweenthesetwoextremes,thepi ose ondregime ap-pears to be an e ient ompromise. First, the timing ondition(1) iseasilymetbyusingFBG ltersshowing mu h larger bandwidths than in CW, i.e. on the or-derof100pm[17℄. Se ond,thisallowstakingadvantage of o-the-shelf InGaAs avalan he photodiodes (APD). Last, when ompared to the femtose ond regime, nar-rower bandwidthsloosen the onstraints on both path-lengthstabilizationfortheinterferingphotonsandlaser syn hronization to subpi ose ond a ura y [15, 16℄ to obtainhigh-visibilityHong-Ou-Mandel interferen e. All thesekeypointsmotivateinvestigatingthisregime.
Assumingsyn hronization is fairly feasible, we there-forefo usedoureortsat in reasingtheoverallspe tral brightness by employing a pi ose ond pump laser asso- iated with state-of-the-art waveguide sour es emitting at tele om wavelengths. As shown in FIG.2, the pump laser (Coherent MIRA 900-D) provides 1.2ps-duration, time-bandwidthlimited(
∆λ
p
=
0.25nm) pulses, at the wavelength ofλ
p
=
768nm and at a repetition rate of 76MHz. Thepulses aresentto aBS whoseoutputsare dire tedtowardstwo10mm-longPPLNwaveguides fab-ri atedin ourlaboratoryusing thesoft-protonex hange te hnique[21℄. Thesedevi esaresinglemodeattele om wavelengths enabling only one nonlinear pro ess to o - uravoidinganyadditionnalba kgroundnoisewithinthe bandwidthofinterestwhi hisakeyadvantage ompared to four-wave mixing sour es [6, 17℄ limited by Raman ba kgroundphotons. Bothsamplesopto-geometri al pa-rameterswere designed to produ e identi al degenerate pairedphotonsat1536nmwithin abandwidthof50nm whenpumpedat768nminthepi ose ondregime. Sin e the lterbandwidthsaremu h narrowerthanthe down onversion bandwidth (∆λ
spdc
∆λ
f ilter
≥ 10
2
), experimentally, the two sour es are simply independently stabilized at thetemperatureof343Ktowithin 0.1K.
Fromea h sour e,wesele tpairs of photonsmeeting theFouriertransform riterionusingnarrowband demul-tiplexers made of two opti al ir ulators and a pair of FBG lters. Thesearesettore e tenergy-mat hedpairs of wavelength at 1537.4nm and 1534.6nm, i.e. around degenera y, asso iated with bandwidths of 800pm and 250pm, respe tively. A slightlywider (800pm) lter is used onthe longwavelength(1537nm) photons mainly to minimizeoverall losses. This lteringsolution based on standardtele om omponentsprovides a lever way to separate deterministi allythe photonsat theoutput of the waveguides and makes them ea h available in a singlemode ber[5℄. The250pm ltersbandwidth has been hoosenfortworeasons: (a)itisnarrowenoughto a ommodatethe1.2ps-durationofthelaserpulsesand anypossiblejitterupto4psduetolasersyn hronization
FIG.2: Coales en eexperimentbasedontwoPPLN waveg-uidespumpedbyasinglepi ose ondlaser. Averyweakpart of thisbeamis sentto asili on APD (not represented) em-ployedas alaser lo k randomdivider. Itsele tri al output isusedtotriggerfourInGaAs-APDs.
Λ
: polingperiodofthe PPLN waveguides; I:isolator; R: retro-re e tor; C: ir ula-tor;&: AND-gate. TheInGaAs-APDsfeature10%quantum e ien yandadark ountprobabilityofabout10
−5
ns
−1
.
stabilizetheir entralwavelengthusingtwoindependent proportional-integral-derivativetemperature ontrollers. To observe the two-photon interferen e, four-fold o-in iden es are dete ted thanks to four InGaAs-APDs (idQuantique201)triggered byaPeltier ooledSi-APD (idQuantique100) pla ed onthe path of an attenuated fra tion of thepump beam. Sin e these InGaAs-APDs are designed to handle a maximum triggering rate of 1MHz,thiste hniqueallowsworkingatanaverage lo k of600kHzrandomlypi kedfromthe76MHzlaser repe-titionrate. Inour ase,fasterInGaAs-APDwouldhighly improvetheoverall oin iden erateasshowninRef.[6℄. The evolutionof the ee tive four-fold oin iden e rate is givenin FIG.3in whi h aremarkabledip isobtained whenthedelaybetweenthetwoinner photonsiszero.
Using only 1mW of mean pump power per sour e, we obtained
4 × 10
3
two-fold oin iden es perhour and persour e whi h orrespondsto about 0.05photon per pulse. In terms of normalizedbrightnessit means that our sour es rea h
1.6 × 10
3
pairs·
s−1
·
pm−1
·
mW−1
after the ltering stage. This brightness orresponds to the state-of-the-art for PPLN waveguides and is basi ally ve orders of magnitude larger than that of bulk rys-tals [21, 22℄. When the two photons are made indis-tinguishable in time (δt
) thanks to the adjustment of the retro-re e tor (R) pla ed in front of onesample, a93%±3%
redu tionintherawfour-fold oin iden e ountisobtained. By orre tlyre ordingthea idental oin i-den es,weareabletodemonstratethatthenetvisibility rea hes
99%±3%
. Thefullwidthathalfmaximumofthe dipisapproximately∆τ ≈ 11
ps,whi hisingood agree-mentwiththe oheren etimeof14psexpe tedfromthe lterbandwidths. This interferen evisibilityis also ex-tremely losetothemaximumvalueof99.9%
al ulated from the theory taking into a ount our ltering band-widths,asproperlyoutlinedinRefs.[8,9,18℄. Notethat theveryhighstabilityoeredbyourguided-waves heme0
5
10
15
20
25
30
35
-50
-40
-30
-20
-10
0
10
20
30
40
0
1000
2000
3000
4000
5000
6000
Four-fold coincidences (per 2 hours)
Two-fold coincidences (per 2 hours)
Relative delay (ps)
Accidental 4-fold coincidences
4-fold coincidences
Gaussian fit V=99%
2-fold coincidences
FIG.3: Four-fold oin iden e rateasa fun tionof the rela-tivedelay,
δt
,between the interferingphotons (∼
0.04
pho-ton/pulse). We learly observeadipforδt = 0
that rea hes thenoise level. TheGaussian tof theinterferen e pattern showsanetvisibilityof99% ± 3%
. Wealsoplottedoneofthe two-fold oin iden esrelated to oneof the sour esafter the BStoverifythis gureis onstant.morethan30pointsevenlyspreadoverthedip.
TableI presentsthe ompared resultsreported inthe literatureforsimilar on gurationswhi h an be dier-entiated by four key points: (i) the type of nonlinear generators,(ii)theemittedphotonwavelengths,(iii)the pumpingregimeanditsasso iatedtimeun ertainty,(iv) and, nally, the applied ltering bandwidths and their asso iated oheren e times. Froma omparisonbetween thereported results, we on lude that the ombination ofthepi ose ondregimeandthesinglemodeproperties of the employed tele ommuni ation omponents is the mostperformant s heme. It allows mat hing e iently theinequality(1)andobtainingnear-perfe ttwo-photon interferen e. For pra ti allongdistan e quantum om-muni ation,itisalsointerestingto omparetheee tive numberofpairsavailableperse ond atAli eand Bob's lo ations. This gureofmeritis al ulatedby normaliz-ingthefour-fold oin iden erateswith respe tto Ali e andBob'sdete tore ien ies. A omparisonwith sim-ilar on gurationsattele omwavelengthsindi atesthat the pi ose ond regime allows maintaining a high ee -tivepair produ tion rate equivalentto that of the fem-tose ondregimebutshowsamu hhighervisibility. One shouldalsonotethetwoordersofmagnituderate dier-en eforequivalentsour ete hnologiesandbrightness be-tweentheCWand pi ose ondregimes. Inaddition,the bestvisibilityreportedinRef.[4℄withphotonsinthe vis-ible omesfromthefa tthatthe lteringbandwidthsare ontheorderofthosene essaryforthepi ose ondregime. Consequently, the overall brightness is very low onsid-eringthe use of e ientSi-APDs. Finally, bright ber sour es are ontaminated by intrinsi spontaneous
Ra-TABLEI:Comparedtwo-photoninterferen evisibilities,sour ebrightness,andoverallexperimental oin iden erates,between reportedworksin ontinuouswave(CW),pi ose ond (ps),andfemtose ond(fs)regimes. Regardingtimeun ertainties, note thatthe rstnumberisalwaysasso iatedwiththepulseduration while the
±
sign orrespondstothe syn hronisationjitter iftwopulsedlasers areinvolved. In ontrast,the timeun ertaintyfor the CW ase is asso iatedwith thedete tor's timing jitters. The oheren etimeis al ulatedforGaussian ltersusingthestandardrelationτ
c
= 0.44
λ
2
c∆λ
. Referen e Sour e Wavelength Brightness Regimea
Filterbandwidth Rate Raw Net
nm
pair/s/pm/mW
(timeun ertainty) ( oheren etime)pair/s
visibility visibility Geneva[5 ℄ PPLN/w 15500.9 × 10
3
2×
CW(70ps) 0.01nm(350ps)3 × 10
−3
NA 77% Ni e PPLN/w 15501.6 × 10
3
1×
ps(1.2ps) 0.25nm(14ps)3 × 10
−1
93% 99% Atsugi[6 ℄ Fiber 1550 NA1×
ps(19ps) 0.2nm(18ps)2
b 64% NA Bristol[17 ℄ Fiber 600 NA1×
ps(1.5ps) 0.3nm(1.8ps)4 × 10
−1
88% NA Geneva[18 ℄ BulkLBO 1310 NA1×
fs(200fs) 5nm(500fs)7 × 10
−1
77% 84% Beijing[3 ℄ BulkBBO 800
1.2 × 10
−2
2×
fs (60±
2fs) 2.8nm(335fs)3 × 10
−2
82% NA Vienna[4℄ BulkBBO 800 NA2×
fs(50±
260fs) 0.4nm(2.3ps)1 × 10
−2
96% NA a
Inthis olumn,the guresasso iatedwiththepumpregimes orrespondtothenumberoflasersemployed. b
Theexperimentoperatesat500MHzrepetitionratethereforein reasingtheavailablephotonpairrate.
Thetwolasersarenottrulyindependentsin etheysharethesameTi:Sapphire rystalKerrmediumfora uratesyn hronization.
andtoredu edvisibilities[6,17℄.
The results demonstrated in this experiment are of broadinterestsin etheobtainednetvisibilityisthebest valuereportedtodatefor omparable on gurations. A - ording to referen e [14℄, this allows, for the rst time, onsideringthepossibilityofusingquantumrelaysin a -tual quantum ryptography networks. In this ontext, wheretwosyn hronizedpi ose ondlasersmustbeused, we notonlyexpe t resultsof thesameorder,thanksto
the high experimental stability and versatility demon-strated, but also be ause of the redu ed- onstraint on thesyn hronizationissue[15,16℄.
The authorsthankM. P.de Mi heliand S. Tas u for the realization of the PPLN waveguides, G. Sauder for te hni alsupport,andA.Beveratos,H.Zbinden,andK. Thyagarajanforfruitfuldis ussions. Thisworkhasbeen supportedbytheCNRS,theUniversityofNi eSophia Antipolis,andtheFren h MinistryofResear h.
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