A photonic quantum information interface

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A photonic quantum information interface

Sébastien Tanzilli, Wolfgang Tittel, Matthaeus Halder, Olivier Alibart, Pascal

Baldi, Nicolas Gisin, Hugo Zbinden

To cite this version:

Sébastien Tanzilli, Wolfgang Tittel, Matthaeus Halder, Olivier Alibart, Pascal Baldi, et al.. A

pho-tonic quantum information interface.

Nature, Nature Publishing Group, 2005, 437, pp.116-120.

�10.1038/nature04009�. �hal-00511973�

arXiv:quant-ph/0509011v1 1 Sep 2005

S. Tanzilli, 1,

∗

W. Tittel, 1 M. Halder, 1 O.Alibart, 2 P. Baldi, 2 Ni olas Gisin, 1

andHugoZbinden 1 1

Group of Applied Physi s, University of Geneva, 20 rue de l'É ole de Méde ine, CH-1211 Geneva 4, Switzerland

2

Laboratoire de Physique de la Matière Condensée,

Université de Ni eSophia Antipolis, Par Valrose, 06108 Ni e Cedex 2, Fran e

Quantum ommuni ation is the art of transferring quantum states[1℄, or quantum bits of information (qubits), from one pla e to another. On the fundamental side, thisallowsoneto distributeentanglementand demonstratequantumnonlo ality over signi ant distan es[2, 3, 4 ,5, 6℄. On the more applied side, quantum ryptography oers, for the rst time in human history, a provably se ure way to establish a on-dentialkey betweendistant partners[7℄. Photons represent the natural ying qubit arriers forquantum ommuni ation,andthe presen eoftele omopti al bresmakes thewavelengthsof1310and1550

nm

parti ularysuitablefordistributionoverlong dis-tan es. However,tostore and pro ess quantum information, qubits ouldbeen oded into alkaline atoms that absorb and emit at around 800

nm

wavelength[8, 9℄. Hen e, futurequantuminformationnetworksmadeoftele om hannelsandalkalinememories willdemandinterfa esabletoa hievequbittransfersbetweentheseusefulwavelengths while preserving quantum oheren e and entanglement[9,10, 11℄. Herewe reporton a qubit transfer betweenphotons at 1310 and 710

nm

via a nonlinear up- onversion pro esswith asu essprobability greaterthan5%. Inthe eventofa su essfulqubit transfer,we observestrongtwo-photoninterferen e betweenthe 710

nm

photon anda third photon at 1550

nm

, initially entangled with the 1310

nm

photon, although they neverdire tly intera ted. The orrespondingdelityishigher than98%.

PACSnumbers:

Superposition ofquantum statesandentanglementarethefundamental resour esofquantum ommuni ationand quantuminformation pro essing[1℄. Fromanabstra t pointof view,thenatureof the arryingparti leis irrelevant sin eonlyamplitudesandrelativephasesareexploitedtoen odetheelementaryquantumbitsofinformation(qubits). Histori ally, experiments have proven many times that the fas inating properties of quantum orrelations an be observedwithpairsof photons[1℄, trappedions[12, 13,14℄, trappedatoms[15℄,or oldgases[16℄. However,themost appropriate arrierandasso iateden odingobservabledependonthespe i task. Photonshavebeenprovensuitable to transmitquantuminformation[1, 2,3, 4,5, 6, 7℄,and atomsorionsto store[17℄ andpro ess[13, 14℄ it. Photoni entanglementoften relies on polarization[4, 5, 6, 18, 19, 20℄, energy-time[2℄, or time-bin[3℄ oding. Depending on thequantum ommuni ation hannel, thewavelengthof thephotoni arrieris also important. Theuse oftele om wavelengths (1310 and 1550

nm

) is parti ularly advantageous when employing opti al bres[2, 3℄ while free spa e transmissionismostlybasedonshorterwavelengths[5,6℄.

Future realizationsofquantum networks, ontainingelementaryquantumpro essorsandmemories, onne tedby ommuni ation hannels,requirequantuminterfa es apable oftransferring qubitstatesfrom onetypeof arrierto another. Thisdemandsthereversiblemappingbetweenphotonsandatomswhi halsoin ludesthemappingbetween photonsofdierentwavelengths[9℄. However,asopposedtothereprodu tionof lassi alinformationbetweendierent media,itisnotpossibletomerelymeasurethepropertiesofagivenquantumsystemandrepli atethema ordingly, asaresultoftheno- loningtheorem[21℄. Neverthelessitispossibletoresorttoatransferofthequantuminformation basedonanintera tionthat maintainsthe oheren epropertiesoftheinitialquantumsystem.

Inthisletter,wedemonstrateadire tquantuminterfa eforphotoni qubitsatdierentwavelengths. Asillustrated ingure1,anarbitraryqubit arriedbyayingtele omphotonat 1310

nm

(

λ

B

),initiallyentangledwithaphoton at 1550

nm

(

λ

A

),is oherentlytransferredto anotherphotonat awavelength of710

nm

(

λ

B

′

)usingsumfrequen y generation(SFG).Thenalwavelengthis losetothat ofalkalineatomi transitions.

In the following, we rst present the theoreti al des ription of our quantum interfa e. We then re all how to reateand hara terizemaximallyenergy-timeentangledphoton-pairs. The oherentquantum informationtransfer, taking advantageof an up- onversionstage, is then introdu ed. Finally, we showhow this operationpreservesthe initial entanglement bymeasuringthe two-photoninterferen e betweenthe710and 1550

nm

photons, and thus we demonstrateauniversalqubit transfer.

oftheele tro-magneti eld inopti albers. Initially,B'isin theva uumstatedenoted

|0i

,whileAand Bmaybe entangled,withS hmidt oe ients

c

1

and

c

2

:

|Ψi

in

= c

1

|α

1

i

A

⊗ |β

1

i

B

+ c

2

|α

2

i

A

⊗ |β

2

i

B

⊗ |0i

B

′

(1)

Thepairsoforthogonalstates

|α

j

i

and

|β

j

i

,

j ∈ {1, 2}

, spanAli eandBob'squbit spa e,respe tively. Forexample,

|α

j

i

ouldrepresentonephotonineither theverti al(j=1)orhorizontal(j=2)polarizationstate,or,asin our ase, onephotoninthe rst(j=1)or se ond (j=2)time-binstate. Thedesiredstateafter asu essful transferfrom Bto B'reads:

|Ψi

transf er

= |0i

B

⊗ c

1

|α

1

i

A

⊗ |β

1

i

B

′

+ c

2

|α

2

i

A

⊗ |β

2

i

B

′

(2) Su hatransferisa hievedbyanintera tiondes ribedbytheee tiveHamiltonian

H =

11

A

⊗



g

1

|0i

B

hβ

1

| ⊗ |β

1

i

B

′

h0| + g

2

|0i

B

hβ

2

| ⊗ |β

2

i

B

′

h0|



+ H.c.

(3)

where

g

1

and

g

2

are oupling onstants. The evolution aneasily be omputed (assumingan intera tion time of 1 unit):

e

−iH

|Ψi

in

= cos(|g

1

|)c

1

|α

1

i

A

⊗ |β

1

i

B

⊗ |0i

B

′

− i

sin(|g

1

|)

|g

1

|

g

1

c

1

|α

1

i

A

⊗ |0i

B

⊗ |β

1

i

B

′

(4)

+ cos(|g

2

|)c

2

|α

2

i

A

⊗ |β

2

i

B

⊗ |0i

B

′

− i

sin(|g

2

|)

|g

2

|

g

2

c

2

|α

2

i

A

⊗ |0i

B

⊗ |β

2

i

B

′

(5)

Note thatin these ond termsin (4)and(5), the oe ients

c

j

are multipliedby

g

j

. This impliesthat thetransfer preservesthequantum oheren e,i.e. isaQuantumInformation(QI)transfer,ifandonlyifthetwo oupling onstants areequal,bothinamplitudeandphase:

g

1

= g

2

≡ g

. Inotherwords,thetransfer anbea hievedwithperfe tdelity providedthis onditionissatised. Insu ha asetheevolutionsimpliesto:

e

−iH

|Ψi

in

= cos(|g|)|Ψi

in

− ig

sin(|g|)

|g|

|Ψi

transf er

(6)

Consequently,thetransferprobabilityis1for

|g| = π/2

.

Figure2presentsour2-photonsour e

S

. It onsistsofaCWpumplaserdiodewitha oheren elength

L

c

p

ex eeding 300

m

, attenuated down to a few

µW

at

λ

p

= 711.6

nm

, and of aquasi-phase mat hed periodi allypoled lithium niobatewaveguide[23,24℄(

P P LN/W 1

). Thepolingperiodis hosensoasto reatedown- onvertedpairsofphotons whose wavelengths are entered at 1312 and 1555

nm

, respe tively. After separation at a ber-opti wavelength divisionmultiplexer (WDM),the1555and1312

nm

photonsaresentto Ali eandBob,respe tively,using standard tele omopti albres.

The long oheren e length of the pump laser implies that ea h pump photon has a very well dened energy. A ordingly, thedown- onversionpro ess yields pairsof photons that are energy orrelated asgoverned byenergy onservation: ea hphotonfromapairhasanun ertainenergy,un ertainintheusualquantumsense,butthesumof theenergiesofthetwophotonsfromapairisverywelldened. Inaddition,thetwophotonsarealsotime orrelated, sin etheyareemittedsimultaneouslywithin their oheren etime. However,theemissiontimeofapairisun ertain within thepump laser oheren e. Hen e thepairedphotonsare entangled àlaEPR[25℄, energyand timerepla ing thehistori alpositionandmomentumvariables,respe tively. Morepre isely,thepro essesofphoton-pair reationat dierenttimesseparatedby

∆τ

are oherentaslongas

∆τ << L

c

p

/c

,where

L

c

p

isthepump laser's oheren elength. Theunderlyingquantum oheren eofthephoton-pairstherefore omesfromthepumplaseritself.

Inordertorevealand hara terizethe oheren e arriedbytheseenergy-timeentangledphoton-pairsweperforma Franson-typeexperiment[22℄andinferthedegreeofentanglementfromthemeasuredtwo-photoninterferen evisibility. Forthispurposethetwophotonsaresenttotwoanalyzers,oneonAli e'sandtheotheronBob'sside,that onsist,in our ase,ofunbalan edMi helsoninterferometersmadefromstandardtele ombres,ber-opti beam-splitters,and Faradaymirrors[1℄ asdepi tedin gure 2. Toprevent singlephotoninterferen e, theopti alpath lengthdieren e in ea hinterferometer (

∆L

A

and

∆L

B

, respe tively) hastobemu h greaterthan the oheren e lengthofthesingle photons,

L

c

s,i

≃ 150 µm

,dedu edfromthe15

nm

spe tralbandwidthofthedown- onvertedlight. Ea hsinglephoton thereforehasanequalprobabilityof

1

interferen e,these analyzershavetobeequallyunbalan ed,

∆L

A

≃ ∆L

B

= ∆L

,within the oheren elengthofthe singlephotons. Atthe sametime, however,sin ean entangled pair representsthequantum obje tto beanalyzed, both

∆L

(

∼ 20 cm

in our ase)havetobemu h smallerthanthe oheren elengthofthepairwhi hisgivenbythe pumplaser(

L

c

p

> 300 m

).

Attheoutputofea hinterferometer,thesinglephotonsaredete ted. Usingatime-resolved oin iden edete tion between Ali eand Bob as depi ted in gure 2, one anpost-proje tthe initial energy-time entangled stateonto a time-binentangledstateoftheform

|Ψi

post

=

1

√

2



|s

A

, s

B

i + e

i(φ

A

+φ

B

)

|l

A

, l

B

i



(7) where

φ

A

and

φ

B

arephasesasso iatedwiththepathlengthdieren e

∆L

A

and

∆L

B

oftherelatedinterferometers. The state (7) orrespondsto the eventswhere the down- onverted photons both travel through the samearms of theinterferometers,i.e.

s

A

, s

B

or

l

A

, l

B

,these twopossibilitiesbeingundistinguishable. Thus,varying the ombined phase(

φ

A

+ φ

B

),weobservesinusoidalinterferen efringesinthe oin iden eratewithnet(a idental oin iden es dis arded) andrawvisibilitiesof97.0

±1.1

%and87.4

±1.1

%,respe tively(seeFig. 2). Be ause almostalla idental oin iden esareduetodete tordark ounts,thepurityofthe reatedstate,andhen etheperforman eofthesour e, shouldbe hara terizedbythenetvisibility. Sin ethisgureofmeritisvery losetothe100%predi tedbyquantum theory,we on ludethatoursour eprovidesastate losetoapuremaximallyentangledstate,in parti ularsuitable toviolatetheBellinequalities[25℄. Notethatthemissing3%inthenetvisibilityismostlikelyduetothemeasurement te hniqueratherthanthestatepreparation[1℄.

Now thatwehave hara terizedour entanglementresour e,wepro eed tothe quantum information transfer,i.e. thetransferofthequbit arriedbyBob'sparti leto anotherphotonofshorter wavelength. Tothisend, werepla e theber-opti interferometerandtheGe-APDonBob'ssidebyanothernonlinearPPLNwaveguide(

P P LN/W 2

),a bulk-opti Mi helsoninterferometer,andaSili on avalan hephotodiode(Si-APD,D

B

)asdepi tedin gure3. This se ondnonlinear rystal,togetherwithapowerreservoir(PR) onsisting ofaCW oherentlaserat

λ

P R

= 1560 nm

and an erbium-doped ber amplier (EDFA), servesas an up- onversion stage for the in oming 1312

nm

photons produ edbythepreviouslydes ribedsour e

S

. Asaresultofthisoperation,whi histheoppositepro ess to down- onversion,photonsat 1312 and 1560

nm

are annihilated and photonsat 712.4

nm

are reatedon Bob'sside. The oheren eofthePRdire tlyrelatesto thephasedieren ebetweenthe oe ientsintheee tiveHamiltonian(3):

g

1

= ξ(t

l

)

and

g

2

= ξ(t

s

)

, where

ξ(t)

is proportional to the PR ele tri eld at times

t

l

and

t

s

, respe tively. The quantity

c · (t

l

− t

s

)

orrespondsto the imbalan e of theinterferometers, using thenotation of equation (7). The previousgeneral ondition

g

1

= g

2

ofequation(6)thereforetranslates,inthe aseofup- onversion,asa onstrainton thePR's oheren e:

L

P R

c

/c ≫ t

l

− t

s

. Inourexperimentthis onditionisdenitelysatisedas

L

P R

c

> 1 km

,whi his learlymu hgreater thanthe

20 cm

path lengthdieren eofBob'sanalyzer. A ordingly, theresultingphotonsat

λ

B

′

shouldbe omeentangled withAli ephotonsat

λ

A

,althoughtheyneverdire tlyintera ted.

ThePRdelivers700

mW

in astandardtele omber. Wemeasured,with lassi alelds, aninternalup- onversion e ien y of 80%/Watt of reservoirpowerat optimum phase-mat hing[27, 28, 29℄. Note that this valueis underes-timatedsin ethe lossesin thewaveguideitself arenegle ted. Thisup- onversione ien y de reasesby afa torof 2when hangingthe singlephoton wavelengthby

±1 nm

. Taking advantageofthe energy orrelationbetweenthe photonsofapair,weredu edthebandwidthbylteringthespe trumof thephotonsat1555

nm

travellingtoAli e to 1.5

nm

(

BP F

A

)[1℄ and in reased the powerof the sour e pump laser in order to haveareasonable photon-pair produ tionrateinthisbandwidth. Hen e,theoverallup- onversionprobabilityisonlylimitedbytheavailablepump power and oupling losses between the waveguideand single mode opti al bers at input and output fa es. From this lassi al onversione ien y,takingintoa ountthereservoirpower,arealisti 40% ouplinge ien y intothe waveguide(bothforthePRandthequbit tobetransferred)andtheratiobetweentheinitialandnalwavelengths, weestimatetheprobabilityofasu essfulQItransfertobe:

P

success

= 80% W

−

1

_{·0.7 W ·(0.4)}

2

_·

712 nm

1312 nm

≈ 5%

. Letus mentionthatindire tqubittransfer analsobea hievedviateleportation[26℄. However,in ontrasttoteleportation, QItransferisa hievedwithamu hhighersu essprobability.

Aftertheup- onversionstage,weneedtoseparatethe

λ

B

′

photonsfromthehugeowofPRphotonsat1560

nm

, about

5 · 10

9

photonsper

ns

,andmeasurethetransferdelity. Forthis,weuseabandpasslter enteredat712

nm

(

BP F

B

,

∆λ = 10 nm

,morethan30

dB

attenuationaround1550

nm

),asdepi tedingure3. Wealsotakeadvantage of thefa t that the Si-APD(

D

B

)is essentiallyinsensitiveto the1560

nm

wavelength. The 712.4

nm

photonsthen enterthetemperaturestabilizedbulk-opti Mi helsoninterferometer. Attheoutportofthis interferometertheyare oupled, for optimal mode overlap, into a single-mode bre adapted to visible light. The orresponding oupling e ien y is greater than 60%. Finally these photons are sent to the dete tor

D

B

. The ele troni signal from this

APD triggersthedete tor of the 1555

nm

photons that arriveon Ali e's side (InGaAsAPD previouslyintrodu ed andoperatedingatedmode). Finallywere ordthe oin iden eevents.

Figure 3showstheinterferen e patternof the oin iden e eventsasafun tion of the ombined phase(

φ

A

+ φ

B

) obtainedwithourtime-resolved oin iden edete tion. Weobserveasinusoidalos illationwithnetandraw visibili-tiesof 96.2

±0.4

% and86.4

±0.4

%,respe tively, representinga learsignatureof thepreserved oheren e duringthe quantuminformation transfer. Thenetvisibility

V

net

is againvery loseto the100%predi tedbyquantumtheory. Assumingthattheentireredu tionofthisvisibility anbeattributedtoanimperfe tquantumstatetransfer,wend adelity

F =

1 + V

net

2

of98.5%. Notehoweverthat thisestimationisvery onservativesin ethevisibilityobserved beforetransfer(seegure2)ishardlyhigher.

Notethatthequantuminterfa edemonstratedinthisletterisnotlimitedtothespe i wavelengths hosen.Indeed, suitablemodi ationsofphase-mat hing onditionsandpumpwavelengthsenabletuningtoanydesiredwavelengths. Inparti ularitallowsmappingoftele ommuni ationphotoni qubitsontoqubitsen odedatawavelength orrespond-ing toalkalineatomi transitions. It isparti ularly interestingto takeadvantageof periodi ally poledwaveguiding stru tures, asemployed here. Firstly, phase mat hing onditions aneasily be tuned overabroadrange by hang-ing thegratingperiod. Se ondly, these omponentsyield veryhigh upand down- onversione ien ies[23, 24, 27℄. This permits the use of a modest reservoir power to a hieve a reasonable qubit transfer probability. In the ase ofappli ations requiringverynarrowphotonbandwidths, forinstan ewhen transferring quantum informationonto atoms[9℄, bright down- onverters make very narrow spe tral ltering possible while maintaining high photon-pair reationrateswith reasonablepump powers. Inaddition, thenarrowlteringensures optimalphase-mat hingover thewholebandwidthofthephotonstobeup- onverted,hen emaximizingthispro ess.

Inthisworkwedemonstrated,inthemostgeneralway,therstdire tquantuminformationinterfa ebetweenqubits arriedbyphotonsofwidelydierentwavelengths. Tothisend,weveriedthatentanglementremainsunae ted,even thoughoneofthetwoentangledphotonsissubmittedtoawavelengthup- onversionpro ess. Thisinterfa emaynd appli ations inquantum networks,where mappingoftravellingqubits, e.g. qubits en oded onto tele ommuni ation photons,andstationaryatomi qubitsfeaturingresonan eatmu hshorterwavelengths,isne essary.

WethankD.B. Ostrowskyforvaluabledis ussions.

Finan ialsupportsbytheSwissNationalCenterforQuantumPhotoni sandtheEuropeanIST proje tRamboQare a knowledged.

S.T.a knowledgesnan ialsupportfromtheEuropeanS ien eFoundationprogram"QuantumInformationTheory andQuantum Computation".

Theauthorsde larethattheyhaveno ompetingnan ialinterests. Corresponden e andrequestsformaterialsshouldbeaddressedtoS.T.

∗

Ele troni address: sebastien.tanzilliphysi s.unige. h

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parametri down- onverter,Phys.Rev.A69,013807(2004).

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Q-Interface

(c

(2)

- SFG)

power reservoir

photon-pair source

(CW laser +

c

(2)

- SPDC)

Final entanglement tested

with 2 interferometers

l

A

l

B

l

A

l

B’

l

PR

Alice

quantum channels

Bob

FIG.1: S hemati illustration ofthe experiment on ept. First,aphoton-pairsour eprodu es,by spontaneous parametri down- onversion(SPDC),energy-timeentangledphotonsatwavelengths

λ

A

and

λ

B

thataresenttoAli eandBob,respe tively. Next,thequbittransferisperformedatBob'spla efromphoton

λ

B

tophoton

λ

B

′

usingsumfrequen ygeneration(SFG).The nalentanglementbetweenthenewly reatedphoton

λ

B

′

andAli e's photon

λ

A

istestedusingtheFranson onguration[22℄ (seegure2).

0

50000

100000

150000

200000

250000

300000

350000

400000

170

171

172

173

174

175

delay between start and stop (ns)

n

u

m

b

e

r

o

f

c

o

in

c

id

e

n

c

e

s

/5

s

Legend

free space

sm fibre

lenses

electronics

PPLN/W1

lp=

711.6 nm

Pump CW

Laser

WDM

SPDC

TDC

HPF

D

B

f

A

D

A

FM

DF

ISO

L

L

L

Alice

1555 nm

1312 nm

coincidences

InGaAs-APD

Ge-APD

DLA

S

s /l

A

B

l /s

A

B

s /s

+ l /l

A

B

A

B

start

stop

elec.delay

FM

fiber interf.

100

150

200

0

100

200

300

400

500

n

u

m

b

e

r

o

f

c

o

in

c

id

e

n

c

e

/5

s

f

A

+f

B

R

C

time-window

f

B

FM

Bob

DLB

FM

fiber interf.

f +

A

f

B

(mod 2 )

p

p

0

2p

3p

-p

FIG.2: ExperimentalFranson-typesetup usedforthe reationandanalysisofenergy-timeentangledpairsofphotons. Ali e andBob'sanalyzersare equallyunbalan edMi helsoninterferometers.

Thesour eis omposedofaCWlaser(Topti aPhotoni s DL100),anonlinearPPLNwaveguide,aWDM andsuitableber ouplinglenses. Italsoin ludesahigh-passwavelengthlter(

HP F

)behindthe rystaltodis ardtheremainingpumpphotons, andlenses(

L

)to ouplethe reatedphotonsintoasinglemodebre. Thedown- onversionquasi-phasemat hingisobtained withaspe i polingperiod

Λ

of14.1

µm

andatatemperatureof85

o

C

. This1

cm

longwaveguidefeaturesadown- onversion e ien ygreaterthan10

−7

. AtBob'spla etheGe-APD(NEC,

D

B

)isliquidNitrogen ooledandpassivelyquen hed,whilethe InGaAs-APD(idQuantiqueid200,

D

A

)atAli e'sisoperatedingatedmode. TheAPDsshowquantumdete tione ien ies ofabout10%and14%andprobabilitiesofdark ountsofaround

3

· 10

−5

and

10

−5

pernanose ond,respe tively.

TheoutputsfromtheseAPDsprovidethestartandstopsignalsforatime-to-digital onverter(TDC)thatre ordsa oin iden e histogram(lefthandside)asafun tionofthetimedieren ebetweenitstwoinputs. Thispi tureis omposedofthreedierent peakswhi harisefromdierent ombinationsofphotontransmissionstroughtheir respe tiveinterferometer,eithertheshort (

s

)orthelong(

l

)arm.Theeventswherebothphotonstakethesamearms(

s

A

/s

B

and

l

A

/l

B

)areindistinguishableasdes ribed byequation(7),leadingtophoton-pairinterferen e. They anbedis riminatedfromtheotherpossibilities(

l

A

/s

B

and

s

A

/l

B

) bymeansofatime-resolved oin iden edete tion. The orresponding oin iden e ountrate

R

C

asafun tionofthe ombined phase(

φ

A

+

φ

B

) isshownintheright handinset. Wendsinusoidalinterferen efringeswithmore than97%net visibility. Notenallythatthevariationofthe ombinedphaseisobtainedby hangingthetemperatureinBob's interferometer.

TDC

D

B

f

A

D

A

FM

Alice

1555 nm

1312 nm

coincidences

InGaAs-APD

Si-APD

DL

A

start

stop

elec.delay

S

712.4 nm

PPLN/W2

WDM

f

B

SFG

bulk interf.

Quantum Interface

Bob

DLB

M

M

BPF

B

BPF

A

FM

l

PR

=1560 nm

Power Res.

CW laser

EDFA

fiber interf.

200

300

0

100

200

300

400

500

600

700

800

n

u

m

b

e

r

o

f

co

in

ci

d

e

n

ce

s

/1

0

0

s

f

A

+ f

B

R

_C

p

0

2p

3p

4p

5p

6p

7p

f +

A

f

B

(mod 2 )

p

FIG.3: Experimental setup for the oherent transfer of quantumentanglement. OnBob's side, PPLN/W2, pumpedbya high- oheren eCW700

mW

powerreservoirat1560

nm

(laser8168AfromHP+EDFAfromKeopsys),ensuresthetransferof thequbitfrom1312to712.4

nm

viatheup- onversionpro ess. Thisse ond1

cm

longPPLNwaveguideisofthesamekindas theoneusedfor thedown- onversion. Theywerebothfabri atedusingthe te hniqueofsoftprotonex hange[23 ℄andfeature almostidenti alphase-mat hing onditions. Both1312 and1560

nm

wavelengthsare mixedatase ondWDMwhoseoutput portisdire tlybutt- oupledto theinput fa eof PPLN/W2withoutanyadditional opti s. At theoutputofthe waveguide, thenewly reatedphotonsat712.4

nm

are oupledintoasinglemodeberanddete tedusingaSili onavalan hephotodiode (EG&GAQ-141-FC,

D

B

).

The oheren eofthetransferisveriedbymeasuringphoton-pairinterferen ebetweenthe712.4and1555

nm

photonsinthe Franson onguration. TheanalysisonBob'ssideutilizesanunbalan ed,bulk-opti interferometerthatisalignedwithrespe t toAli e's. Althoughthe oin iden erateissubstantiallyredu edduetoalimitedup- onversionprobability andlossesinthe interferometers, theanalysis ofthe oin iden eeventsyieldsinterferen e fringeswithnet andrawvisibilities of 96and 86%, respe tively.This onrmsthepreviouslyobtainedresultwithoutthequantuminterfa eanddemonstrates,inthemostgeneral way, oherenttransfer ofquantuminformationbetweentwophotons.