Table Of ContentA New and Feasible Protocol for Semi-quantum Key
Distribution
MichelBoyer1,3,MattyKatz2,RotemLiss2,4,andTalMor2,5
1 DépartementIRO,UniversitédeMontréal,Montréal(Québec)H3C3J7,Canada
2 ComputerScienceDepartment,Technion,Haifa3200003,Israel
7 3 [email protected]
1 4 [email protected]
0 5 [email protected]
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7 Abstract. QuantumKeyDistribution(QKD)protocolsmakeitpossiblefortwo
0 quantumpartiestogenerateasecretsharedkey.Semi-quantumKeyDistribution
1. (SQKD) protocols, such as “QKD with classical Bob” and “QKD with classi-
0 calAlice”(thathavebothbeenprovenrobust),achievethisgoal evenifoneof
7 thepartiesisclassical.However,thecurrentlyexistingSQKDprotocolsarenot
1 experimentallyfeasiblewiththecurrenttechnology.Herewesuggestanewpro-
: tocol (“Classical Alice with a controllable mirror”) that can be experimentally
v
implementedwiththecurrenttechnology,andproveittoberobust.
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2 MichelBoyer,MattyKatz,RotemLiss,andTalMor
1 Introduction
Quantum Key Distribution (QKD) makes it possible for two legitimate parties, Alice
andBob,togenerateaninformation-theoreticallysecurekey[1],thatissecureagainst
any possible attack allowed by the laws of quantum physics. Alice and Bob use an
insecure quantum channel and an authenticated classical channel. The adversary Eve
mayinterferewiththequantumchannelandislimitedonlybythelawsofnature;she
maynot,however,modifythedatasentintheauthenticatedclassicalchannel(shecan
onlylistentoit).
Semi-quantum Key Distribution (SQKD) protocols limit one of the parties to be
classical,yetgivingasecurekey[4].ThefirstSQKDprotocolwas“QKDwithclassical
Bob”[4];later,the“QKDwithclassicalAlice”[22,8]protocolwassuggested,aswell
as various other SQKD protocols (see for example [12,19,21]). Most of the SQKD
protocolshavebeenproven“robust”:namely[4],anysuccessfulattackbyanadversary
necessarilyinducessomenoisethatthelegitimatepartiesmaynotice(seealsotheformal
definitioninSect.2.3).Afewofthemhavealsobeenprovensecure[11].
However,tothebestofourknowledge,allthecurrentlyexistingSQKD protocols
cannotbeexperimentallyconstructedinasecurewaybyusingthecurrenttechnology,
asexplainedinSect.1.3.Inotherwords,despitethefactthatSQKDprotocolsshould
have been easier to implement than QKD protocols (because only one party requires
quantumabilities),itturnsoutthatsomeofthe“classical”operationsareveryhardto
implementinasecureway.
WepresentanewSemi-quantumKeyDistributionprotocolthatcanbeexperimen-
tally constructedbyusinga “controllablemirror”.Itisbasedon“QKD with classical
Alice”[22,8],butitismorecomplicated,becauseitallowsAlicetochooseoneoffour
operations(insteadoftwo).Weprovethisprotocoltoberobust.
1.1 QuantumKeyDistributionProtocols
QKD protocols achieve the classically-impossible goal of distributing a secret key to
two parties (Alice and Bob), in a way that is secure against all the possible attacks.
Moreover,the key shared by Alice and Bob remainssecret even if weaknesses in the
devices(currentlyunknownto anyone,includingthe adversary)are discoveredin the
future:namely,fortheadversaryEvetofindthe key,she mustattackwhenAlice and
Bobapplytheprotocol,andnotlater(whileforencryptionmethodssuchasRSA,Eve
may keep the ciphertextuntil she is able to find the privatekey, e.g., by factorizinga
largenumber).
ThefirstQKDprotocolwasBB84[1]:
Protocol1 (BB84). The BB84 protocol, operated by the two parties Alice and Bob,
consistsofthefollowingsteps:
1. AlicesendstoBobNquantumstates,allofthemrandomlychosenfromthefollowing
set: 0 , 1 , + , |0i+|1i, − , |0i−|1i
{| i | i | i √2 | i √2 }
2. Bobmeasuresallthereceived states;for eachofthe states, hechoosesrandomly
whether to measure it in the computational basis 0 , 1 or in the Hadamard
{| i | i}
ANewandFeasibleProtocolforSemi-quantumKeyDistribution 3
basis + , − .IfBobmeasuresinthecomputationalbasis,heidentifies 0 and
1 wi{th|ceirt|aini}ty,butgetsarandomresultif + or − issent;theconverse|isitrue
| i | i | i
fortheHadamardbasis.
3. Now Alice and Bobeach holdsa (classical) bit string: Alice holdsthe list of bits
shesent(bit0correspondingtothestates 0 and + ,andbit1correspondingto
the states 1 and − ), and Bob holds the| liist of|bitis he measured (with similar
| i | i
interpretationsasAlice).Inaddition,Aliceknowsthebasissheusedtosendeach
state,andBobknowsthebasisheusedtomeasureeachstate.
4. AliceandBobreveal(byusingtheclassicalchannel)theirbasischoices,anddiscard
allthestatesthatBobmeasuredinabasisdifferentfromtheonesentbyAlice.
5. Alice and Bob reveal some random subset of their bit string, compare the bits,
and estimate the error rate. They abort the protocol if the error rate is above a
specified threshold (in BB84, the asymptotic threshold (for infinite key-length) is
11%[17,18]).Theydiscardtherevealedbits.
6. NowAlice andBobkeep onlythestring ofbitsthatwere measuredbyBobinthe
samebasistheyweresentbyAlice(andthatwerenotdiscarded).Ifthereisnonoise
oreavesdropping,thisbitstringshouldbethesameforAliceandBob.
7. AlicesendstoBoberrorcorrectioninformation,andBobcorrectstheerrorsinhis
bitstring,sothatitisthesameasAlice’s.
8. AliceandBobperformaprivacyamplificationprocess,yieldingafinalkeythatis
identicalandisfullysecurefromanyeavesdropper.
The notion of “(composable)full security” of a protocol(informally)means that,
exceptwithanexponentially-smallprobabilitye ,oneofthetwofollowingeventshap-
pens:theprotocolisaborted,orthesecretkeygeneratedbytheprotocolisthesameasa
perfectkeythatisuniformlydistributed,isthesameforbothparties,andisindependent
oftheadversary’sinformation[17,16](seeearlierdefinitionsof“fullsecurity”,thatwere
notcomposable,in[14,2,18]).
Many QKD protocols have been provenfully (and unconditionally)secure in the
theoreticalsense[17,16].
However,practicalimplementationsdeviatefrom the theoreticaldescriptions,and
may thus be insecure. Two important attacks that take advantage of this fact are the
“PhotonNumberSplitting”attack[10,9]andthe“BrightIllumination”attack[13].The
“PhotonNumberSplitting”attacktakesadvantageofthefactthatAlicecannotgenerate
onlyone-photonpulses,butsometimesgeneratespulsesoftwo(ormore)photons:Eve
can, under certain conditions,getfull informationon the secret key withoutinducing
errors. The “Bright Illumination”attack uses a weakness of Bob’s detectors, existing
in some practical implementations, to get full information on the secret key without
inducingerrors.
OtherQKDprotocols,eithersimilartoBB84oronesthatusedifferentapproaches,
havealsobeensuggested,andinsomecaseshavealsobeenprovenfullysecure.
1.2 ExistingSemi-quantumKeyDistributionProtocols
Thenotionofa “Semi-quantumKeyDistribution”(SQKD)protocol,in whichoneof
thepartiesusesonlyclassicaloperations,wasintroducedin[4,5].
4 MichelBoyer,MattyKatz,RotemLiss,andTalMor
ForthepurposeofthedefinitionofSQKDprotocols,theterm“classicaloperations”
referstothefollowingoperations[3]:
1. Measuringastateinthecomputationalbasis 0 , 1 .
2. Generatingastateinthecomputationalbasis{| 0i |, 1i} andsendingittotheother
{| i | i}
party.
3. Reordering quantum states, without measurement. (This operation is not used in
this paper, but is useful for randomization-basedprotocols, such as the protocols
describedin[5,3].)
4. Movingorreflectingaquantumsystemorsubsystem,withoutchangingit.
5. Choosingarandombit.
6. Alltheregularoperationsontheclassicalchannel,classicalcomputation,etc.
Theoperationsare“classical”inthesensethattheyonlytreatthecomputationalbasis
0 , 1 (or do nothing). If both Alice and Bob are classical in that sense, and the
{| i | i}
only interaction is between them, then the protocol is classical, and thus secure key
distributionisimpossible;however,if onepartyis classicalandthe otheris quantum,
thekeydistributionprotocolmaybesecure.
QuantumKeyDistributionwithClassicalBob. TheSQKDprotocol“QuantumKey
DistributionwithclassicalBob”[4]isdefinedasfollows:
Definition2 (Classical Operations in “QKD with Classical Bob”). The classical
operationsofBobinthe“QKDwithclassicalBob”protocolare:
CTRL Return the received state to Alice, without modifying or measuring it. (Here,
Bobusestheclassicaloperation4.)
SIFT(measure+resend) Measurethereceivedstateinthecomputationalbasis 0 , 1 ,
prepare a newstate (0 or 1 ) accordingto the measured bit, and send{th|ein|ewi}
| i | i
statebacktoAlice.(Here,Bobusestheclassicaloperations1+2.)
Protocol3 (“QKD with Classical Bob”). The “QKD with classical Bob” protocol
(usingatwo-wayquantumchannel),operatedbythetwopartiesAliceandBob(where
Bobisclassical,inthesensedefinedabove),consistsofthefollowingsteps:
1. AlicesendstoBobNquantumstates,allofthemrandomlychosenfromthefollowing
set: 0 , 1 , + , |0i+|1i, − , |0i−|1i
{| i | i | i √2 | i √2 }
2. Foreachofthereceivedstates,Bobrandomlychooses(byusingtheclassicaloper-
ation5)oneofthetwoclassicaloperationslistedinDefinition2(CTRLorSIFT).
3. Alicemeasuresallthestatesshereceivesinthesamebasisshesentthem.
4. Now Alice and Bobeach holdsa (classical) bit string: Alice holdsthe list of bits
shesent(bit0correspondingtothestates 0 and + ,andbit1correspondingto
the states 1 and − ), and Bob holdsthe|liist of b|itsihe measured when he used
| i | i
SIFT. In addition,Alice knows the basis she used to send each state; Bob knows
theoperationheusedforeachstate(SIFTorCTRL);andAliceholdsabitstring
representingherfinalmeasurements,thatwillbeusedlaterforcheckingtheerror
rate.
ANewandFeasibleProtocolforSemi-quantumKeyDistribution 5
5. AliceandBobreveal(byusingtheclassicalchannel)Alice’sbasischoicesandBob’s
operationchoices.
6. AlicecheckstheerrorrateintheCTRLbits.
7. Alice and Bob reveal some random subset of the SIFT bits sent by Alice in the
computationalbasis, compare them, and estimate the error rate (both in the way
fromAlicetoBobandinthewayfromBobbacktoAlice).Theyaborttheprotocolif
theerrorrateinthoseSIFTbitsorintheCTRLbitsisaboveaspecifiedthreshold.
Theydiscardtherevealedbits.
8. NowAliceandBobkeeponlythestringofSIFTbitsthatweresentbyAliceinthe
computationalbasis (and that were not discarded). If there is no noise or eaves-
dropping,thisbitstringshouldbethesameforAliceandBob.
9. AlicesendstoBoberrorcorrectioninformation,andBobcorrectstheerrorsinhis
bitstring,sothatitisthesameasAlice’s.
10. AliceandBobperformaprivacyamplificationprocess,yieldingafinalkeythatis
identicalandisfullysecurefromanyeavesdropper.
Thisprotocolwasprovedtoberobustin[4],andwaslaterprovedtobesecure[11].
QuantumKeyDistributionwithClassicalAlice. Anextensionto“QKDwithclassical
Bob”,inwhichtheoriginatoralwayssendsthesamestate + , wassuggestedin[22].
| i
Following[8],we prefertocalltheoriginatorin[22]Bob(andnotAlice),andtocall
theclassicalpartyAlice.Thus,wecalltheSQKDprotocolof[22]“QKDwithclassical
Alice”.
Definition4 (Classical Operations in “QKD with Classical Alice”). The classical
operationsofAliceinthe“QKDwithclassicalAlice”protocolare:
CTRL ReturnthereceivedstatetoBob,withoutmodifyingormeasuringit.
SIFT(measure+resend) Measurethereceivedstateinthecomputationalbasis 0 , 1 ,
prepare a newstate (0 or 1 ) accordingto the measured bit, and send{th|ein|ewi}
| i | i
statebacktoBob.
Protocol5 (“QKDwithClassicalAlice”).The“QKDwithclassicalAlice”protocol
(usingatwo-wayquantumchannel),operatedbythetwopartiesAliceandBob(where
Aliceisclassical,inthesensedefinedabove),consistsofthefollowingsteps:
1. BobsendstoAliceN quantumstates,allequalto + , |0i+|1i.
| i √2
2. For each of the received states, Alice randomly chooses one of the two classical
operationslistedinDefinition4(CTRLorSIFT).
3. Bobmeasuresallthestateshereceives,choosingrandomlyforeachstatewhetherto
measureitinthecomputationalbasis 0 , 1 orintheHadamardbasis + , − .
{| i | i} {| i | i}
4. Now Alice and Bobeach holdsa (classical) bit string: Alice holdsthe list of bits
she measured when she used SIFT (bit 0 corresponding to the state 0 , and bit
1 corresponding to the state 1 ), and Bob holds the list of bits he m|eaisured. In
| i
addition,Alice knowsthe operationshe used foreachstate (SIFTorCTRL), and
Bobknowsthebasisheusedtomeasureeachstate.
6 MichelBoyer,MattyKatz,RotemLiss,andTalMor
5. AliceandBobreveal(byusingtheclassicalchannel)Alice’soperationchoicesand
Bob’sbasischoices.
6. BobcheckstheerrorrateintheCTRLbitsthathemeasuredintheHadamardbasis.
7. AliceandBobrevealsomerandomsubsetoftheSIFTbitsmeasuredbyBobinthe
computationalbasis, compare them, and estimate the error rate (in the way from
AlicebacktoBob).TheyaborttheprotocoliftheerrorrateinthoseSIFTbitsorin
theCTRLbitsisaboveaspecifiedthreshold.Theydiscardtherevealedbits.
8. Now Alice and Bob keep only the string of SIFT bits that were measured by Bob
in the computational basis (and that were not discarded). If there is no noise or
eavesdropping,thisbitstringshouldbethesameforAliceandBob.
9. AlicesendstoBoberrorcorrectioninformation,andBobcorrectstheerrorsinhis
bitstring,sothatitisthesameasAlice’s.
10. AliceandBobperformaprivacyamplificationprocess,yieldingafinalkeythatis
identicalandisfullysecurefromanyeavesdropper.
Notethatadifferentprotocol(alsonamed“ClassicalAlice”)wassuggestedin[12],
independentlyof[22].
As proven in [8], “QKD with classical Alice” [22] is completely robust against
eavesdropping.Theproofofrobustnesswasextendedin[7]toincludephotonicimple-
mentationsandmulti-photonpulses.
1.3 TheExperimentalInfeasibilityoftheSIFTOperationinSQKDProtocols
InmanySQKDprotocols(suchas“QKDwithclassicalBob”and“QKDwithclassical
Alice”describedabove),itisassumedthattheclassicalpartycaneitherdonothing(the
CTRLoperation)ormeasureinthecomputationalbasis 0 , 1 andthenresend(the
{| i | i}
SIFT operation).In practical (photonic)implementations, and especially if limited to
theexistingtechnology,itisquiteimpossiblefortheclassicalpartytodothat,andthe
photongeneratedbyhimorherduringtheSIFToperationwillprobablybeatadifferent
timingorfrequency,thusleakinginformationtotheeavesdropper;seecommenton[4]
andthereply[20,6].
For example, let us look at the “QKD with classical Alice” protocol, and assume
thatthetwoclassicalstates, 0 and 1 ,describetwopulses(intwodistincttime-bins)
| i | i
onthe same arm,such thatthe photoncan eitherbe inone pulse,in theother,orin a
superposition(thatisanon-classicalstate).
Giventhatimplementation,itisindeedverydifficultforAlicetoregeneratetheSIFT
photonsattherighttiming.Furthermore,in[20]itisshownthatevenifAlicecouldhave
the machinery to SIFT with perfect timing, Eve can make use of the fact that Alice
does not detect the state with perfect qubit-detectors: Eve can modify the frequency
of the photon generated by Bob. Then, if Alice SIFTs, she generates a photon in the
originalfrequency,whileifsheperformstheCTRLoperation,thereflectedphotonisin
thefrequencymodifiedbyEve.Then,bymeasuringthefrequency,Evecantellwhether
AliceusedtheSIFTortheCTRLoperation;ifEvefindsoutthatAliceusedCTRL,she
shiftsthefrequencybacktotheoriginalfrequency,whileifshefindsoutthatAliceused
SIFT,shecancopythebitsentbyAliceinthecomputationalbasis.This“tagging”attack
makesitpossibleforEvetogetfullinformationonthekeywithoutinducingnoise.
ANewandFeasibleProtocolforSemi-quantumKeyDistribution 7
1.4 OurContribution
WesuggestanewSQKDprotocol,similarto“QKDwithclassicalAlice”,thatisexper-
imentallyfeasible:intheoriginalprotocolof“QKDwithclassicalAlice”,Alicecould
choose only between two operations (CTRL and SIFT); in our new protocol, Alice
may choose between four operations (CTRL, SIFT-1, SIFT-0, and SIFT-ALL), some
ofthem(SIFT-1andSIFT-0)correspondingtopossiblereflectionsofpulsesbyusinga
controllablemirror,ratherthanreflectingaqubitasawhole(CTRL).
Wecandescribethenewprotocolinthetermsofphotonpulsesthatcorrespondto
distincttime-bins:thestate 0 correspondstoonephotoninthefirsttime-bin;thestate
1 correspondstoonephot|oniinthesecondtime-bin;andthestates + , |0i+|1i and
| i | i √2
− , |0i−|1i correspondtosuperpositionsofpulsesinthetwotime-bins.Therefore, 0
| i √2 | i
and 1 canbeseenasclassicalstates,while + and − arestrictlyquantumstates.Inthis
| i | i | i
case,theCTRLoperationcorrespondstooperatingthemirroronbothpulses(reflecting
bothpulsesbacktotheoriginator,Bob);theSIFT-1(SIFT-0)operationcorrespondsto
operatingthe mirroronlyonthe 0 (1 )pulse, while measuringthe otherpulse;and
| i | i
theSIFT-ALLoperationcorrespondstomeasuringallthepulses,withoutreflectingany
ofthem.
Thisprotocolisexperimentallyfeasibleandissafe againstthe attackdescribedin
[20].Moreover,weprovethisprotocoltobecompletelyrobustagainstanattackerEve
that is allowed to do anythingallowedby the laws of quantumphysics, includingthe
possibility of sending multi-photon pulses (namely, assuming that Eve may use any
quantumstateconsistingofthetwomodes(i.e.,twoqubit-states) 0 and 1 ).
| i | i
2 Preliminaries
2.1 TheNotationsofQuantumInformation
Inquantuminformation,informationisrepresentedbyaquantumstate.Aquantumpure
stateisdenotedby y ,andisanormalizedvectorinaHilbertspace.ThequbitHilbert
spaceisH =Span| 0i , 1 ,with 0 and 1 beingtwoorthonormalvectors;twoother
2
importantstatesinH{|2iar|ei|}+i, |0|i√+i2|1i an|di|−i, |0i√−2|1i.
A quantum mixed state is a probability distribution of several pure states, and is
represented by a density matrix: r =(cid:229) p y y , where p is the probability that
j j j j j
the system is in the pure state y (this de|finiithion|should not be confused with the
j
| i
probabilities of measurement results). For example, if the mixed state of a system is
r = 1 0 0 +2 + +,thismeansthatthesystemisinthe 0 statewithprobability 1
1 3| ih | 3| ih | | i 3
andinthe + statewithprobability 2.
| i 3
ThemostgeneraloperationsallowedbyquantumphysicsfortheHilbertspaceH
are:performinganyunitarytransformationU:H H;addinganancillarystateinside
→
anotherHilbertspace; measuringa state with respectto some orthonormalbasis; and
tracingoutaquantumsystem(namely,ignoringandforgettingaquantumsystem).
See[15]formorebackgroundaboutquantuminformation.
8 MichelBoyer,MattyKatz,RotemLiss,andTalMor
2.2 TheFockSpaceNotations
TheFockspacenotations,thatserveasanextensionofthequbitstates,areasfollows:
theFockbasisvector 0,1 standsforasinglephotoninaqubit-state 0 ,andtheFock
basisvector 1,0 stand|sfoirasinglephotoninaqubit-state 1 .Natural|lyi,theHadamard
| i | i
basisqubit-statesaregivenbythesuperpositionsofthoseFockstates,sothat |0,1i±|1,0i
√2
stand fora single photonin a qubit-state ± . The generalstate ofthis photonicqubit
canthenbewrittenasa 0,1 +b 1,0 ,w|ithi a 2+ b 2=1.
| i | i | | | |
Qubitsareembeddedinthe2-modeFockspace
F =Span m1,m0 m1 0,m0 0 , (1)
{| i| ≥ ≥ }
wherem1andm0arenon-negativeintegers.Thestate m1,m0 representsm1indistin-
guishablephotonsinthequbit-state 1 andm0 indistin|guishabilephotonsinthequbit-
state 0 . | i
M|oireformally,thelinearembeddings :H F ofthequbitsintheFockspace
2
isdefinedbys 0 = 0,1 ands 1 = 1,0 ,and→consequently
| i | i | i | i
H s (H )=Span 0,1 , 1,0 . (2)
2 2
≃ {| i | i}
Inparticular,thestate 0,0 F isusedfordescribingabsenceofphotons(the“vacuum
| i∈
state”).
2.3 RobustnessandSecurityofQKDProtocols
A QKD (or SQKD) protocol is operated by two legitimate parties, normally called
AliceandBob,thattrytogenerateasecretkeysharedbythem.Theadversary(Eve)is
computationallyandtechnologicallyunlimitedandcandoanythingallowedbythelaws
ofnature.
Following[4],therobustnessofQKDandSQKDprotocolsisdefinedasfollows:
Definition6 (RobustnessofaQKDProtocol).
– AQKDorSQKDprotocoliscompletelyrobustif,assumingthattheparties’proba-
bilityoffindinganyerrorinthebitstestedbytheprotocolequalstozero,Evecannot
obtainanyinformationontherawkey(namely,onthebitstringheldbyAliceand
Bob before the error correction and the privacy amplificationsteps, thatgive the
finalkey).
– A QKD orSQKD protocolis completelynonrobust if, assumingthatthe parties’
probabilityoffindinganyerrorinthebitstestedbytheprotocolequalstozero,Eve
canstillgetfullinformationontherawkey.
– AQKDorSQKDprotocolispartlyrobustif,assumingthattheparties’probability
offindinganyerrorinthebitstestedbytheprotocolequalstozero,Evecanacquire
somelimitedinformationontherawkey.
Incontrast,a“security”ofaQKDprotocol(informally)meansthat,exceptwithan
exponentially-smallprobabilitye , the protocoleither aborts or generatesa secret key
ANewandFeasibleProtocolforSemi-quantumKeyDistribution 9
thatisthesameasaperfectkey:namely,itisuniformlydistributed,isthesameforAlice
andBob,andisindependentofEve’sinformation[17,16].
Assaidin[4],a completelynonrobustprotocolisautomaticallyinsecure,because
Evemaystealthewholerawkey,whileAliceandBobcannotnoticethatandwillnot
aborttheprotocol.However,acompletelyrobustprotocolisnotnecessarilysecure.
3 The “ClassicalAlicewitha ControllableMirror” Protocol
3.1 DescriptionoftheProtocol
TheClassicalOperations.
Definition7 (ClassicalOperationsin“ClassicalAlicewithaControllableMirror”).
The classical operations of Alice in the “Classical Alice with a controllable mirror”
protocol,givenherinitialprobestate 0,0 AandastatesentfromBob m1,m0 B(both
| i | i
representedbyusingtheFockspacenotations),are:
I(CTRL) Donothing:(usingtheclassicaloperation4describedinSect.1.2)
I 0,0 A m1,m0 B= 0,0 A m1,m0 B (3)
| i | i | i | i
S1 (SIFT-1) SwaphalfofAlice’sprobewiththe m1 B halfofBob’sstate:(usingthe
| i
classicaloperations1+4describedinSect.1.2,eachappliedononeofthetwopulses
only)
S1 0,0 A m1,m0 B= m1,0 A 0,m0 B (4)
| i | i | i | i
S0 (SIFT-0) SwaphalfofAlice’sprobewiththe m0 B halfofBob’sstate:(usingthe
| i
classicaloperations1+4describedinSect.1.2,eachappliedononeofthetwopulses
only)
S0 0,0 A m1,m0 B= 0,m0 A m1,0 B (5)
| i | i | i | i
S(SIFT-ALL) SwaptheentireprobeofAlicewiththeentirestate m1,m0 B ofBob:
| i
(usingtheclassicaloperation1describedinSect.1.2)
S 0,0 A m1,m0 B= m1,m0 A 0,0 B (6)
| i | i | i | i
Aftereachofthoseoperations,Alicemeasuresherprobe(theAstate)inthecomputa-
tionalbasisandsendstoBobtheBstate.
Intheprotocol,Alice’sactionsaredescribedasattachingaprobeinFockspaceF,
applyinga swap transformation,and performinga measurementin the computational
basis. This description is meant to match the general framework of measurements in
quantuminformation,anditcorrespondstotheoperationperformedbyAlice:thisisa
gooddescriptionof the usage of a mirror(with 0 and 1 beingtwo photonpulses),
| i | i
suchthatAlicecandecidewhetherthemirrorreflectsbothpulses(CTRL),justthefirst
pulse(SIFT-1),justthesecondpulse(SIFT-0),ornoneofthepulses(SIFT-ALL).
10 MichelBoyer,MattyKatz,RotemLiss,andTalMor
ALimitationoftheMeasurementDevices. Inthecurrentreliableimplementationsof
QKDthatusethecurrenttechnology,AliceandBobarelimitedinthesensethatthey
cannotcountthenumberofphotonsineachqubit-state(e.g.,counthowmanyphotons
they detectin the qubit-state 0 and how manyphotonsthey detectin the qubit-state
1 ),butcanonlycheckwhethe|ritheygetanyphotoninthequbit-state 0 ornot,andalso
|chieckwhethertheygetanyphotoninthequbit-state 1 ornot.Forou|riprotocol(andits
| i
robustnessanalysis)to bepractical,we assume thatAlice andBob areindeedlimited
inthatsense.Therefore,whenAliceandBobmeasureinthecomputationalbasis,their
measurementresultsaredenotedasmˆ1mˆ0,withmˆ0,mˆ1 0,1 .Similarly,whenBob
∈{ }
measuresintheHadamardbasis,hismeasurementresultismˆ mˆ ,withmˆ ,mˆ 0,1 .
+ +
− −∈{ }
Thislimitationleadstothefollowingdefinition:
Definition8 (“Count”ofaMeasurementResult).Letuslookatameasurementresult
ofAliceorBob(thatis00,01,10,or11).The“count”ofthismeasurementresultisthe
numberofdistinctqubit-statesdetectedduringthemeasurement.
TheabovedefinitionsaresummarizedinTable1.
Table1.ThefourpossiblemeasurementresultsbyAliceorBob(measuringinthecomputational
basis),dependingonthestateobtainedhimorher(thatisrepresentedintheFockspacenotations)
ObtainedState MeasurementResult “Count”
0,0 00 0
| i
0,m0 (m0>0) 01 1
| i
m1,0 (m1>0) 10 1
| i
m1,m0 (m1>0,m0>0) 11 2
| i
TheProtocol.
Protocol9 (“ClassicalAlicewithaControllableMirror”).The“ClassicalAlicewith
a controllable mirror” protocol(using a two-way quantum channel),operated by the
twopartiesAliceandBob(whereAliceisclassical,inthesensedefinedinsection1.2),
consistsofthefollowingsteps:
1. BobsendstoAliceN quantumstates,allofthemequalto |+iB, |0iB√+2|1iB;or,in
theFockspacenotations,tos |+iB= |0,1iB√+2|1,0iB.
2. Foreachofthereceivedstates,Aliceaddsaprobestate 0,0 (sothattheglobal
A
stateshouldbe 0,0 s + ),andthenrandomlychoose|sonieofthefourclassical
A B
| i | i
operationslistedinDefinition7(CTRL,SIFT-1,SIFT-0,orSIFT-ALL).(SeeTable2
foralistofthestatesthatshouldbeobtainedbyBobafterthisstep.)
3. Bobmeasuresallthestateshereceives,choosingrandomlyforeachstatewhetherto
measureitinthecomputationalbasis 0 , 1 orintheHadamardbasis + , − .
{| i | i} {| i | i}
