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Marcos Curty1,2
Coauthors: Tobias Moroder2,3, and Norbert Lütkenhaus2,3 1. Center for Quantum Information and Quantum Control (CQIQC), University of Toronto2. Institute for Quantum Computing, University of Waterloo3. Max-Plank-Forschungsgruppe, Institut für Optik, Information und Photonik, Universität
Erlangen-Nürnberg
On One-way and Two-way Classical Post-Processing Quantum Key Distribution
• Quantum Key Distribution (QKD)• Precondition for secure QKD (Two-way & One-
way)• Witness Operators (Two-way & One-way QKD)• Semidefinite Programming• Evaluation
Overview
Quantum Key Distribution (QKD)
Phase I: Physical Set-Up
Mathematical Model
AiBj
Pr(Ai,Bj)=Tr(Ai Bj )AB
AB=i Pr(Ai)1/2AiAi with AB= AB
AB
Bj
Ai
Ai
Pr(Ai,Bj)=Pr(Ai)Tr(Bj )AiAi
Reduced density matrix of Alice fixed Add: A= TrB(AB) Ai 1
Quantum Key Distribution (QKD)
Phase II: Classical Communication Protocol
Pr(Ai,Bj) Secret key
• Advantage distillation (e.g. announcement of bases in BB84 protocol)• Error Correction ( Alice and Bob share the same key)• Privacy Amplification ( generates secret key shared by Alice and Bob)
Authenticated Classical Channel
Two-way
Quantum Key Distribution (QKD)
Phase II: Classical Communication Protocol
Pr(Ai,Bj) Secret key
• Advantage distillation (e.g. announcement of bases in BB84 protocol)• Error Correction ( Alice and Bob share the same key)• Privacy Amplification ( generates secret key shared by Alice and Bob)
Authenticated Classical Channel
One-way (Reverse Reconciliation: RR)
Quantum Key Distribution (QKD)
Phase II: Classical Communication Protocol
Pr(Ai,Bj) Secret key
• Advantage distillation (e.g. announcement of bases in BB84 protocol)• Error Correction ( Alice and Bob share the same key)• Privacy Amplification ( generates secret key shared by Alice and Bob)
Authenticated Classical Channel
One-way (Direct communication: DC)
Quantum Key Distribution (QKD)
Which type of correlations Pr(Ai,Bj) are useful for QKD?
secret bitsper signal
Distance (channel model)
Not secure (proven) Protocol independent
Regime of Hope
secure(proven)protocol
Talk: T. Moroder
This talk
Talk: G. O. Myhr
Precondition for Secure QKD
Theorem (Two-way QKD)
AB
Pr(Ai,Bj)
AB separable No Key
MC, M. Lewenstein and N. Lütkenhaus, Phys. Rev. Lett. 92, 217903 (2004)
Ai Bj
AB is separable if AB = i pi |aiai|A|bibi|B
Precondition for Secure QKD
Theorem (One-way QKD)
AB
Pr(Ai,Bj)
AB has a symmetric extension to two-copies of system B
(A), then the secret key rate for direct communication (reverse reconciliation) vanishes.
T. Moroder, MC and N. Lütkenhaus, quant-ph/0603270.
Ai Bj
Precondition for Secure QKD
AB with symmetric extension to two copies of system B
AB
TrE(ABE)= AB
ABE
A B
E
A B
E
AB
A B
E
TrB(ABE)= AE = AB AB
Witness Operators (Two-way QKD)
AB separable?
TrWAB < 0 TrWAB 0 AB comp.with
separable
Witness Operators
TrWAB = ij cij P(Ai,Bj )
MC, M. Lewenstein and N. Lütkenhaus, Phys. Rev. Lett. 92, 217903 (2004)MC, O. Gühne, N. Lewenstein, N. Lütkemhaus, Phys. Rev. A 71, 022306 (2005)
restricted knowledge
compatiblewith sep.
verifiableentangled
• AB
W
Accesible witnesses: W = ij cij Ai Bj
Optimal Wopt
Wopt
Witness Operators (One-way QKD)
AB symmetric extension?
T. Moroder, MC and N. Lütkenhaus, quant_ph/0603270.
TrWAB = ij cij P(Ai,Bj )
TrWAB < 0 TrWAB 0 AB comp. with symmetric extension
restricted knowledge
compatiblewith symmetric
extension.
Without symmetric extension
• AB
Wopt
Witness Operators
Accesible witnesses: W = ij cij Ai Bj
Witness Operators (Two-way QKD)
Evaluation: 4-state QKD protocol
Uses two mutually unbiased bases:e.g. X,Z direction in Bloch sphere
|0|1|1
|0
Error Rate: 36 %
0.07987 0.04516 0.00913 0.11591 0.04508 0.07986 0.11593 0.00901 0.11599 0.00909 0.08001 0.04507 0.00897 0.11593 0.04505 0.07985
0101
0 1 0 1A\B
Pr(Ai,Bj)
| e=cos(X)|00+sin(X)(cos(Y)|01+sin(Y)(cos(Z)|10+sin(Z)|11))
Systematic Search
MC, M. Lewenstein and N. Lütkenhaus, Phys. Rev. Lett. 92, 217903 (2004)
W4 = 1/2(|ee| + |ee|)
Witness Operators (Two-way QKD)
(only parameter combinations leading to negative expectation values are marked)
MC, O. Gühne, N. Lewenstein, N. Lütkemhaus, Phys. Rev. A 71, 022306 (2005)MC, O. Gühne, N. Lewenstein, N. Lütkemhaus, Proc. SPIE Int. Soc. Opt. Eng. 5631, 9-19 (2005).J. Eisert, P. Hyllus, O. Gühne, MC, Phys. Rev. A 70, 062317 (2004).
Evaluation: 4-state QKD protocol
TrWAB = ij cij P(Ai,Bj )
Other QKD protocols (including higher dimensional QKD schemes)
Witness Operators (Two-way and One-way QKD)
• One witness: Sufficient condition as a first step towards the demonstration of the feasibility of a particular experimental implementation of QKD. This criterion is independent of any chosen communication protocol in Phase II.
• All witnesses: Systematic search for quantum correlations (or symmetric extensions) for a given QKD setup. Ideally the main goal is to obtain a compact description of a minimal verification set of witnesses (Necessary-and Sufficient).
Advantages: Witnesses operators
Disadvantages: Witnesses operators
• Too many tests: To guarantee that no secret key can be obtained from the observed data it is necessary to test all the members of the minimal verification set.
• How to find them?: To find a minimal verification set of EWs, even for qubit-based QKD schemes, is not always an easy task, and it seems to require a whole independent analysis for each protocol.
Semidefinite Programming (SDP)
Primal problem
minimise cTxsubject to F0+i xi Fi ≥ 0
with x=(x1, ..., xt)T the objective variable, c is fixed by the optimisation problem, and the matrices Fi are Hermitian
SDPs can be efficiently solved
Equivalent class of states S
S = {AB such as Tr(Ai Bj AB) = Pr(Ai,Bj) i,j}
Qubit-based QKD (with losses): AB H2H3
Semidefinite Programming (SDP)
Two-way QKD
AB S with AB 0 No Key
AB
Pr(Ai,Bj) Ai Bj
SDP
Feasibility problemc = 0
minimise 0subject to AB(x) 0 AB
(x) 0
AB(x) SS
MC, T. Moroder, and N. Lütkenhaus, in preparation (2006)
Dual problem maximise -Tr(F0 Z)
subject to Z ≥ 0 Tr(Fi Z) = ci for all i
where the Hermitian Z is the objective variable
Semidefinite Programming (SDP)SDP: One-way QKD
MC, T. Moroder, and N. Lütkenhaus, in preparation (2006)
minimise 0subject to AB(x) SS PABA’(x)P = ABA’(x)
ABA’(x) 0 TrA’[ABA’(x)] = AB(x)
with P the swap operator: P|ijkABA’ = |kjiABA’
Dual problem (one way & two-way) Witness operator optimal for Pr(Ai,Bj)
Evaluation
• We need experimental data Pr(Ai,Bj)
• Channel Model:
AB = (1-p)[(1-e)|AB|+e/2 A1B] + p A|vacBvac|
p: probability Bob receives the vacuum state |vacB e: depolarizing rate1B: 1B- |vacBvac|
Evaluation
Six-state protocol:
|0
|00
|1|1
|0
|11Alice and Bob:
Bruß, Phys. Rev. Lett. 81, 3018 (1998).
Four-state protocol:
|0
|00
|1
|11Alice and Bob:
C.H. Bennett and G. Brassard, Proc. IEEE Int. Conf. On Computers, System and Signal Processing, 175 (1984).
QBER: 33 %
QBER: 16.66 %
H. Bechmann-Pasquinucci, and N. Gisin, Phys. Rev. A 59, 4238 (1999).
QBER: 25 %
QBER: 14.6 %
C. A. Fuchs, N. Gisin, R. B. Griffiths, C.-S. Niu, andA. Peres, Phys. Rev. A 56, 1163 (1997); J. I. Cirac, and N. Gisin, Phys. Lett. A 229, 1 (1997).
Evaluation Two-state protocol:
Alice:
|0 = |0+|1
|1 = |0-|1 C. H. Bennett, Phys. Rev. Lett. 68, 3121 (1992).
Bob:
B0 = 1/(22)|11
|
B1 = 1/(22)|00
|
B? = |00|+|11|-B0-B1
Bvac = |vacvac|
Limit USD p1-22
e=0Four-plus-two-state protocol:
|0|1|1
|0
Like 2 two-state protocols:
B. Huttner, N. Imoto, N. Gisin, and T. Mor, Phys. Rev. A 51, 1863 (1995).
Inflexion pointe constant
p=1-22
(USD)
Other QKD protocols MC, T. Moroder, and N. Lütkenhaus, in preparation (2006)
Summary
Interface Physics – Computer Science: Classical Correlated Data with a Promise Necessary condition for secure QKD (Two-way & One-way).
Relevance for experiments: show the presence of entanglement (states without symmetric extension)
• No need to enter details of classical communication protocols• Prevent oversights in preliminary analysis• One properly constructed proof suffices
Evaluation: Semidefinite programming (qubit-based QKD protocols in the presence of loss).
Task for Theory: Develop practical tools for realistic experiments ( for given measurements).