830 research outputs found
Improved one-way rates for BB84 and 6-state protocols
We study the advantages to be gained in quantum key distribution (QKD)
protocols by combining the techniques of local randomization, or noisy
preprocessing, and structured (nonrandom) block codes. Extending the results of
[Smith, Renes, and Smolin, quant-ph/0607018] pertaining to BB84, we improve the
best-known lower bound on the error rate for the 6-state protocol from 14.11%
for local randomization alone to at least 14.59%. Additionally, we also study
the effects of iterating the combined preprocessing scheme and find further
improvements to the BB84 protocol already at small block lengths.Comment: 17 pages, to appear in Quantum Information & Computation. Replaced by
accepted versio
Higher Security Thresholds for Quantum Key Distribution by Improved Analysis of Dark Counts
We discuss the potential of quantum key distribution (QKD) for long distance
communication by proposing a new analysis of the errors caused by dark counts.
We give sufficient conditions for a considerable improvement of the key
generation rates and the security thresholds of well-known QKD protocols such
as Bennett-Brassard 1984, Phoenix-Barnett-Chefles 2000, and the six-state
protocol. This analysis is applicable to other QKD protocols like Bennett 1992.
We examine two scenarios: a sender using a perfect single-photon source and a
sender using a Poissonian source.Comment: 6 pages, 2 figures, v2: We obtained better results by using reverse
reconciliation as suggested by Nicolas Gisi
Key distillation from quantum channels using two-way communication protocols
We provide a general formalism to characterize the cryptographic properties
of quantum channels in the realistic scenario where the two honest parties
employ prepare and measure protocols and the known two-way communication
reconciliation techniques. We obtain a necessary and sufficient condition to
distill a secret key using this type of schemes for Pauli qubit channels and
generalized Pauli channels in higher dimension. Our results can be applied to
standard protocols such as BB84 or six-state, giving a critical error rate of
20% and 27.6%, respectively. We explore several possibilities to enlarge these
bounds, without any improvement. These results suggest that there may exist
weakly entangling channels useless for key distribution using prepare and
measure schemes.Comment: 21 page
Tomography increases key rates of quantum-key-distribution protocols
We construct a practically implementable classical processing for the BB84
protocol and the six-state protocol that fully utilizes the accurate channel
estimation method, which is also known as the quantum tomography. Our proposed
processing yields at least as high key rate as the standard processing by Shor
and Preskill. We show two examples of quantum channels over which the key rate
of our proposed processing is strictly higher than the standard processing. In
the second example, the BB84 protocol with our proposed processing yields a
positive key rate even though the so-called error rate is higher than the 25%
limit.Comment: 13 pages, 1 figure, REVTeX4. To be published in PRA. Version 2 adds
many references, a closed form key rate formula for unital channels, and a
procedure for the maximum likelihood channel estimatio
On the performance of two protocols: SARG04 and BB84
We compare the performance of BB84 and SARG04, the later of which was
proposed by V. Scarani et al., in Phys. Rev. Lett. 92, 057901 (2004).
Specifically, in this paper, we investigate SARG04 with two-way classical
communications and SARG04 with decoy states. In the first part of the paper, we
show that SARG04 with two-way communications can tolerate a higher bit error
rate (19.4% for a one-photon source and 6.56% for a two-photon source) than
SARG04 with one-way communications (10.95% for a one-photon source and 2.71%
for a two-photon source). Also, the upper bounds on the bit error rate for
SARG04 with two-way communications are computed in a closed form by considering
an individual attack based on a general measurement. In the second part of the
paper, we propose employing the idea of decoy states in SARG04 to obtain
unconditional security even when realistic devices are used. We compare the
performance of SARG04 with decoy states and BB84 with decoy states. We find
that the optimal mean-photon number for SARG04 is higher than that of BB84 when
the bit error rate is small. Also, we observe that SARG04 does not achieve a
longer secure distance and a higher key generation rate than BB84, assuming a
typical experimental parameter set.Comment: 48 pages, 10 figures, 1 column, changed Figs. 7 and
Security of quantum key distribution protocols using two-way classical communication or weak coherent pulses
We apply the techniques introduced in [Kraus et. al., Phys. Rev. Lett. 95,
080501, 2005] to prove security of quantum key distribution (QKD) schemes using
two-way classical post-processing as well as QKD schemes based on weak coherent
pulses instead of single-photon pulses. As a result, we obtain improved bounds
on the secret-key rate of these schemes
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