615 research outputs found
Quantum and Classical Noise in Practical Quantum Cryptography Systems based on polarization-entangled photons
Quantum-cryptography key distribution (QCKD) experiments have been recently
reported using polarization-entangled photons. However, in any practical
realization, quantum systems suffer from either unwanted or induced
interactions with the environment and the quantum measurement system, showing
up as quantum and, ultimately, statistical noise. In this paper, we investigate
how ideal polarization entanglement in spontaneous parametric downconversion
(SPDC) suffers quantum noise in its practical implementation as a secure
quantum system, yielding errors in the transmitted bit sequence. Because all
SPDC-based QCKD schemes rely on the measurement of coincidence to assert the
bit transmission between the two parties, we bundle up the overall quantum and
statistical noise in an exhaustive model to calculate the accidental
coincidences. This model predicts the quantum-bit error rate and the sifted key
and allows comparisons between different security criteria of the hitherto
proposed QCKD protocols, resulting in an objective assessment of performances
and advantages of different systems.Comment: Rev Tex Style, 2 columns, 7 figures, (a modified version will appear
on PRA
Coherent pulse implementations of quantum cryptography protocols resistant to photon number splitting attacks
A new class of quantum cryptography (QC) protocols that are robust against
the most general photon number splitting attacks in a weak coherent pulse
implementation has been recently proposed. In this article we give a quite
exhaustive analysis of several eavesdropping attacks on these schemes. The
eavesdropper (Eve) is supposed to have unlimited technological power while the
honest parties (Alice and Bob) use present day technology, in particular an
attenuated laser as an approximation of a single-photon source. They exploit
the nonorthogonality of quantum states for decreasing the information
accessible to Eve in the multi-photon pulses accidentally produced by the
imperfect source. An implementation of some of these protocols using present
day technology allow for a secure key distribution up to distances of
150 km. We also show that strong-pulse implementations, where a strong pulse is
included as a reference, allow for key distribution robust against photon
number splitting attacks.Comment: 16 pages, 11 figure
Experimental quantum key distribution with source flaws
Decoy-state quantum key distribution (QKD) is a standard technique in current
quantum cryptographic implementations. Unfortunately, existing experiments have
two important drawbacks: the state preparation is assumed to be perfect without
errors and the employed security proofs do not fully consider the finite-key
effects for general attacks. These two drawbacks mean that existing experiments
are not guaranteed to be secure in practice. Here, we perform an experiment
that for the first time shows secure QKD with imperfect state preparations over
long distances and achieves rigorous finite-key security bounds for decoy-state
QKD against coherent attacks in the universally composable framework. We
quantify the source flaws experimentally and demonstrate a QKD implementation
that is tolerant to channel loss despite the source flaws. Our implementation
considers more real-world problems than most previous experiments and our
theory can be applied to general QKD systems. These features constitute a step
towards secure QKD with imperfect devices.Comment: 12 pages, 4 figures, updated experiment and theor
Security of quantum key distribution with imperfect devices
We prove the security of the Bennett-Brassard (BB84) quantum key distribution
protocol in the case where the source and detector are under the limited
control of an adversary. Our proof applies when both the source and the
detector have small basis-dependent flaws, as is typical in practical
implementations of the protocol. We derive a general lower bound on the
asymptotic key generation rate for weakly basis-dependent eavesdropping
attacks, and also estimate the rate in some special cases: sources that emit
weak coherent states with random phases, detectors with basis-dependent
efficiency, and misaligned sources and detectors.Comment: 22 pages. (v3): Minor changes. (v2): Extensively revised and
expanded. New results include a security proof for generic small flaws in the
source and the detecto
Multi-Photon Quantum Key Distribution Based on Double-Lock Encryption
This paper presents a multi-stage, multi-photon quantum key distribution
protocol based on the double-lock cryptography. It exploits the asymmetry in
the detection strategies between the legitimate users and the eavesdropper. The
security analysis of the protocol is presented with coherent states under the
intercept-resend attack, the photon number splitting attack, and the
man-in-the-middle attack. It is found that the mean photon number can be much
larger than one. This complements the recent interest in multi-photon quantum
communication protocols that require a pre-shared key between the legitimate
users
Quantum Cryptography
Quantum cryptography could well be the first application of quantum mechanics
at the individual quanta level. The very fast progress in both theory and
experiments over the recent years are reviewed, with emphasis on open questions
and technological issues.Comment: 55 pages, 32 figures; to appear in Reviews of Modern Physic
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