11,787 research outputs found
Experimental demonstration of long-distance continuous-variable quantum key distribution
Distributing secret keys with information-theoretic security is arguably one
of the most important achievements of the field of quantum information
processing and communications. The rapid progress in this field has enabled
quantum key distribution (QKD) in real-world conditions and commercial devices
are now readily available. QKD systems based on continuous variables present
the major advantage that they only require standard telecommunication
technology, and in particular, that they do not use photon counters. However,
these systems were considered up till now unsuitable for long-distance
communication. Here, we overcome all previous limitations and demonstrate for
the first time continuous-variable quantum key distribution over 80 km of
optical fibre. The demonstration includes all aspects of a practical scenario,
with real-time generation of secret keys, stable operation in a regular
environment, and use of finite-size data blocks for secret information
computation and key distillation. Our results correspond to an implementation
guaranteeing the strongest level of security for QKD reported to date for such
long distances and pave the way to practical applications of secure quantum
communications
Using quantum key distribution for cryptographic purposes: a survey
The appealing feature of quantum key distribution (QKD), from a cryptographic
viewpoint, is the ability to prove the information-theoretic security (ITS) of
the established keys. As a key establishment primitive, QKD however does not
provide a standalone security service in its own: the secret keys established
by QKD are in general then used by a subsequent cryptographic applications for
which the requirements, the context of use and the security properties can
vary. It is therefore important, in the perspective of integrating QKD in
security infrastructures, to analyze how QKD can be combined with other
cryptographic primitives. The purpose of this survey article, which is mostly
centered on European research results, is to contribute to such an analysis. We
first review and compare the properties of the existing key establishment
techniques, QKD being one of them. We then study more specifically two generic
scenarios related to the practical use of QKD in cryptographic infrastructures:
1) using QKD as a key renewal technique for a symmetric cipher over a
point-to-point link; 2) using QKD in a network containing many users with the
objective of offering any-to-any key establishment service. We discuss the
constraints as well as the potential interest of using QKD in these contexts.
We finally give an overview of challenges relative to the development of QKD
technology that also constitute potential avenues for cryptographic research.Comment: Revised version of the SECOQC White Paper. Published in the special
issue on QKD of TCS, Theoretical Computer Science (2014), pp. 62-8
Nonlinear limits to the information capacity of optical fiber communications
The exponential growth in the rate at which information can be communicated
through an optical fiber is a key element in the so called information
revolution. However, like all exponential growth laws, there are physical
limits to be considered. The nonlinear nature of the propagation of light in
optical fiber has made these limits difficult to elucidate. Here we obtain
basic insights into the limits to the information capacity of an optical fiber
arising from these nonlinearities. The key simplification lies in relating the
nonlinear channel to a linear channel with multiplicative noise, for which we
are able to obtain analytical results. In fundamental distinction to the linear
additive noise case, the capacity does not grow indefinitely with increasing
signal power, but has a maximal value. The ideas presented here have broader
implications for other nonlinear information channels, such as those involved
in sensory transduction in neurobiology. These have been often examined using
additive noise linear channel models, and as we show here, nonlinearities can
change the picture qualitatively.Comment: 1 figure, 7 pages, submitted to Natur
Calculation of Mutual Information for Partially Coherent Gaussian Channels with Applications to Fiber Optics
The mutual information between a complex-valued channel input and its
complex-valued output is decomposed into four parts based on polar coordinates:
an amplitude term, a phase term, and two mixed terms. Numerical results for the
additive white Gaussian noise (AWGN) channel with various inputs show that, at
high signal-to-noise ratio (SNR), the amplitude and phase terms dominate the
mixed terms. For the AWGN channel with a Gaussian input, analytical expressions
are derived for high SNR. The decomposition method is applied to partially
coherent channels and a property of such channels called "spectral loss" is
developed. Spectral loss occurs in nonlinear fiber-optic channels and it may be
one effect that needs to be taken into account to explain the behavior of the
capacity of nonlinear fiber-optic channels presented in recent studies.Comment: 30 pages, 9 figures, accepted for publication in IEEE Transactions on
Information Theor
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