5 research outputs found
Field test of a continuous-variable quantum key distribution prototype
We have designed and realized a prototype that implements a
continuous-variable quantum key distribution protocol based on coherent states
and reverse reconciliation. The system uses time and polarization multiplexing
for optimal transmission and detection of the signal and phase reference, and
employs sophisticated error-correction codes for reconciliation. The security
of the system is guaranteed against general coherent eavesdropping attacks. The
performance of the prototype was tested over preinstalled optical fibres as
part of a quantum cryptography network combining different quantum key
distribution technologies. The stable and automatic operation of the prototype
over 57 hours yielded an average secret key distribution rate of 8 kbit/s over
a 3 dB loss optical fibre, including the key extraction process and all quantum
and classical communication. This system is therefore ideal for securing
communications in metropolitan size networks with high speed requirements.Comment: 15 pages, 6 figures, submitted to New Journal of Physics (Special
issue on Quantum Cryptography
Improvement of continuous-variable quantum key distribution systems by using optical preamplifiers
Continuous-variable quantum key distribution protocols, based on Gaussian
modulation of the quadratures of coherent states, have been implemented in
recent experiments. A present limitation of such systems is the finite
efficiency of the detectors, which can in principle be compensated for by the
use of classical optical preamplifiers. Here we study this possibility in
detail, by deriving the modified secret key generation rates when an optical
parametric amplifier is placed at the output of the quantum channel. After
presenting a general set of security proofs, we show that the use of
preamplifiers does compensate for all the imperfections of the detectors when
the amplifier is optimal in terms of gain and noise. Imperfect amplifiers can
also enhance the system performance, under conditions which are generally
satisfied in practice.Comment: 11 pages, 7 figures, submitted to J. Phys. B (special issue on Few
Atoms Optics
Entangled Quantum Key Distribution with a Biased Basis Choice
We investigate a quantum key distribution (QKD) scheme which utilizes a
biased basis choice in order to increase the efficiency of the scheme. The
optimal bias between the two measurement bases, a more refined error analysis,
and finite key size effects are all studied in order to assure the security of
the final key generated with the system. We then implement the scheme in a
local entangled QKD system that uses polarization entangled photon pairs to
securely distribute the key. A 50/50 non-polarizing beamsplitter with different
optical attenuators is used to simulate a variable beamsplitter in order to
allow us to study the operation of the system for different biases. Over 6
hours of continuous operation with a total bias of 0.9837/0.0163 (Z/X), we were
able to generate 0.4567 secure key bits per raw key bit as compared to 0.2550
secure key bits per raw key bit for the unbiased case. This represents an
increase in the efficiency of the key generation rate by 79%.Comment: v2: Revised paper based on referee reports, Theory section was
revised (primarily regarding finite key effects), Results section almost
completely rewritten with more experimental data. 16 pages, 5 figures. v1: 14
pages, 6 figures, higher resolution figures will be available in the
published articl
High rate, long-distance quantum key distribution over 250km of ultra low loss fibres
We present a fully automated quantum key distribution prototype running at
625 MHz clock rate. Taking advantage of ultra low loss fibres and low-noise
superconducting detectors, we can distribute 6,000 secret bits per second over
100 km and 15 bits per second over 250km
Composability in quantum cryptography
In this article, we review several aspects of composability in the context of
quantum cryptography. The first part is devoted to key distribution. We discuss
the security criteria that a quantum key distribution protocol must fulfill to
allow its safe use within a larger security application (e.g., for secure
message transmission). To illustrate the practical use of composability, we
show how to generate a continuous key stream by sequentially composing rounds
of a quantum key distribution protocol. In a second part, we take a more
general point of view, which is necessary for the study of cryptographic
situations involving, for example, mutually distrustful parties. We explain the
universal composability framework and state the composition theorem which
guarantees that secure protocols can securely be composed to larger
applicationsComment: 18 pages, 2 figure