1,209 research outputs found
Reconciliation of a Quantum-Distributed Gaussian Key
Two parties, Alice and Bob, wish to distill a binary secret key out of a list
of correlated variables that they share after running a quantum key
distribution protocol based on continuous-spectrum quantum carriers. We present
a novel construction that allows the legitimate parties to get equal bit
strings out of correlated variables by using a classical channel, with as few
leaked information as possible. This opens the way to securely correcting
non-binary key elements. In particular, the construction is refined to the case
of Gaussian variables as it applies directly to recent continuous-variable
protocols for quantum key distribution.Comment: 8 pages, 4 figures. Submitted to the IEEE for possible publication.
Revised version to improve its clarit
Cloning and Cryptography with Quantum Continuous Variables
The cloning of quantum variables with continuous spectra is investigated. We
define a Gaussian 1-to-2 cloning machine, which copies equally well two
conjugate variables such as position and momentum or the two quadrature
components of a light mode. The resulting cloning fidelity for coherent states,
namely , is shown to be optimal. An asymmetric version of this Gaussian
cloner is then used to assess the security of a continuous-variable quantum key
distribution scheme that allows two remote parties to share a Gaussian key. The
information versus disturbance tradeoff underlying this continuous quantum
cryptographic scheme is then analyzed for the optimal individual attack.
Methods to convert the resulting Gaussian keys into secret key bits are also
studied. The extension of the Gaussian cloner to optimal -to- continuous
cloners is then discussed, and it is shown how to implement these cloners for
light modes, using a phase-insensitive optical amplifier and beam splitters.
Finally, a phase-conjugated inputs -to- continuous cloner is
defined, yielding clones and anticlones from replicas of a
coherent state and replicas of its phase-conjugate (with ).
This novel kind of cloners is shown to outperform the standard -to-
cloners in some situations.Comment: 8 pages, 3 figures, submitted to the special issue of the European
Physical Journal D on "Quantum interference and cryptographic keys: novel
physics and advancing technologies", proceedings of the conference QUICK
2001, Corsica, April 7-13 2001. Minor correction, references adde
Perturbation of orthogonal polynomials on an arc of the unit circle
Orthogonal polynomials on the unit circle are completely determined by their
reflection coefficients through the Szeg\H{o} recurrences. We assume that the
reflection coefficients converge to some complex number a with 0 < |a| < 1. The
polynomials then live essentially on the arc {e^(i theta): alpha <= theta <= 2
pi - alpha} where cos alpha/2 = sqrt(1-|a|^2) with 0 <= alpha <= 2 pi. We
analyze the orthogonal polynomials by comparing them with the orthogonal
polynomials with constant reflection coefficients, which were studied earlier
by Ya. L. Geronimus and N. I. Akhiezer. In particular, we show that under
certain assumptions on the rate of convergence of the reflection coefficients
the orthogonality measure will be absolutely continuous on the arc. In
addition, we also prove the unit circle analogue of M. G. Krein's
characterization of compactly supported nonnegative Borel measures on the real
line whose support contains one single limit point in terms of the
corresponding system of orthogonal polynomials
Security of Quantum Key Distribution with Coherent States and Homodyne Detection
We assess the security of a quantum key distribution protocol relying on the
transmission of Gaussian-modulated coherent states and homodyne detection. This
protocol is shown to be equivalent to a squeezed state protocol based on a CSS
code construction, and is thus provably secure against any eavesdropping
strategy. We also briefly show how this protocol can be generalized in order to
improve the net key rate.Comment: 7 page
Secure Coherent-state Quantum Key Distribution Protocols with Efficient Reconciliation
We study the equivalence between a realistic quantum key distribution
protocol using coherent states and homodyne detection and a formal entanglement
purification protocol. Maximally-entangled qubit pairs that one can extract in
the formal protocol correspond to secret key bits in the realistic protocol.
More specifically, we define a qubit encoding scheme that allows the formal
protocol to produce more than one entangled qubit pair per coherent state, or
equivalently for the realistic protocol, more than one secret key bit. The
entanglement parameters are estimated using quantum tomography. We analyze the
properties of the encoding scheme and investigate its application to the
important case of the attenuation channel.Comment: REVTeX, 11 pages, 2 figure
Sexual and marital trajectories and HIV infection among ever-married women in rural Malawi.
OBJECTIVE: To explore how sexual and marital trajectories are associated with HIV infection among ever-married women in rural Malawi. METHODS: Retrospective survey data and HIV biomarker data for 926 ever-married women interviewed in the Malawi Diffusion and Ideational Change Project were used. The associations between HIV infection and four key life course transitions considered individually (age at sexual debut, premarital sexual activity, entry into marriage and marital disruption by divorce or death) were examined. These transitions were then sequenced to construct trajectories that represent the variety of patterns in the data. The association between different trajectories and HIV prevalence was examined, controlling for potentially confounding factors such as age and region. RESULTS: Although each life course transition taken in isolation may be associated with HIV infection, their combined effect appeared to be conditional on the sequence in which they occurred. Although early sexual debut, not marrying one's first sexual partner and having a disrupted marriage each increased the likelihood of HIV infection, their risk was not additive. Women who both delayed sexual debut and did not marry their first partner are, once married, more likely to experience marital disruption and to be HIV-positive. Women who marry their first partner but who have sex at a young age, however, are also at considerable risk. CONCLUSIONS: These findings identify the potential of a life course perspective for understanding why some women become infected with HIV and others do not, as well as the differentials in HIV prevalence that originate from the sequence of sexual and marital transitions in one's life. The analysis suggests, however, the need for further data collection to permit a better examination of the mechanisms that account for variations in life course trajectories and thus in lifetime probabilities of HIV infection
Experimental linear-optical implementation of a multifunctional optimal qubit cloner
We present the first experimental implementation of a multifunctional device
for the optimal cloning of one to two qubits. Previous implementations have
always been designed to optimize the cloning procedure with respect to one
single type of a priori information about the cloned state. In contrast, our
"all-in-one" implementation is optimal for several prominent regimes such as
universal cloning, phase-covariant cloning, and also the first ever realized
mirror phase-covariant cloning, when the square of the expected value of
Pauli's Z operator is known in advance. In all these regimes the experimental
device yields clones with almost maximum achievable average fidelity (97.5% of
theoretical limit). Our device has a wide range of possible applications in
quantum information processing, especially in quantum communication. For
instance, one can use it for incoherent and coherent attacks against a variety
of cryptographic protocols, including the Bennett-Brassard 1984 protocol of
quantum key distribution through the Pauli damping channels. It can be also
applied as a state-dependent photon multiplier in practical quantum networks.Comment: 9 pages, 6 figures, accepted to Phys. Rev. A (Rapid Communications
QKD in Standard Optical Telecommunications Networks
To perform Quantum Key Distribution, the mastering of the extremely weak
signals carried by the quantum channel is required. Transporting these signals
without disturbance is customarily done by isolating the quantum channel from
any noise sources using a dedicated physical channel. However, to really profit
from this technology, a full integration with conventional network technologies
would be highly desirable. Trying to use single photon signals with others that
carry an average power many orders of magnitude bigger while sharing as much
infrastructure with a conventional network as possible brings obvious problems.
The purpose of the present paper is to report our efforts in researching the
limits of the integration of QKD in modern optical networks scenarios. We have
built a full metropolitan area network testbed comprising a backbone and an
access network. The emphasis is put in using as much as possible the same
industrial grade technology that is actually used in already installed
networks, in order to understand the throughput, limits and cost of deploying
QKD in a real network
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