681 research outputs found
Unidimensional continuous-variable quantum key distribution
We propose the continuous-variable quantum key distribution protocol based on
the Gaussian modulation of a single quadrature of the coherent states of light,
which is aimed to provide simplified implementation compared to the
symmetrically modulated Gaussian coherent-state protocols. The protocol waives
the necessity in phase quadrature modulation and the corresponding channel
transmittance estimation. The security of the protocol against collective
attacks in a generally phase-sensitive Gaussian channels is analyzed and is
shown achievable upon certain conditions. Robustness of the protocol to channel
imperfections is compared to that of the symmetrical coherent-state protocol.
The simplified unidimensional protocol is shown possible at a reasonable
quantitative cost in terms of key rate and of tolerable channel excess noise.Comment: 7 pages, 5 figures, close to the published versio
Trusted Noise in Continuous-Variable Quantum Key Distribution: a Threat and a Defense
We address the role of the phase-insensitive trusted preparation and
detection noise in the security of a continuous-variable quantum key
distribution, considering the Gaussian protocols on the basis of coherent and
squeezed states and studying them in the conditions of Gaussian lossy and noisy
channels. The influence of such a noise on the security of Gaussian quantum
cryptography can be crucial, even despite the fact that a noise is trusted, due
to a strongly nonlinear behavior of the quantum entropies involved in the
security analysis. We recapitulate the known effect of the preparation noise in
both direct and reverse-reconciliation protocols, as well as the detection
noise in the reverse-reconciliation scenario. As a new result, we show the
negative role of the trusted detection noise in the direct-reconciliation
scheme. We also describe the role of the trusted preparation or detection noise
added at the reference side of the protocols in improving the robustness of the
protocols to the channel noise, confirming the positive effect for the
coherent-state reverse-reconciliation protocol. Finally, we address the
combined effect of trusted noise added both in the source and the detector.Comment: 25 pages, 9 figure
Distributing Secret Keys with Quantum Continuous Variables: Principle, Security and Implementations
The ability to distribute secret keys between two parties with
information-theoretic security, that is, regardless of the capacities of a
malevolent eavesdropper, is one of the most celebrated results in the field of
quantum information processing and communication. Indeed, quantum key
distribution illustrates the power of encoding information on the quantum
properties of light and has far reaching implications in high-security
applications. Today, quantum key distribution systems operate in real-world
conditions and are commercially available. As with most quantum information
protocols, quantum key distribution was first designed for qubits, the
individual quanta of information. However, the use of quantum continuous
variables for this task presents important advantages with respect to qubit
based protocols, in particular from a practical point of view, since it allows
for simple implementations that require only standard telecommunication
technology. In this review article, we describe the principle of
continuous-variable quantum key distribution, focusing in particular on
protocols based on coherent states. We discuss the security of these protocols
and report on the state-of-the-art in experimental implementations, including
the issue of side-channel attacks. We conclude with promising perspectives in
this research field.Comment: 21 pages, 2 figures, 1 tabl
General entropy-like uncertainty relations in finite dimensions
We revisit entropic formulations of the uncertainty principle for an
arbitrary pair of positive operator-valued measures (POVM) and , acting
on finite dimensional Hilbert space. Salicr\'u generalized
-entropies, including R\'enyi and Tsallis ones among others, are used
as uncertainty measures associated with the distribution probabilities
corresponding to the outcomes of the observables. We obtain a nontrivial lower
bound for the sum of generalized entropies for any pair of entropic
functionals, which is valid for both pure and mixed states. The bound depends
on the overlap triplet with (resp. ) being the
overlap between the elements of the POVM (resp. ) and the
overlap between the pair of POVM. Our approach is inspired by that of de
Vicente and S\'anchez-Ruiz [Phys.\ Rev.\ A \textbf{77}, 042110 (2008)] and
consists in a minimization of the entropy sum subject to the Landau-Pollak
inequality that links the maximum probabilities of both observables. We solve
the constrained optimization problem in a geometrical way and furthermore, when
dealing with R\'enyi or Tsallis entropic formulations of the uncertainty
principle, we overcome the H\"older conjugacy constraint imposed on the
entropic indices by the Riesz-Thorin theorem. In the case of nondegenerate
observables, we show that for given , the bound
obtained is optimal; and that, for R\'enyi entropies, our bound improves
Deutsch one, but Maassen-Uffink bound prevails when .
Finally, we illustrate by comparing our bound with known previous results in
particular cases of R\'enyi and Tsallis entropies
Theoretical and Practical Advances in Computer-based Educational Measurement
This open access book presents a large number of innovations in the world of operational testing. It brings together different but related areas and provides insight in their possibilities, their advantages and drawbacks. The book not only addresses improvements in the quality of educational measurement, innovations in (inter)national large scale assessments, but also several advances in psychometrics and improvements in computerized adaptive testing, and it also offers examples on the impact of new technology in assessment. Due to its nature, the book will appeal to a broad audience within the educational measurement community. It contributes to both theoretical knowledge and also pays attention to practical implementation of innovations in testing technology
Enhanced Uplink Quantum Communication with Satellites via Downlink Channels
In developing the global Quantum Internet, quantum communication with
low-Earth-orbit satellites will play a pivotal role. Such communication will
need to be two way: effective not only in the satellite-to-ground (downlink)
channel but also in the ground-to-satellite channel (uplink). Given that losses
on this latter channel are significantly larger relative to the former,
techniques that can exploit the superior downlink to enhance quantum
communication in the uplink should be explored. In this work we do just that -
exploring how continuous variable entanglement in the form of two-mode squeezed
vacuum (TMSV) states can be used to significantly enhance the fidelity of
ground-to-satellite quantum-state transfer relative to direct uplink-transfer.
More specifically, through detailed phase-screen simulations of beam evolution
through turbulent atmospheres in both the downlink and uplink channels, we
demonstrate how a TMSV teleportation channel created by the satellite can be
used to dramatically improve the fidelity of uplink coherent-state transfer
relative to direct transfer. We then show how this, in turn, leads to the
uplink-transmission of a higher alphabet of coherent states. Additionally, we
show how non-Gaussian operations acting on the received component of the TMSV
state at the ground station can lead to even further enhancement. Since TMSV
states can be readily produced in situ on a satellite platform and form a
reliable teleportation channel for most quantum states, our work suggests
future satellites forming part of the emerging Quantum Internet should be
designed with uplink-communication via TMSV teleportation in mind
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