4,107 research outputs found
Robustness of the BB84 quantum key distribution protocol against general coherent attacks
It is demonstrated that for the entanglement-based version of the
Bennett-Brassard (BB84) quantum key distribution protocol, Alice and Bob share
provable entanglement if and only if the estimated qubit error rate is below
25% or above 75%. In view of the intimate relation between entanglement and
security, this result sheds also new light on the unconditional security of the
BB84 protocol in its original prepare-and-measure form. In particular, it
indicates that for small qubit error rates 25% is the ultimate upper security
bound for any prepare-and-measure BB84-type QKD protocol. On the contrary, for
qubit error rates between 25% and 75% we demonstrate that the correlations
shared between Alice and Bob can always be explained by separable states and
thus, no secret key can be distilled in this regime.Comment: New improved version. A minor mistake has been eliminate
Applications of single-qubit rotations in quantum public-key cryptography
We discuss cryptographic applications of single-qubit rotations from the
perspective of trapdoor one-way functions and public-key encryption. In
particular, we present an asymmetric cryptosystem whose security relies on
fundamental principles of quantum physics. A quantum public key is used for the
encryption of messages while decryption is possible by means of a classical
private key only. The trapdoor one-way function underlying the proposed
cryptosystem maps integer numbers to quantum states of a qubit and its
inversion can be infeasible by virtue of the Holevo's theorem.Comment: to appear in Phys. Rev.
Dynamics of matter-wave and optical fields in superradiant scattering from Bose-Einstein condensates
We study superradiant scattering off Bose-Einstein condensates by solving the
semiclassical Maxwell-Schroedinger equations describing the coupled dynamics of
matter-wave and optical fields. Taking the spatial dependence of these fields
along the condensate axis into account, we are able to reproduce and explain
many of the characteristic features observed in the experiments of Inouye et
al. [Science 285, 571 (1999)] and Schneble et al. [Science 300, 475 (2003)],
such as the shape of the atomic side-mode distributions for forward and
backward scattering, the spatial asymmetry between forward and backward side
modes, and the depletion of the condensate center observed for forward
scattering.Comment: 4 pages, 2 figure
Effective forecasting for supply-chain planning: an empirical evaluation and strategies for improvement
Demand forecasting is a crucial aspect of the planning process in supply-chain companies. The most common approach to forecasting demand in these companies involves the use of a simple univariate statistical method to produce a forecast and the subsequent judgmental adjustment of this by the company's demand planners to take into account market intelligence relating to any exceptional circumstances expected over the planning horizon. Based on four company case studies, which included collecting more than 12,000 forecasts and outcomes, this paper examines: i) the extent to which the judgmental adjustments led to improvements in accuracy, ii) the extent to which the adjustments were biased and inefficient, iii) the circumstances where adjustments were detrimental or beneficial, and iv) methods that could lead to greater levels of accuracy. It was found that the judgmentally adjusted forecasts were both biased and inefficient. In particular, market intelligence that was expected to have a positive impact on demand was used far less effectively than intelligence suggesting a negative impact. The paper goes on to propose a set of improvements that could be applied to the forecasting processes in the companies and to the forecasting software that is used in these processes
Analysis and minimization of bending losses in discrete quantum networks
We study theoretically the transfer of quantum information along bends in
two-dimensional discrete lattices. Our analysis shows that the fidelity of the
transfer decreases considerably, as a result of interactions in the
neighbourhood of the bend. It is also demonstrated that such losses can be
controlled efficiently by the inclusion of a defect. The present results are of
relevance to various physical implementations of quantum networks, where
geometric imperfections with finite spatial extent may arise as a result of
bending, residual stress, etc
- …