Quantum key distribution with realistic states: photon-number statistics in the photon-number splitting attack

Quantum key distribution can be performed with practical signal sources such as weak coherent pulses. One example of such a scheme is the Bennett-Brassard protocol that can be implemented via polarization of the signals, or equivalent signals. It turns out that the most powerful tool at the disposition of an eavesdropper is the photon-number splitting attack. We show that this attack can be extended in the relevant parameter regime such as to preserve the Poissonian photon number distribution of the combination of the signal source and the lossy channel.Comment: 4 page

Experimental demonstration of four-party quantum secret sharing

Secret sharing is a multiparty cryptographic task in which some secret information is splitted into several pieces which are distributed among the participants such that only an authorized set of participants can reconstruct the original secret. Similar to quantum key distribution, in quantum secret sharing, the secrecy of the shared information relies not on computational assumptions, but on laws of quantum physics. Here, we present an experimental demonstration of four-party quantum secret sharing via the resource of four-photon entanglement

Measurable nonlocal effect of bipartite system during a local cyclic evolution of its subsystem

In this letter, a nonlocal effect for a bipartite system which is induced by a local cyclic evolution of one of its subsystem is suggested. This effect vanishes when the system is at a disentangled pure state but can be observed for some disentangled mixed states. As a paradigm, we study the effect for the system of two qubits in detail. It is interesting that the effect is directly related to the degree of entanglement for pure state of qubit pairs. Furthermore, we suggest a Bell-type experiment to measure this nonlocal effect for qubit pairs.Comment: 5 pages, 2 figure

Experimental fully contextual correlations

Quantum correlations are contextual yet, in general, nothing prevents the existence of even more contextual correlations. We identify and test a noncontextuality inequality in which the quantum violation cannot be improved by any hypothetical postquantum theory, and use it to experimentally obtain correlations in which the fraction of noncontextual correlations is less than 0.06. Our correlations are experimentally generated from the results of sequential compatible tests on a four-state quantum system encoded in the polarization and path of a single photon.Comment: REVTeX4, 6 pages, 3 figure

Unconditional security at a low cost

By simulating four quantum key distribution (QKD) experiments and analyzing one decoy-state QKD experiment, we compare two data post-processing schemes based on security against individual attack by L\"{u}tkenhaus, and unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our results show that these two schemes yield close performances. Since the Holy Grail of QKD is its unconditional security, we conclude that one is better off considering unconditional security, rather than restricting to individual attacks.Comment: Accepted by International Conference on Quantum Foundation and Technology: Frontier and Future 2006 (ICQFT'06

Méthodes de traitement d'antenne revisitées

Dans cette étude nous développons une nouvelle méthode pour la détection et la localisation de sources par les techniques dites à haute résolution. Elle exploite les propriétés algébriques des matrices issues des factorisations de la matrice interspectrale des signaux reçus. Le gros avantage de cette méthode est l'amélioration de la localisation en présence de faible rapport signal sur bruit

Méthodes de Traitement d'Antenne pour la caractérisation d'objets en mouvement à partir d'une séquence d'images

Au cours de ces dernières années, un certain nombre de méthodes déterministes ou statistiques ont été proposées pour caractériser des droites à partir d'une séquence d'images. En général, les performances de ces méthodes se dégradent en présence d'un bruit ou de pixels isolés et surtout en présence de droites de pentes très voisines. Dans cette étude, nous utilisons les méthodes de traitement d'antenne pour améliorer le pouvoir séparateur afin d'identifier au mieux des droites de pentes voisines. Nous proposons aussi une extension de ces méthodes pour estimer les paramètres cinématiques (coordonnées et vitesses) des objets en mouvement. Cette extension est possible dans le cas où les paramètres cinématiques varient lentement au cours du temps

Performance of various quantum key distribution systems using 1.55 um up-conversion single-photon detectors

We compare the performance of various quantum key distribution (QKD) systems using a novel single-photon detector, which combines frequency up-conversion in a periodically poled lithium niobate (PPLN) waveguide and a silicon avalanche photodiode (APD). The comparison is based on the secure communication rate as a function of distance for three QKD protocols: the Bennett-Brassard 1984 (BB84), the Bennett, Brassard, and Mermin 1992 (BBM92), and the coherent differential phase shift keying (DPSK). We show that the up-conversion detector allows for higher communication rates and longer communication distances than the commonly used InGaAs/InP APD for all the three QKD protocols.Comment: 9 pages, 9 figure

Hand gesture recognition system based in computer vision and machine learning

"Lecture notes in computational vision and biomechanics series, ISSN 2212-9391, vol. 19"Hand gesture recognition is a natural way of human computer interaction and an area of very active research in computer vision and machine learning. This is an area with many different possible applications, giving users a simpler and more natural way to communicate with robots/systems interfaces, without the need for extra devices. So, the primary goal of gesture recognition research applied to Human-Computer Interaction (HCI) is to create systems, which can identify specific human gestures and use them to convey information or controlling devices. For that, vision-based hand gesture interfaces require fast and extremely robust hand detection, and gesture recognition in real time. This paper presents a solution, generic enough, with the help of machine learning algorithms, allowing its application in a wide range of human-computer interfaces, for real-time gesture recognition. Experiments carried out showed that the system was able to achieve an accuracy of 99.4% in terms of hand posture recognition and an average accuracy of 93.72% in terms of dynamic gesture recognition. To validate the proposed framework, two applications were implemented. The first one is a real-time system able to help a robotic soccer referee judge a game in real time. The prototype combines a vision-based hand gesture recognition system with a formal language definition, the Referee CommLang, into what is called the Referee Command Language Interface System (ReCLIS). The second one is a real-time system able to interpret the Portuguese Sign Language. Sign languages are not standard and universal and the grammars differ from country to country. Although the implemented prototype was only trained to recognize the vowels, it is easily extended to recognize the rest of the alphabet, being a solid foundation for the development of any vision-based sign language recognition user interface system.(undefined

Quantum dense coding over Bloch channels

Dynamics of coded information over Bloch channels is investigated for different values of the channel's parameters. We show that, the suppressing of the travelling coded information over Bloch channel can be increased by decreasing the equilibrium absolute value of information carrier and consequently decreasing the distilled information by eavesdropper. The amount of decoded information can be improved by increasing the equilibrium values of the two qubits and decreasing the ratio between longitudinal and transverse relaxation times. The robustness of coded information in maximum and partial entangled states is discussed. It is shown that the maximum entangled states are more robust than the partial entangled state over this type of channels