74 research outputs found
Impact of the slit geometry on the performance of wire-grid polarisers
Wire-grid polarisers are versatile and scalable components which can be
engineered to achieve small sizes and extremely high extinction ratios. Yet the
measured performances are always significantly below the predicted values
obtained from numerical simulations. Here we report on a detailed comparison
between theoretical and experimental performances. We show that the discrepancy
can be explained by the true shape of the plasmonic structures. Taking into
account the fabrication details, a new optimisation model enables us to achieve
excellent agreement with the observed response and to re-optimise the grating
parameters to ensure experimental extinction ratios well above 1,000 at 850 nm.Comment: 8 pages, 6 figure
Integrated Quantum Key Distribution sender unit for hand-held platforms
Mastering the generation, propagation and detection of electro-magnetic waves has enabled
a technological breakthrough that has changed our entire society. World-wide communication through the telephone and the internet has become an integral part of our daily-life,
which is expected to grow even further with the emergence of the internet of things. While
secure communication was of concern mostly for governmental and financial institutions,
digital security has now caught the attention of the general public. The weaknesses of cur-
rent encryption protocols, such as the existence of back-doors or the predicted breakdown
of popular algorithms such as RSA, reveal the need for alternative encryption schemes
ensuring unconditional security on all types of devices.
Quantum Key Distribution (QKD) has emerged as a powerful option to ensure a private
communication between two users. Based on the laws of quantum mechanics, this class of
protocols offers the possibility to detect the presence of a third party trying to intercept the
key during its distribution, and even to quantify the amount of leaked information. While
most research projects focus on long distance applications, little attention has been devoted
to short distance schemes such as wireless payment, network access and authentication,
which could highly benefit from QKD-enhanced security.
This thesis focuses on the development of a miniature QKD sender add-on that could
be embedded either in mobile devices or in existing optical communication platforms,
thus allowing for a secure key exchange with a shared dedicated receiver over a free-
space link. The proposed optics architecture (35 Ă— 20 Ă— 8 mm 3 ) is optimised for BB84-like
protocols and uses an array of four Vertical-Cavity Surface-Emitting Lasers with highly
similar properties to generate 40 ps long near-infrared faint coherent pulses at 100 MHz
repetition rate. Under strong modulation, the polarisation of the pulses is not well defined
and enables an external control of each diode’s emission by a wire-grid polariser. The
four beams are spatially overlapped in a polarisation-insensitive femtosecond laser written
waveguide array, and combined with a red beacon laser using an external beamsplitter to
ensure a stable, synchronised optical link with the receiver.
The complete module is compatible with current smartphone technology, allowing to
run the classical post-processing over WLAN in the future. First tests with a free-space
receiver indicate an average error ratio of 3.3 % and an asymptotic secure key rate of
54 kHz under static alignment. For the first time, a secure key exchange between a mobile
platform held by a user and a receiver equipped with a dynamic alignment system could
be demonstrated with an error ratio of 4.1 % and a secure key rate of 31 Hz. The further
optimisation of the experimental parameters and the implementation of a decoy protocol
will enhance the key generation rate as well as the general security of the system. The
results of this thesis pave the way towards unprecedented security in wireless optical
networks, as examplified for the communication between a mobile device and a dedicated
receiver.Die Fähigkeit, elektromagnetische Strahlung kontrolliert zu erzeugen, gerichtet zu emittieren sowie zu detektieren stellte einen technologischen Durchbruch dar, der die ganze
Gesellschaft verändert hat. Weltweite Kommunikation und Datenübertragung mittels
Radiowellen, dem Telefon und dem Internet wurde zu einem wesentlichen Bestandteil des
täglichen Lebens. Es ist zu erwarten, dass durch das Internet der Dinge die übertragene
Datenmenge weiter zunehmen wird. Während zunächst vor allem Regierungen und Banken
an Methoden für sichere Datenübertragung interessiert waren, wurde die Frage nach digitaler Sicherheit mit der Entwicklung mobiler Geräte, die immer mehr persönliche Daten
sammeln und übertragen, in die breite Offentlichkeit getragen. Die Schwachpunkte aktueller Verschlüsselungstechnologien, wie z.B mögliche Hintertüren in existierenden Implementierungen oder das in absehbarer Zeit erwartete Brechen des weit verbreiteten RSA-
Algorithmus, zeigen die Notwendigkeit alternativer Verfahren, deren Sicherheit nicht von
zusätzlichen, mitunter nicht überprüfbaren Annahmen abhängt.
Quantenschlüsselübertragung (engl. Quantum Key Distribution, QKD) stellt eine leistungsfähige Alternative dar, um verschlüsselte Kommunikation zwischen zwei Benutzern mithilfe eines gemeinsamen sicheren Schlüssels zu ermöglichen. Aufbauend auf den Gesetzen der Quantenmechanik ermöglicht es diese Klasse von Protokollen, eine dritte Partei beim Abhören des Schlüssels zu detektieren. Ebenso kann die Menge der möglicherweise abgefangenen Daten quantifiziert werden. Die meisten Forschungsprojekte konzentrierten sich bisher auf die Kommunikation über weite Strecken, wohingegen Anwendungen über
kurze Entfernungen, wie z.B. handybasierte Bezahlmethoden oder Zugang und Authentifizierung in einem Netzwerk weitgehend vernachlässigt wurden, obwohl auch diese Anwendungen von der verbesserten Sicherheit durch QKD profitieren könnten.
Die vorliegende Arbeit beschäftigt sich mit der Entwicklung einer miniaturisierten QKD
Sendeeinheit fĂĽr den sicheren SchlĂĽsselaustausch ĂĽber eine Freistrahlverbindung mit einem
Empfänger, wie sie als Erweiterung für mobile Geräte oder bestehende optische Kommunikationsinfrastruktur verwendet werden könnte. Das vorgeschlagene Design des optischen
Chips (35 × 20 × 8 mm³) ist für Protokolle, die sich an BB84 anlehnen optimiert und verwendet eine Anordnung von vier praktisch nicht zu unterscheidenden Oberflächenemittern
(Vertical-Cavity Surface-Emitting Laser, VCSEL) die schwache, kohärente Pulse mit 40 ps
Länge im nahen Infrarotbereich mit einer Wiederholrate von 100 MHz erzeugen. Bei starker Modulation ist die Polarisation der Pulse unbestimmt und kann daher mithilfe
eines Gitterpolarisators fĂĽr jede Diode separat eingestellt werden. Die vier Strahlen werden
räumlich in einem optischen Wellenleiter, der mit einem Femtosekundenlaser geschrieben
wurde und unabhängig von der Polarisation arbeitet, überlappt. Anschließend werden
sie an einem externen Strahlteiler mit einem weiteren Laser im sichtbaren Bereich zusammengeführt. Dieser dient dazu, eine synchronisierte Verbindung zum Empfänger herzustellen.
Das komplette Modul ist kompatibel mit aktueller Smartphonetechnik, wodurch ein klassischer Kanal, der für die Datennachbearbeitung benötigt wird, über LTE oder WLAN zur Verfügung gestellt werden kann. Erste Freistrahltests, bei denen Sender und Empfänger fixiert waren, ergaben eine durchschnittliche Fehlerrate von 3,3 % und eine asymptotische sichere Schlüsselrate von 54 kHz. Zum ersten Mal konnte auch ein sicherer Schlüsselaustausch zwischen einem vom Nutzer in der Hand gehaltenem mobilen Gerät und einem mit einem dynamischen Justagesystem ausgestatteten Empfänger gezeigt werden. Die Fehlerrate lag hierbei bei 4,1 % und die sichere Schlüsselrate bei 31 Hz. Durch eine weitere Optimierung der experimentellen Parameter sowie der Implementierung eines sogenannten
Decoyprotokolls wird sich die SchlĂĽsselrate sowie die Sicherheit des Systems noch deutlich
erhöhen lassen. Zusammenfassend stellen diese Ergebnisse, exemplarisch gezeigt anhand der Kommunikation zwischen einem mobilen Gerät und einem stationren Empfänger, einen ersten
Schritt hin zu bisher unerreichter Sicherheit in drahltlosen Netzwerken dar
ISO-SWS calibration and the accurate modelling of cool-star atmospheres - II. General results
The fine calibration of the ISO-SWS detectors (Infrared Space Observatory -
Short Wavelength Spectrometer) has proven to be a delicate problem. We
therefore present a detailed spectroscopic study in the 2.38 -- 12 micron
wavelength range of a sample of 16 A0 -- M2 stars used for the calibration of
ISO-SWS. By investigating the discrepancies between the ISO-SWS data of these
sources, the theoretical predictions of their spectra, the high-resolution
FTS-KP (Kitt Peak) spectrum of Alpha Boo and the solar FTS-ATMOS (Atmospheric
Trace Molecule Spectroscopy) spectrum, both calibration problems and problems
in computing the theoretical models and the synthetic spectra are revealed. The
underlying reasons for these problems are sought for and the impact on the
further calibration of ISO-SWS and on the theoretical modelling is discussed
extensively.Comment: 17 pages, 19 figures; Astronomy and Astrophysics, in press; preprints
can be obtained by contacting [email protected] or via WWW on
http://www.ster.kuleuven.ac.be/~leen or via anonymous ftp on
ftp://ftp.ster.kuleuven.ac.be/dist/leen/latex/h3317 Appendix electronically
available (3 pages, 7 figures
IRAK4 gene polymorphism and odontogenic maxillary sinusitis
Objectives This study aimed to evaluate whether a specific interleukin-1 receptor-associated kinase-4 (IRAK4) gene polymorphism had any influence on the development of changes in maxillary sinus, particularly in the presence of etiological factors of dental origin.Materials and methods The study population included 153 Portuguese Caucasians that were selected from a database of 504 retrospectively analysed computed tomography (CT) scans. A genetic test was performed, and a model was created through logistic analysis and regression coefficients. The statistical methodologies included were the independent Chi test, Fisher's exact test, binary logistic regression and the receiver operating characteristic (ROC) curve.Results The estimated prevalence of IRAK4 gene polymorphism found in a Portuguese Caucasian population was 26.8 % (CI 95 %) [20.1, 34.7 %]. A model to predict the inflammatory response in the maxillary sinus in the presence etiological factors of dental origin was constructed. This model had the following as variables: previously diagnosed sinusitis, sinus pressure symptoms, cortical bone loss observed on CT, positive genetic test result and radiographic examination that revealed the roots of the teeth communication with the maxillary sinus, which are interpreted as risk factors.Conclusions The constructed model should be considered an initial clinical tool. The area under the ROC curve found, AUC=0.91, revealed that the model correctly predicts the outcome in 91.1 % of cases.info:eu-repo/semantics/publishedVersio
Early-life respiratory tract infections and the risk of school-age lower lung function and asthma: A meta-analysis of 150 000 European children
BACKGROUND: Early-life respiratory tract infections might affect chronic obstructive respiratory diseases, but conclusive studies from general populations are lacking. Our objective was to examine if children with early-life respiratory tract infections had increased risks of lower lung function and asthma at school age. METHODS: We used individual participant data of 150 090 children primarily from the EU Child Cohort Network to examine the associations of upper and lower respiratory tract infections from age 6 months to 5 years with forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), FEV1/FVC, forced expiratory flow at 75% of FVC (FEF75%) and asthma at a median (range) age of 7 (4-15) years. RESULTS: Children with early-life lower, not upper, respiratory tract infections had a lower school-age FEV1, FEV1/FVC and FEF75% (z-score range: -0.09 (95% CI -0.14- -0.04) to -0.30 (95% CI -0.36- -0.24)). Children with early-life lower respiratory tract infections had a higher increased risk of school-age asthma than those with upper respiratory tract infections (OR range: 2.10 (95% CI 1.98-2.22) to 6.30 (95% CI 5.64-7.04) and 1.25 (95% CI 1.18-1.32) to 1.55 (95% CI 1.47-1.65), respectively). Adjustment for preceding respiratory tract infections slightly decreased the strength of the effects. Observed associations were similar for those with and without early-life wheezing as a proxy for early-life asthma. CONCLUSIONS: Our findings suggest that early-life respiratory tract infections affect development of chronic obstructive respiratory diseases in later life, with the strongest effects for lower respiratory tract infections
L'effet de l'information nouvelle erronée
Summary : The postevent information effect.
There is a renewal of interest in eyewitness testimony research in the experimental psychology of memory. However, as far as the postevent information effect is concerned, « mnesic » explanations have been seriously called into question by the Response Bios hypoihesis (McCloskey and Zaragoza, 1985). In this paper, we propose reasons for going beyond the criticisms suggested in this context and to examine the mechanisms responsible for interferences according to the Coexistence hypothesis.
Key-words : postevent information effect, integration, coexistence, differential accessibility.Résumé
Depuis une quinzaine d'années, les recherches sur le témoignage oculaire connaissent un renouveau en psychologie expérimentale de la mémoire. Cependant, en ce qui concerne l'effet de l'information nouvelle erronée, les explications « mnésiques » ont été sévèrement remises en question par l'hypothèse de Biais de Réponse (McCloskey et Zaragoza, 1985). Dans cet article on propose des raisons d'aller au-delà des critiques émises dans ce cadre pour examiner les mécanismes avancés par l'hypothèse de la Coexistence.
Mots clés : effet de l'information nouvelle erronée, intégration, coexistence, accessibilité différentielle.Mélen Marc. L'effet de l'information nouvelle erronée. In: L'année psychologique. 1989 vol. 89, n°3. pp. 393-410
Recent Characteristic Budgets of Inorganic Chlorine and Fluorine above the Jungfraujoch Station
Within the frame of NDSC (Network for the Detection of Stratospheric Change)-related monitoring activities conducted at the International Scientific Station of the Jungfraujoch (Swiss Alps, 46.5°N, 8.0°E, 3580 m a.s.l.), long-term investigations of the inorganic chlorine and fluorine budgets have been pursued during the last years. They are based on the analysis of high-resolution infrared solar spectra, recorded with grating and Fourier transform state-of-the-art spectrometers. Nonlinear least squares fittings of synthetic spectra to the observations are performed over selected intervals, encompassing characteristic absorptions of HCl, ClONO2, HF and COF2; this allows the retrieval of their total vertical column abundances above the Jungfraujoch and the evaluation of their budgets.GIRPA
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