116 research outputs found
On the equilibrium problem and infinitesimal mechanisms of class theta tensegrity systems
No full textThis work presents a study on the equilibrium problem and the infinitesimal mechanisms of class θ= 1 tensegrity prisms. Local solutions of the self-equilibrium problem are numerically obtained through Newton-Raphson iterations. The presented results suggest that the analyzed structures can be usefully employed as building blocks of novel tensegrity metamaterials, due to their rich kinematic response and the considerably large number of infinitesimal mechanisms. © 2019 Author(s)
Goodness-of-fit tests in conditional duration models
No full textWe propose specification tests for the innovation distribution in conditional duration models. The new tests are based either on the cumulative distribution function, or on exponential transforms such as the Laplace transform and the characteristic function, or on characterizations of the innovation-distribution under test. We study the finite-sample performance of the proposed procedures in comparison with alternative tests which employ nonparametric density estimates as well as with tests based on entropy. A bootstrap version of the tests is utilized in order to study the small sample behavior of the procedures. A real-data example illustrates the applicability of our method and confirms conclusions drawn by earlier author
Full 3D CAD procedure for the speedy evaluation of the seismic vulnerability of masonry towers
No full textA very straightforward 3D CAD approach for the speedy evaluation of the seismic vulnerability of existing masonry towers is presented. The procedure requires only the detailed 3D geometric model of the structure and automatically calculates the collapse acceleration on a user defined failure mechanism. In this paper, few pre-assigned mechanisms are tested, as for instance vertical splitting, simple overturning at the base, rocking with inclined yield lines and combined rocking and vertical splitting. The restriction of the possible tower failure within such a few mechanisms grounds on previous numerical research in the field and post-earthquake surveys experience. In any case, any user can define his own mechanisms according to the specificity of the case-study under consideration, directly shaping distinct volumes inside the CAD software. The procedure is automatized and the direct application of the principle of virtual works-assuming that masonry behaves as a no-tension material-allows the immediate evaluation of the horizontal acceleration at collapse. The mechanism associated to the minimum acceleration, in agreement with the kinematic theorem of limit analysis, is that most probably would occur in reality during a seismic event. The approach allows a straightforward evaluation of the seismic vulnerability of a tower and can be used even by practitioners not familiar with advanced FE computations and limit analysis concepts, so adapting well to the heterogeneous community involved in cultural heritage preservation. The automatized procedure is applied in this paper to a historical tower located in central Italy, to show the capabilities of the approach. © 2019 Author(s)
Study of all-optical wavelength conversion and regeneration subsystems for use in wavelength division multiplexing (WDM) telecommunication networks
No full textIn this thesis, we study all-optical processing techniques based on non-linear semiconductor optical amplifiers for the functionalities of second and third generation optical networks. In particular, devices for all-optical wavelength conversion with regenerative properties have been investigated. The configurations under investigation are based on the non-linear four-wave mixing process which arises when fields with proper spectral and power arrangement are coupled into the active medium. The performance characteristics of the devices are determined by the properties of the optical amplifier as well as by the operating conditions like the gain of the optical amplifier, the power levels and the frequency detuning of the pump and the information signal. In the first part of this thesis, the noise properties of the converted signal generated by the four-wave mixing based wavelength conversion process were investigated. The investigation of the noise characteristics was carried out by a theoretical analysis and the corresponding experimental confirmation, including the separate study of noise induced by the four wave mixing process itself and the amplified spontaneous emission noise from the amplifier. The study of the noise properties at the new wavelength was performed for the intensity as well as the phase noise of the converted signal. The useful conclusions from this study were used for the investigation of the regenerative properties of the mixing process. The second part of this thesis starts with the experimental investigation and the confirmation of the regenerative properties of the four wave mixing process in a semiconductor optical amplifier. The regeneration and simultaneous wavelength conversion of 2.5 Gbps optical signals is experimentally demonstrated. Furthermore, the second part includes the design and optimization by numerical simulation of a wavelength conversion system with regenerative properties, for use in nodes of wavelength division multiplexing optical networks. In this thesis, an alternative configuration for the pump and data signal is proposed for the first time, in order to obtain non-linear response from the optical amplifier. Based on this approach the new signal exhibits improved noise characteristics and lower bit error rate. The numerical investigation showed successful regenerative operation at 40 Gbps.Σε αυτή τη διατριβή μελετούμε νέες τεχνικές αμιγώς οπτικής επεξεργασίας σήματος για οπτικά δίκτυα δεύτερης και τρίτης γενιάς, οι οποίες βασίζονται σε μη γραμμικούς ημιαγωγικούς οπτικούς ενισχυτές. Συγκεκριμένα, διερευνήθηκαν διατάξεις οι οποίες επιτελούν λειτουργίες μετατροπής μήκους κύματος με αναγεννητικά χαρακτηριστικά. Οι διατάξεις εκμεταλλεύονται το μη γραμμικό φαινόμενο της μίξης τεσσάρων κυμάτων που εγείρεται εφόσον κύματα κατάλληλης ισχύος και φασματικής απόστασης εισαχθούν σε ενεργό μέσο. Οι επιδόσεις και οι ιδιότητες των διατάξεων επεξεργασίας σήματος καθορίζονται από τα χαρακτηριστικά των οπτικών ενισχυτών, καθώς και από τις συνθήκες λειτουργίας όπως η απολαβή του οπτικού ενισχυτή, η ισχύς και η φασματική απόσταση των κυμάτων άντλησης και πληροφορίας που χρησιμοποιούνται. Στο πρώτο μέρος της εργασίας αυτής παρέχεται μια θεωρητική ανάλυση και η πειραματική επιβεβαίωση των ιδιοτήτων θορύβου της μετατροπής μήκους κύματος μέσω της διαδικασίας της μίξης τεσσάρων κυμάτων. Μελετώνται ξεχωριστά η επίδραση της διαδικασίας της μίξης τεσσάρων κυμάτων και του θορύβου αυθόρμητης εκπομπής από τον οπτικό ενισχυτή στα χαρακτηριστικά θορύβου του σήματος στο νέο μήκος κύματος, για δύο περιπτώσεις. Η πρώτη περίπτωση αφορά τον θόρυβο πλάτους και η δεύτερη τον θόρυβο φάσης του σήματος στην έξοδο. Από την μελέτη εξάγονται χρήσιμα συμπεράσματα τα οποία χρησιμοποιούνται για την διερεύνηση των αναγεννητικών χαρακτηριστικών της διαδικασίας της μίξης. Στο δεύτερο μέρος της εργασίας αυτής πραγματοποιείται η πειραματική επιβεβαίωση των αναγεννητικών χαρακτηριστικών της διαδικασίας της μείξης τεσσάρων κυμάτων σε οπτικό ημιαγωγικό ενισχυτή. Επιτυγχάνεται πειραματικά η αναγέννηση διαμορφωμένων οπτικών σημάτων ρυθμού 2.5 Gbps με ταυτόχρονη μετατροπή μήκους κύματος. Στην συνέχεια πραγματοποιείται η σχεδίαση και η βελτιστοποίηση μέσω αριθμητικής προσομοίωσης ενός συστήματος μετατροπής μήκους κύματος με ιδιότητες αναγέννησης διαμορφωμένων οπτικών σημάτων, για χρήση σε κόμβους δικτύων πολυπλεξίας μήκους κύματος. Στην εργασία αυτή προτείνεται για πρώτη φορά μια εναλλακτική διάταξη των κυμάτων άντλησης και πληροφορίας για την εκμετάλλευση της μη γραμμικής απόκρισης του οπτικού ενισχυτή και προκειμένου το νέο κύμα να φέρει βελτιωμένα χαρακτηριστικά θορύβου και άρα μικρότερο ρυθμό σφαλμάτων. Η θεωρητική διερεύνηση έδειξε επιτυχή λειτουργία σε ρυθμούς μέχρι 40 Gbps
Sub-wavelength resolution dynamics of semiconductor passively mode-locked lasers induced by optical feedback
No full textQuantum dot arrays in silicon and germanium
No full textElectrons and holes confined in quantum dots define excellent building blocks for quantum emergence, simulation, and computation. Silicon and germanium are compatible with standard semiconductor manufacturing and contain stable isotopes with zero nuclear spin, thereby serving as excellent hosts for spins with long quantum coherence. Here, we demonstrate quantum dot arrays in a silicon metal-oxide-semiconductor (SiMOS), strained silicon (Si/SiGe), and strained germanium (Ge/SiGe). We fabricate using a multi-layer technique to achieve tightly confined quantum dots and compare integration processes. While SiMOS can benefit from a larger temperature budget and Ge/SiGe can make an Ohmic contact to metals, the overlapping gate structure to define the quantum dots can be based on a nearly identical integration. We realize charge sensing in each platform, for the first time in Ge/SiGe, and demonstrate fully functional linear and two-dimensional arrays where all quantum dots can be depleted to the last charge state. In Si/SiGe, we tune a quintuple quantum dot using the N + 1 method to simultaneously reach the few electron regime for each quantum dot. We compare capacitive crosstalk and find it to be the smallest in SiMOS, relevant for the tuning of quantum dot arrays. We put these results into perspective for quantum technology and identify industrial qubits, hybrid technology, automated tuning, and two-dimensional qubit arrays as four key trajectories that, when combined, enable fault-tolerant quantum computation.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QCD/Veldhorst LabQCD/Vandersypen LabQCD/Scappucci LabBUS/Quantum DelftQN/Vandersypen La
Hot qubits in silicon for quantum computation
No full textThe understanding of quantum mechanics enabled the development of technology such as transistors and has been the foundation of today’s information age. Actively using quantum mechanics to build quantum technology may cause a second revolution in handling information. However, to execute meaningful algorithms, largescale quantum computers have to be built. Such systems are constructed from many qubits, the quantum version of the classical bit. While exciting progress is being made across a range of different qubit platforms, achieving the radical scalability that is necessary to build a largescale processor could be a roadblock. Huge challenges are put on reproducibility, inand output connectivity and material quality. Qubits based on the spins of electrons and holes confined in semiconductor quantum dots may have an important advantage in constructing quantum processors. This platform can profit from the advanced semiconductor industry that was responsible for the first computing revolution. Group IV semiconductors such as silicon and germanium have a high compatibility with industrial semiconductor manufacturing and contain stable isotopes with zero nuclear spin. The materials can be isotopically purified and serve as excellent hosts for spins with long quantum coherence. In Chapter 3 we present quantum dot arrays in silicon metaloxidesemiconductor (SiMOS), strained silicon (Si/SiGe) and strained germanium (Ge/SiGe). A nearly identical integration scheme based on an overlapping gate structure can be used to define quantum dots in each platform. Each platform has its own opportunities, which are carefully assessed. By employing charge sensing we confirm that all quantum dots can be depleted to the singleelectron regime. We compare capacitive crosstalk and find it to be the smallest in SiMOS, relevant for the tuning of quantum dot arrays. Using this crossplatform integration, we can study qubits in each platform with minimal overhead. Long coherence times, excellent singlequbit gate fidelities and twoqubit logic have been demonstrated with SiMOS spin qubits, making it one of the leading platforms for quantum information processing. However, due to the high disorder at the Si/SiO2 interface compared to Ge/SiGe and Si/SiGe interface, quantum dots defined in SiMOS are small and achieving sufficient control over single electrons has been a long standing challenge. In Chapter 5 we show experiments on a double quantum dot that can be isolated from its reservoir. We demonstrate a tunable tunnel coupling between single electrons up to 13 GHz and tunable tunnel rates down to below 1 Hz. These results mark an important step towards the required degree of control over the location of and coupling between quantum dots, necessary for the operation of a large array.QCD/Veldhorst La
Compact optical displacement sensing by detection of microwave signals generated from a monolithic passively mode-locked laser under feedback
No full textCompact optical displacement sensing by detection of microwave signals generated from a monolithic passively mode-locked laser under feedback
No full textA monolithic passively mode-locked laser is proposed as a compact optical sensor for displacements and vibrations of a reflecting object. The sensing principle relies on the change of the laser repetition frequency that is induced by optical feedback from the object under measurement. It has been previously observed that, when a semiconductor passively mode locked laser receives a sufficient level of optical feedback from an external reflecting surface it exhibits a repetition frequency that is no more determined by the mode-locking rule of the free-running operation but is imposed by the length of the external cavity. Therefore measurement of the resulting laser repetition frequency under self-injection permits the accurate and straightforward determination of the relative position of the reflecting object. The system has an inherent wireless capability since the repetition rate of the laser can be wirelessly detected by means of a simple antenna which captures the microwave signal generated by the saturable absorber and is emitted through the wiring of the laser. The sensor setup is very simple as it requires few optical components besides the laser itself. Furthermore, the deduction of the relative position of the reflecting object is straightforward and does not require any processing of the detected signal. The proposed sensor has a theoretical sub-wavelength resolution and its performance depends on the RF linewidth of the laser and the resolution of the repetition frequency measurement. Other physical parameters that induce phase changes of the external cavity could also be quantified
- …
