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• optoelectronic oscillator
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• Over the last few decades, quantum cryptography has been one of the most important quantum technologies developed. It holds the promise of communicating secure information over untrusted channels. In particular, quantum cryptography is immune to the eventual threats posed by large-scale quantum computers, which is not the case for our current classical cryptography infrastructures. In a world where information, communication and privacy are of paramount importance, the development of secure quantum communication infrastructures becomes imperative. So far, most of the quantum cryptographic systems developed to date are based on two-dimensional encoding schemes, analogous to classical bits, known as qubits. Nevertheless, other than for the polarization of light, photonic degrees of freedom, i.e. position, momentum, time and frequency, naturally occur as high-dimensional quantum systems. Moreover, by considering high-dimensional encoding schemes beyond qubits, advantages in terms of information capacity and noise tolerance are predicted in theory. In this thesis, the basic components of a high-dimensional quantum cryptographic system are investigated. We begin by reviewing important developments in quantum cryptography and high-dimensional quantum information. As a starting point, two different quantum information tasks, i.e. optimal quantum cloning and quantum metrology, are experimentally investigated for high-dimensional quantum systems in order to demonstrate the numerous advantages of performing quantum tasks beyond qubits. The photonic degree of freedom used in these experiments, known as transverse spatial modes, is obtained by combining the position and momentum photonics degrees of freedom. Thus, we develop a novel method to measure high-dimensional quantum states encoded in arbitrary spatial modes. Using a similar characterization technique, several high-dimensional quantum communication channels are characterized to perform high-dimensional quantum cryptography. The tools developed in the laboratory are then brought to realistic conditions in order to test the feasibility of high-dimensional quantum cryptography. Two different types of quantum channels are experimentally investigated, namely an intra-city 300 m long free-space link and a short 3 m outdoor underwater link. Finally, we investigate and compare novel high-dimensional protocols to overcome current limitations.
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• The present thesis attempts to develop new techniques for testing analog parts of embedded cores-based mixed signal integrated circuits and systems. In particular, the oscillation based test methodologies have been investigated in the thesis. In the oscillation based test methods, the circuit under test (CUT) is first converted to an oscillator in the test mode and the oscillation parameters, viz. frequency, amplitude, etc. are then measured. Any deviation of these parameters causes either the oscillation frequency of the converted CUT to differ from its nominal value, or the converted CUT stops oscillation altogether. For evaluation purpose, a program has been written in C to help us in simulating our test methodologies. The program is used to inject faults to the circuit under test. The detailed experimental results provided give frequency and amplitude measurements data performed on the individual circuit blocks together with fault coverage. In this work, however, only catastrophic faults were considered. The simulation experiments carried out on different circuits not only demonstrate that the developed approaches are quite feasible but show in addition that the fault coverage is quite satisfactory (100%) in all cases.
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• High peak power 10 ns near-infrared coherent radiation has been obtained from frequency conversion of a dye laser beam in a LiNbO$\sb3$ crystal. The observed tuning range spanned the 0.734 to 2.853 $\mu$m wavelength range. An examination of optical parametric oscillator frequency conversion revealed that the production of the monochromatic laser radiation at the signal wavelength was only obtainable for a limited portion of the accessible pump wavelengths. In contrast, optical parametric amplifier frequency conversion resulted in the production of bichromatic infrared radiation for the entire accessible pump wavelength range. It appears that the crystal pump acceptance bandwidth leading to reduce power conversion can be described by a model in which the signal wave is assumed to have zero bandwidth.
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• The nonlinear dynamics of one-dimensional pulsating detonations were studied numerically using a simple two-step chain branching model with separate induction and exothermic reaction zones. The stability boundary was found for a wide range of non-dimensional activation energy and heat release. The wave dynamics within the detonation structure responsible for non-linear pulsating instability were studied using the three families of characteristic curves on x-t diagrams, representing the trajectories of pressure waves and particle paths. This clarified the dynamics responsible for loss of stability and the period of the pulsating instability. Four main regimes of pulsations were observed: the high frequency, very high frequency, low frequency and transition regimes. The high and very high frequency modes tend to manifest itself for low values of activation energy, while the low frequency mode tends to appear for higher activation energy. The very high frequency pulsations are governed by a cycle of expansion waves traveling across the induction zone along the C- characteristics coupled with the compression waves traveling across the induction zone along the C+ characteristics. The high frequency pulsations are controlled by a coupling between the particle path originating from the leading shock traveling across the induction zone and the compression wave traveling across the induction zone via the C+ characteristics. The low frequency oscillations involve the C- characteristics or particle paths traveling across the entire detonation from the shock followed by the C+ characteristics traveling toward the leading shock. The mechanisms governing the pulsating instability and the periods of oscillation were found to be in good qualitative agreement with Toong's phenomenological model based on the wave dynamics in a square wave model.
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• Double barrier heterostructures of Al$\sb{\rm x}$Ga$\sb{\rm 1-x}$As/GaAs are studied. The presence of a quantized level in the well is confirmed by the observation of a region of negative differential resistance (NDR) in the current-voltage (I-V) characteristics of the heterostructures. A spectrum analyser and a network analyser are thus used to measure the oscillator properties of the samples and to investigate the effects of the external circuit on these properties and on the I-V characteristics. For the sample with AlAs barriers, electron oscillations, of frequency greater than 1 GHz, are measured at room temperature; the observation of these frequencies depends on the matching of the impedances of the external circuit to the device under test. Photoconductivity spectra and photocurrent-voltage (PC-V) characteristics at fixed wavelengths are measured in order to study the effects of light incident on the sample. I-V measurements are carried out in a quantizing magnetic field parallel to the tunneling current. With increasing magnetic field, the voltage periodic structures are shifted to higher bias positions at a similar rate of change. (Abstract shortened by UMI.)
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• optoelectronic oscillator
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• Document