Ce manuscrit présente l'étude de l'effet photorefractif dans le proche infrarouge et, plus particulièrement, l'étude des matériaux sensibles dans cette gamme de longueurs d'onde. la première partie du travail a consiste a étudier les matériaux existants, provenant de la microélectronique, le GaAs et l'InP. Les études réalisées, tant expérimentales que théoriques, ont permis de comprendre leurs propriétés et de mettre en évidence leurs limitations, notamment pour les applications dans la gamme de longueurs d'onde autour de 1,3 m. au vu de ces résultats, nous avons propose une technique d'optimisation des performances de gaas utilisant l'irradiation électronique. L'irradiation induit une légère variation du niveau de fermi, qui doit favoriser l'effet photorefractif a 1,3 m. Les résultats obtenus ont montre que l'effet attendu était fortement contrebalance par la création au milieu de la bande interdite, d'un défaut d'irradiation. L'influence directe de ce défaut a été établie grâce au développement d'un modèle théorique de l'effet photorefractif prenant en compte deux niveaux de pièges profonds. En parallèle a cette étude de l'effet d'irradiation, nous avons travaille sur les composes ii-vi, comme le CdTe. Les premiers cristaux étudiés présentent des gains photorefractifs intéressants avec des faisceaux de faible puissance. Ces résultats confirment les promesses de ces cristaux pour une extension de l'effet photorefractif vers 1,5 m. Pour finir, nous présentons une technique d'amplification du gain photorefractif qui utilise l'application d'un champ alternatif carre. une augmentation du gain d'un ordre de grandeur est obtenue.

Made available in DSpace on 2015-05-14T12:14:06Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1577961 bytes, checksum: c71d976d783495df566e0fa6baadf8ca (MD5) Previous issue date: 2009-08-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / We start this work studying the appearance of geometric quantum phases as in the relativistic as in the non-relativistic quantum dynamics of a neutral particle with permanent magnetic and electric dipole moment which interacts with external electric and magnetic fields in the presence of linear topological defects. We describe the linear topological defects using the approach proposed by Katanaev and Volovich, where the topological defects in solids are described by line elements which are solutions of the Einstein's equations in the context of general relativity. We also analyze the in uence of non-inertial effects in the quantum dynamics of a neutral particle using two distinct reference frames for the observers: one is the Fermi-Walker reference frame and another is a rotating frame. As a result, we shall see that the difference between these two reference frames is in the presence/absence of dragging effects of the spacetime which makes its in uence on the phase shift of the wave function of the neutral particle. In the following, we shall use our study of geometric quantum phases to make an application on the Holonomic Quantum Computation, where we shall show a new approach to implement the Holonomic Quantum Computation via the interaction between the dipole moments of the neutral particle and external fields and the presence of linear topological defects. Another applications for the Holonomic Quantum Computation is based in the structure of the topological defects in graphene layers. In the presence of topological defects, a graphene layer shows two distinct phase shifts: one comes from the mix of Fermi points while the other phase shift comes from the topology of the defect. To provide a geometric description for each phase shift in the graphene layer, we use the Kaluza-Klein theory where we establish that the extra dimension describes the Fermi points in the graphene layer. Hence, we can implement the Holonomic Quantum Computation through the possibility to build cones and anticones of graphite in such way we can control the quantum uxes in graphene layers. In the last part of this work, we study the Landau quantization for neutral particles as in the relativistic dynamics and non-relativistic dynamics. In the non-relativistic dynamics, we study the Landau quantization in the presence of topological defects as in an inertial as in a non-inertial reference frame. In the relativistic quantum dynamics, we start our study with the Landau quantization in the Minkowisky considering two different gauge fields. At the end, we study the relativistic Landau quantization for neutral particles in the Cosmic Dislocation spacetime. / Neste trabalho estudamos inicialmente o surgimento de fases geometricas nas dinâmicas quânticas relativística e não-relativística de uma partícula neutra que possui momento de dipolo magnético e elétrico permanente interagindo com campos elétricos e magnéticos externos na presença de defeitos topológicos lineares. Para descrevermos defeitos topológicos lineares usamos a aproximação proposta por Katanaev e Volovich, onde defeitos lineares em sólidos são descritos por elementos de linha que são soluções das equações de Einstein no contexto da relatividade geral. Analisamos também a inuência de efeitos não-inerciais na dinâmica quântica de uma partícula neutra em dois tipos distintos de referenciais para os observadores: um é o referencial de Fermi-Walker e outro é um referencial girante. Vemos que a diferença entre dois referenciais está na presença/ausência de efeitos de arrasto do espaço-tempo que irá influenciar diretamente na mudança de fase na funçãao de onda da partícula neutra. Em seguida, usamos nosso estudo de fases geométricas para fazer aplicações na Computação Quântica Holonômica onde mostramos uma nova maneira de implementar a Computação Quântica Holonômica através da interação entre momentos de dipolo e campos externos e pela presença de defeitos topológicos lineares. Outra aplicação para a Computação Quântica Holonômica está baseada na estrutura de defeitos topológicos em um material chamado grafeno. Na presença de defeitos topológicos lineares, esse material apresenta duas fases quânticas de origens distintas: uma da mistura dos pontos de Fermi e outra da topologia do defeito. Para dar uma descrição geométrica para a origem de cada fase no grafeno usamos a Teoria de Kaluza-Klein, onde a dimensão extra sugerida por esta teoria descreve os pontos de Fermi no grafeno. Portanto, a implementação da Computação Quântica Holonômica no grafeno está baseada na possibilidade de construir cones e anticones de grafite de tal maneira que se possa controlar os fluxos quânticos no grafeno. Na última parte deste trabalho estudamos a quantização de Landau para partículas neutras tanto na dinâmica não-relativística quanto na dinâmica relativística. Na dinâmica não-relativítica, estudamos a quantização de Landau na presença de defeitos em um referecial inercial e, em seguida, em um referencial nãoo-inercial. Na dinâmica relativística, estudamos inicialmente a quantização de Landau no espaço-tempo plano em duas configurações de campos diferentes. Por fim, estudamos a quantização de Landau relativística para partículas neutras no espaço-tempo da deslocação cósmica.

Derek Parfit är kanske en av vår tids mest kända moralfilosofer. Parfit inleder sin första bok Reasons and Persons med att ställa frågan: vad har vi mest skäl att göra? Hans fråga berör vad som egentligen har betydelse, en fråga som han fortsätter att beröra i sin andra bok On What Matters. Filosofen Toby Ord argumenterar i sin bok The Precipice för att den utmaning som definierar vår tid, och bör ha en central prioritering, är utmaningen att skydda mänskligheten emot så kallade existentiella risker. En existentiell risk är en typ av risk som hotar att förstöra, eller förhindra, mänsklighetens långsiktiga potential. Ord menar att vi idag befinner oss vid en kritisk tidpunkt i mänsklighetens historia som kan vara helt avgörande för om det ens kommer existera en framtid för mänskligheten. Men om vi bör skydda mänskligheten emot existentiella risker, så kan en lämplig följdfråga vara i vilken ordning vi bör prioritera olika existentiella risker. Den svenske filosofen Nick Bostrom har liksom Ord länge förespråkat att existentiella risker bör tas på allvar. Han menar att preventiva åtgärder bör vidtas. I sin bok Superintelligens argumenterar Bostrom, både omfattande och väl, för att den existentiella risk som kan te sig som mest brådskande, och kanske allvarligast, är artificiell intelligens. Bostrom menar att vi har goda skäl att tro att utveckling av artificiell intelligens kan eskalera till den grad att mänsklighetens öde kan hamna bortom vår egen kontroll. Det han syftar på är att människan just nu är den dominerande agenten på jorden och därför innehar en stor kontroll, men att så inte alltid behöver vara fallet. Bostroms tes kunde te sig som okonventionell då den presenterades, men kan även te sig så idag vid en första anblick. Han har dock fått explicit medhåll av personer som Bill Gates, Stephen Hawking, Elon Musk, Yuval Noah Harari och Max Tegmark, som antingen håller med eller resonerar i liknande banor. Även jag själv finner Bostroms antaganden välgrundade. Slutsatsen som många drar är därför att vi bör betrakta artificiell intelligens som en existentiell risk som ska prioriteras högt. Jag kommer dock i denna text att argumentera för tesen att vi inte bör betrakta artificiell intelligens som en existentiell risk. Tesen följer från en invändning som jag kommer att kalla för AI-paradoxen. Det tycks enligt invändningen som att artificiell intelligens inte kan leda till en existentiell katastrof givet vissa premisser som flera i debatten om artificiell intelligens tycks acceptera. Texten i uppsatsen är strukturerad på följande sätt. I avsnitt 2 kommer jag att återge det övergripande argumentet som cirkulerar i debatten om artificiell intelligens som ett hot. I avsnittet kommer jag också förklara några viktiga termer och begrepp. I avsnitt 3 börjar jag med att titta på den första premissen i argumentet, samt resonera om dess rimlighet. I avsnitt 4 går jag sedan vidare till den andra premissen i argumentet och gör samma sak med den. Väl i avsnitt 5 så väljer jag att presentera min egen idé som jag kallar för AI-paradoxen, vilket är en invändning mot argumentet. I avsnitt 6 diskuterar jag sedan AI-paradoxens implikationer. Avslutningsvis, i avsnitt 7, så ger jag en övergripande sammanfattning och en slutsats, samt några sista reflektioner. / Derek Parfit is perhaps one of the most famous moral philosophers of our time. Parfit begins his first book Reasons and Persons by asking the question: what do we have most reason to do? His question touches upon what really matters, a question he continues to touch upon in his second book On What Matters. The philosopher Toby Ord argues in his book The Precipice that the challenge that defines our time, and should have a central priority, is the challenge of safeguarding humanity from so-called existential risks. An existential risk is a type of risk that threatens to destroy, or prevent, humanity’s longterm potential. Ord means that today we are at a critical time in the history of humanity that can be absolutely decisive for whether there will even exist a future for humanity. But if we are to safeguard humanity from existential risks, then an appropriate question may be in what order we should prioritize different existential risks. The Swedish philosopher Nick Bostrom, like Ord, has long advocated that existential risks should be taken seriously. He believes that preventive measures should be taken. In his book Superintelligence Bostrom argues, both extensively and well, that the existential risk that may seem most urgent, and perhaps most severe, is artificial intelligence. Bostrom believes that we have good reason to believe that the development of artificial intelligence can escalate to the point that the fate of humanity can end up beyond our own control. What he is referring to is that humans are currently the dominant agent on earth and therefore has great control, but that this does not always have to be the case. Bostrom's thesis may have seemed unconventional when it was presented, but it can also seem so today at first glance. However, he has been explicitly supported by people like Bill Gates, Stephen Hawking, Elon Musk, Yuval Noah Harari and Max Tegmark, who either agree or reason similarly. I myself also find Bostrom's assumptions well-founded. The conclusion that many draw is therefore that we should regard artificial intelligence as an existential risk that should be given a high priority. However, in this text I will argue for the thesis that we should not regard artificial intelligence as an existential risk. The thesis follows from an objection of my own, which I call the AI ​​paradox. According to the objection, it seems that artificial intelligence cannot lead to an existential catastrophe given certain premises that many in the debate about artificial intelligence as a threat seem to accept. The text in the essay is structured as follows. In section 2 I will present the main argument circulating in the debate about artificial intelligence as a threat. In the section I will also explain some important terms and concepts. In section 3 I begin by looking at the first premise in the argument, and also reason about its plausibility. In section 4 I proceed to the second premise in the argument and examine it similarly. Once in section 5 I choose to present my own idea, which I call the AI ​​paradox, which is an objection to the argument. In section 6 I discuss the implications of the AI ​​paradox. Finally, in section 7, I give an overall summary and a conclusion, as well as some last reflections.