Thesis (doctoral)--Universität Bern. / Bibliography: p. vii-viii.

Cette thèse présente les résultats issus de l’investigation de la réponse optique de la transition D2 du Rubidium 87 en fonction de la densité de l’ensemble atomique. Afin de sonder cette transition nous utilisons un faisceau laser proche de la résonance (780nm) sur un échantillon de Rubidium ultra-froid (100µK). Nous observons ainsi la transmission en fonction de la longueur d’onde pour des densités allant de 10^12 atomes/cm^3 à 10^14 atomes/cm^3. Lorsque la densité augmente, on s’attend à ce que les interactions dipôle-dipôle jouent un rôle de plus en plus important du fait de la proximité des éléments diffusants. Quantitativement, les dipôles induits par le faisceau sonde commencent à jouer un rôle important lorsque la densité n atteint n*(lambda/2Pi)^3 = 1, une densité que nous atteignons dans notre système.Deux études systématiques seront présentées. La première montre les résultats obtenus pour un système à 12 niveaux, la deuxième pour un système à 2 niveaux obtenu par polarisation de l’ensemble atomique. Les résultats issus de ces études sont ensuite comparés aux théories existantes. La première approche est microscopique et décrit les interactions des dipôles couplés, la deuxième approche, macroscopique, est donnée par l’équation de Clausius-Mosotti.Les propriétés de propagation d’impulsions à travers ce système sont étudiées et révèlent en particulier un avancement fractionnel de l’impulsion et un indice de groupe inégalés. / This thesis investigates the response of the D2 transition of Rubidium 87 for various densities.To probe this transition we illuminate an ultra cold (100µK) sample of Rubidium 87 with close toresonance of lambda = 780 nm laser light.We observe the transmitted light while scanning the frequency over the atomic resonance. Such a spectrum is taken for peak densities ranging from 10^12 atoms/cm^3 to 10^14 atoms/cm^3. As matter gets denser and denser dipoie-dipole interaction start playing a role due to the close proximity of neighbouring scatteres. These interactions are caused by the probe light induced dipoles and start being important when the density reaches n*(lambda/2Pi)^3 = 1, which for us is the case at the upper end of the explored density range.We start off measuring these transmission data for degenerate 12-level Rubidium 87 and afterwardsfor Rubidium 87, which we first spin polarize and then by lifting the degeneracy generate an artifical2-level system. These results are systematically compared to the two available theories.A microscopic one, which is described by coupled dipoles and a macroscopic one the so calledClausius-Mosotti equation. None of the ab initio theories can explain the results obtained during this thesis.The rigerous comparison of the various acquired datasets shows that the data in itself is consistentand relative changes going from a 12-level system to a 2-level system are understood.Additionally we also investigate the pulse propagation behaviour through such a systemrevealing stunning values for the fractional pulse advancement and the group index.

Call number: LD2668 .T4 1937 H31 / Master of Science

No description available.

Call number: LD2668 .T4 1951 B69 / Master of Science

The idea of cooperation is not new to man. It is as old as man himself. Cooperatives are an almost universal form of organization today found in practically all countries and used by people in many ways. The cooperatives are formed to secure low cost credit, to purchase supplies and equipment for farming and household needs, to market products, even to secure many services, like electric power, irrigation, health, and insurance. Cooperatives can be used in many ways to benefit people in the everyday needs of life. The meaning of cooperation is that isolated and powerless individuals can, by combining with one another, achieve advantages available to the rich and the powerful so that they may advance not only materially but also morally. In other words a cooperative is a business organization that is owned by those who use its services, the control of which rests equally with all the members. It is voluntary and democratic and the moral element is as important as the material one. Furthermore, it recognizes social, educational, and community values. It is easier to understand the concept of the cooperative by knowing its specific objectives. They can be summed as follows: 1) They aim to provide goods and services. 2) They aim to eliminate the unnecessary profits of middlemen in trade and commerce. 3) They seek to prevent the exploitation of the weaker members of society. 4) They aim to protect the rights of people both as producers and consumers. 5) They promote mutual understanding and education among their members and people in general. Mr. W. P. Watkins of International Cooperative Alliance defines cooperation as: "A system of social organization based on the principles of unity, economy, democracy, equity, and liberty." 1

Zugl.: Berlin, Techn. Univ., 2006

<p>In wireless transmissions the communication is often degraded by random fades, noise and other performance reducing phenomena. One way of improving the stability and reducing the error rates is to use relaying techniques where several nodes cooperate in a transmission between two of them. This thesis analyzes some of the available Decode-and-Forward relaying schemes for wireless transmission. The investigated schemes are conventional repetition coding, partial repetition coding and non-collaborative direct transmission. I have developed a three-node communication system using an audio channel to test the performance of repetition coding and direct transmission. This audio communication system can also be used to demonstrate some basic phenomena in wireless transmissions and how different scenarios change the performance of the communication. A theoretical performance analysis and computer simulations of the schemes performance over a Rayleigh fading channel are done as a basis for comparison. As a result we see that in the audio communication system repetition coding actually degrades the performance, compared to direct transmission, when using a relatively slow data rate in comparison to the speed of the fading in the audio channel.</p>

Time-sensitive applications that are also CPU intensive like video games, video playback, eye-candy desktops etc. are increasingly common. These applications run on commodity operating systems that are targeted at diverse hardware, and hence they cannot assume that sufficient CPU is always available. Increasingly, these applications are designed to be adaptive. When executing multiple such applications, the operating system must not only provide good timeliness but also (optionally) allow co-ordinating their adaptations so that applications can deliver uniform fidelity. In this work, we present a starvation-free, fair, process scheduling algorithm that provides predictable and low latency execution without the use of reservations and assists adaptive time sensitive tasks with achieving consistent quality through cooperation. We combine an event-driven application model called cooperative polling with a fair-share scheduler. Cooperative polling allows sharing of timing or priority information across applications via the kernel thus providing good timeliness, and the fair-share scheduler provides fairness and full utilization. Our experiments show that cooperative polling leverages the inherent efficiency advantages of voluntary context switching versus involuntary pre-emption. In CPU saturated conditions, we show that the scheduling responsiveness of cooperative polling is five times better than a well-tuned fair-share scheduler, and orders of magnitude better than the best-effort scheduler used in the mainstream Linux kernel.

This study focuses on identifying communication technology adoption behaviors to provide educational benchmarks for Texas cooperatives. A survey was conducted with questions identifying a range of variables describing adoption behavior of communication technology from the background of cooperative managers to board management policy. The survey categorized 105 different cooperatives by current technology use and management practices. Once the data were collected, a factor analysis to understand underlying relationships of variables was conducted. The survey found that Texas cooperative managers are willing to expand on their current use of communication technology, however a clear definition of how to use new concepts as a powerful tool is needed. In terms of governance, we found that many cooperatives have no stated policies regarding the use of communication technologies. Generally, those cooperatives that had defined technology use policies were more likely to be using more forms of technology. Through a logistic and ordered logistic regression of the data, the study did not reflect our initial hypothesis that age of the respondent and the years working for a cooperative (manger characteristics) would be a significant factor in estimating Texas cooperatives? willingness to adopt new forms of communication technology and social media. However, the cooperatives? technology adoption behavior can best be explained by the data produced from descriptive cooperative information and the existence of employee communication technology policies. Likewise, cooperatives? willingness to adopt social media can best be explained by the data produced from manager attitudes and cooperatives? concerns.