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Orientador: Amir Ordacgi Caldeira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-06T02:34:45Z (GMT). No. of bitstreams: 1 Brito_FredericoBorgesde_D.pdf: 5789237 bytes, checksum: 6648aaced89bad6ff2fc57794c0ae747 (MD5) Previous issue date: 2006 / Resumo: Esta tese teve como objetivo estudar processos de perda de coerência quântica, chamados de descoerência, em sistemas de matéria condensada eleitos pela literatura como possíveis implementações do bit quântico (qubit). Esta perda de coerência quântica ocorre devido ao inevitável acoplamento do sistema de interesse com o seu ambiente. Como os estados de superposição quântica são a chave para a realização de operações baseadas na lógica quântica, tem-se que tais processos limitam ou até mesmo impedem o uso de determinados sistemas na esperada realização física do computador quântico. Os sistemas de matéria condensada padecem de uma dificuldade maior para se isolar o qubit do seu ambiente, o que acarreta, em geral, na observação de tempos de coerência piores do que os encontrados em seus concorrentes. Por outro lado, a possibilidade de construção de vários qubits acoplados e de manipulação de cada um de modo individual, usando-se técnicas convencionais de engenharia, têm impulsionado esforços nestes tipos de dispositivos. Os sistemas abordados na tese foram: qubits supercondutores contendo junções Josephson; e qubits de pontos quânticos. Para a investigação completa do primeiro tipo tivemos que desenvolver o modelo Caldeira-Leggett para o caso de várias fontes de dissipação acopladas ao qubit. Com a prescrição apresentada aqui, pudemos determinar o número de banhos de osciladores necessários para a correta descrição das fontes, e verificar que as taxas totais de relaxação e de descoerência não são necessariamente as somas das taxas devido a cada fonte individualmente. Além disso, pudemos aplicar o formalismo desenvolvido no estudo de um qubit de uxo em investigação. Para o sistema de pontos quânticos seguimos a linha de determinação do banho efetivo visto pelo grau de liberdade de spin do elétron aprisionado no ponto quântico. Determinada a função espectral efetiva, pudemos encontrar soluções analíticas para a dinâmica do valor esperado para cada uma das componentes de spin s x,y,z. o que permitiu uma análise completa dos efeitos de cada parâmetro físico do sistema. Em ambos os casos, fomos capazes de indicar os melhores regimes de operação de cada qubit, e dar estimativas dos tempos de relaxação e de descoerência / Abstract: The purpose of this thesis was to study the process of loss of quantum coherence, named decoherence, in condensed matter systems cited in the literature as possible candidates for the implementation of a quantum bit (qubit). Decoherence occurs due to the inevitable coupling of the system of interest to its environment. Once the quantum superposition states are the key to perform operations based on quantum logic, these processes limit, or even hinder, the utilization of some of those systems in the physical realization of the quantum computer. Relatively to its competitors, condensed matter systems usually present a higher degree of difficulty as one tries to minimize the coupling between the qubit and its environment, which, generally, worsens its coherence time observations. On the other hand, these devices present advantages which stimulates its study, such as: the possibility of construction of several coupled qubits and the possibility of manipulating each one individually, using conventional engineering techniques. The systems studied in this thesis were: superconducting qubits with Josephson junctions; and electronic spins quantum dots. Aiming at a complete investigation of the first system, we developed the Caldeira-Leggett model for the case of several dissipation sources coupled to the qubit. With the prescription presented here, we determine the number of oscillator baths needed to the correct description of the noise sources, and verify that the total relaxation and decoherence rates are not necessarily the sum of the individual rates relative to each source. Moreover, we applied this formalism to the study of a ux qubit currently under investigation. For the quantum dot qubits, we employed the effective bath approach to treat the dynamics of the spin of the electron localized in the quantum dot. As a result, we found analytical solutions for the dynamics of the average value of each one of the spin components s x,y,z . In both cases, we indicated the best operational regime of each qubit and gave estimates of the relaxation and decoherence times / Doutorado / Física da Matéria Condensada / Doutor em Ciências

The southwestern Florida shelf marine environment has often been characterized as oligotrophic, yet these waters can support large, high biomass, persistent phytoplankton blooms, including blooms of the toxin producing dinoflagellate Karenia brevis. Little is known regarding which major nutrient potentially limits primary production in these waters as both inorganic nitrogen and phosphorus concentrations are often near the limits of analytical detection and it is difficult to estimate what percentage of the dissolved organic pool is available for phytoplankton uptake. To assess the nutrient status of phytoplankton populations on the southwest Florida shelf, this project examines the particulate nutrient stoichiometry of ambient phytoplankton assemblages from 1998-2000 as part of the ECOHAB: Florida Program. Particulate C, N, P concentrations and particulate ratios display a large range of values across the West Florida Shelf (WFS). The average particulate stoichiometry is well above the classic Redfield ratio with a geometric mean of 410C:56N:1P. Frequency percentages of particulate ratio values to total sample number binned according to potential nutrient limitation indicate that 39% (C:N) of the data have values suggesting N limitation and that from 88% (N:P) to 95% (C:P) of the data have values which suggest P-limitation. It is difficult to discern whether phytoplankton biomass is truly P-limited as related to the nutrient regime on the WFS or whether detrital contributions, which can potentially be large on this shallow shelf, are skewing the N:P and C:P ratios towards higher values. Errors which could potentially be related to the different methodologies of determining C, N and P concentrations must also be considered when interpreting the particulate nutrient ratios. The data were also analyzed as subsets to determine near-shore to offshore, latitudinal, seasonal, inter-annual and K. brevis bloom versus non-bloom trends. The near-shore to offshore transect indicates decreasing concentrations of particulate C, N, P concentrations and increasing C:N, N:P, C:P ratios with increasing distance offshore. Particulate nutrient concentrations and particulate ratio values are very similar between the Tampa Bay, Sarasota and Fort Meyers transects indicating that these latitudes are not spatially distinct with regards to these variables. There does not appear to be any relationship between the particulate C, N, P concentrations or C:N, N:P, C:P ratios and rainfall as indicated by Spearman Ranking Correlation coefficients. However, there does appear to be monthly trends across the shelf where peak particulate nutrient concentrations and particulate ratio values occur during the spring, summer and fall. The average particulate nutrient concentrations and ratios differ for each year as well as each K. brevis bloom which occurred during the study period. In summary, the particulate C, N, P concentrations and particulate nutrient ratios vary both spatially and temporally on the WFS and are potentially related to the flexibility of phytoplankton uptake kinetics in response to the varying nutrient regimes of the WFS.

Plants and bacteria both have specialized light-harvesting pigment-protein complexes, composed of a network of chromophores encompassed by a protein scaffold, that are involved in photosynthesis. While chromophore, as well as protein, composition and arrangement vary in these light-harvesting complexes, chromophores transfer energy as molecular excitation energy through their complex multi-chromophoric network with near perfect efficiency. Understanding the efficiency of this excitation energy transfer process has been the focus of many interdisciplinary studies. By elucidating the mechanisms involved in efficient excitation energy transfer in biological systems, we are able to guide the design of novel organic materials for their application in photovoltaic systems. Interdisciplinary studies of light-harvesting biological systems leverage advanced spectroscopic techniques and theoretical models to help explain the interaction be- tween excited electronic states. Difficulties in assigning the origin of spectral features in spectroscopy experiments arise from both homogeneous and inhomogeneous effects. Various computational studies have been able to provide theoretical models that help disentangle these effects and provide insight into the origin of some these spectral features. In this work, we present a computational approach that is used to calculate an ensemble of model Hamiltonians for a light-harvesting pigment-protein complex found in algae. To verify the reliability of our model, we compare various computed spec- tra with experimental measurements. Next, we extend our computational approach for parameterizing an ensemble of Hamiltonians for two configurationally unique or- ganic dimers. Finally, we examine the error of some of the approximations made while partitioning “system” and “bath” degrees of freedom when computing molecu- lar properties. Using these methods we are able to provide mechanistic interpretations and explanations of spectral signatures observed in various linear and nonlinear ex- perimental spectra.

The dissolved silicon (DSi) has a crucial role for growth of a large group of primary producers – diatoms and, hence, impact on functioning of the aquatic food web. This thesis contributes to an increased understanding of the modifications of the DSi cycling in the Baltic Sea. The results provide new information about spatial and temporal changes in DSi concentrations and nutrient ratios for the period 1970-2001 as well as during the 20th century. For the period 1970-2001, the declining DSi trends were found at the majority of monitoring stations all over the Baltic Sea. This decrease is assumed to be mainly due to the ongoing eutrophication. It is supported by the increasing trends of inorganic nitrogen and phosphorus. The trends have implications for the nutrient ratios, DSi:DIN and DSi:DIP, which are important indicators of the state of an ecosystem. The long-term retrospective DSi budget has shown that the DSi concentrations before major hydrological alterations and eutrophication were about twice the present ones. This decrease is related to both eutrophication and anthropogenic perturbations in the catchment. The occurrence of DSi concentrations close to the potentially limiting levels has been also analysed. While DSi concentrations are still high in the northern regions of the Baltic, other areas may be at risk of developing Si limitation if the decrease in DSi concentrations persists. The results depict the Baltic Sea journey from being water body with DSi levels sufficient to support diatom production to one that may experience Si limitation and its adverse ecological consequences. / Löst kisel (DSi) har en viktig roll för tillväxten av en stor grupp av primärproducenter – kiselalger, och därmed även påverkar hela den akvatiska näringskedjan. Denna avhandling bidrar till en ökad förståelse av förändringarna i DSi kretsloppet i Östersjön. Resultaten tillhandahåller ny information om rumsliga och tidsmässiga förändringar i DSi koncentrationer såväl för perioden 1970-2001 som för hela 1900-talet. För perioden 1970-2001 återfanns minskade DSi koncentrationer på mätstationer över hela Östersjön. Minskningen antas främst bero på den pågående övergödningen. Detta antagande stöds av stigande halter av oorganiskt kväve och fosfor. Sammantaget har dessa trender en inverkan på ekosystemets tillstånd och näringsämnenas kvoter, DSi: DIN och DSi: DIP. Ur ett längre tidsperspektiv kan man se att innan övergödningen och de stora hydrologiska ombildningar i Östersjöområdet var DSi koncentrationerna ungefär dubbelt så höga som idag. Dagens förekomst av DSi koncentrationer som ligger nära de potentiellt begränsande nivåerna har också analyserats. DSi koncentrationerna är fortfarande höga i norra delar av Östersjön, men är i andra områden i riskzonen för att utveckla Si begränsning om minskningen av DSi koncentrationer fortsätter. Resultaten skildrar Östersjöns resa från att vara ett havsområde med DSi halter som är tillräckliga för att understödja kiselalgernas produktion till ett sådant som kan uppleva Si begränsning och dess negativa ekologiska konsekvenser.

Les activités anthropiques affectent les cycles biogéochimiques globaux, principalement par l'ajout de nutriments dans les écosystèmes. Il est donc crucial de déterminer dans quelle mesure les cycles biogéochimiques globaux peuvent être régulés. Les autotrophes peuvent réguler les réservoirs de nutriments par la consommation des ressources, mais la majorité des ressources leur sont inaccessibles à l'échelle globale. Par des modèles théoriques, nous avons cherché à évaluer la manière dont les autotrophes répondent à la fertilisation à l'échelle globale et leur capacité à réguler les concentrations des nutriments quand leur accessibilité est limitée. Nous avons également étudié les mécanismes qui déterminent la régulation des rapports de Redfield dans l'océan, ainsi que les effets de l'ajout de nutriments sur la production primaire océanique totale. Nous avons montré que les organismes ne régulent pas efficacement les réservoirs de nutriments. Le couplage des cycles biogéochimiques et la compétition entre groupes fonctionnels peuvent altérer, négativement ou positivement, la régulation des cycles biogéochimiques globaux par les organismes. Une régulation inefficace des concentrations de nutriments n'exclut par contre pas une forte régulation des rapports entre ces nutriments, comme dans le cas des rapports de Redfield. La fertilisation des écosystèmes terrestres et océaniques risque donc de fortement impacter la production primaire et les cycles biogéochimiques globaux, à de courtes comme à de grandes échelles de temps. / Anthropogenic activities heavily impact global biogeochemical cycles, mainly through nutrient fertilisation of ecosystems; thus it is crucial to assess the extent to which global biogeochemical cycles are regulated. Autotrophs can regulate nutrient pools locally through resource consumption, but most resources are inaccessible to them at global scales. We used theoretical models to assess how organisms respond nutrient fertilisation at global scales and how they can regulate the concentration of these nutrients when their accessibility of is limited. We further investigated the mechanisms driving the regulation of Redfield ratios in oceans, and the effects of nutrient fertilisation on total oceanic primary production. We showed that organisms cannot efficiently regulate nutrient pools. Mechanisms such as coupling of nutrient cycles and competition between functional groups can alter the strength of biotic regulation of global biogeochemical cycles, either positively or negatively. An inefficient regulation of inaccessible nutrient concentration, however, does not exclude a strong biotic regulation of nutrient ratios, as is the case with Redfield ratios in oceans. Nutrient fertilization of oceanic and terrestrial ecosystems is thus likely to have a strong impact on primary production and global nutrient cycles at both small and long timescales.

Diese Dissertation befasst sich mit Modellrechnungen zur Dynamik vom photoinduzierten Elektrontransfer und Exzitontransfer in Systemen mit vielen Freiheitsgraden. Außerdem trägt diese Arbeit zu einigen theoretischen und numerischen Aspekten der Redfield-Theorie bei. Betrachtet werden der ultraschnelle Elektrontransfer im Farbstoff Betain-30, die Elektroninjektion von einem Chromophormolekül ins Leitungsband von einem Halbleiter, sowie die Exziton-Ausbreitung in einem modellhaften ringförmigen System mit 18 Lokalisierungszentren. Zuerst wird der Einfluss der elektronischen Kopplung auf die Dissipationsterme der Redfield-Gleichung untersucht. Es wird gezeigt, dass bei bestimmten Potenzialkonfigurationen die Vernachlässigung der elektronischen Kopplung (die soganannte diabatische Dämpfungsnäherung) dazu führt, dass das System nicht in das thermische Gleichgewicht mit dem Wärmebad relaxiert. Jedoch verliert diese Näherung ihre Gültigkeit nicht für kleine elektronische Kopplung in einer ganzen Reihe von Fällen, z.B. im Marcus-invertierten Bereich. Die Transfermechanismen, welche jenseits dieser Näherung auftreten, werden mit Hilfe der Störungstheorie erster Ordnung in der elektronischen Kopplung detailliert untersucht. Weiterhin werden direkte Verfahren zur genauen numerischen Lösung zeitlokaler Mastergleichungen implementiert und getestet. Die Effizienz dieser Methoden wird am Beispiel von einem eindimensionalen Elektrontransfer-Modell bestimmt. Desweiteren wird noch ein neues stochastisches Verfahren zur Propagation von Dichtematrizen entwickelt und in den Simulationen verwendet. Der ultraschnelle photoinduzierte Elektrontransfer in Betain-30 wird sowohl mit einer einzelnen Reaktionsmode als auch mit zwei Reaktionsmoden modelliert. Anhand der reduzierten Dichtematrix lässt sich die Gesamtpolarisation berechnen und somit ist es möglich, ein Pump-Probe-Experiment zu simulieren. Die Rechenergebnisse werden mit experimentellen Daten verglichen.

The purpose of this study was to determine the limiting nutrient in a set of coastal areas and estuaries in the Baltic Sea. Although the subject as been studied for several decades, no clear consensus has been reached in the scientific community as to whether primary production is limited by phosphorus or nitrogen. A total of five coastal areas, all located on the east coast of Sweden, were assessed regarding their limiting nutrient by using three methods. The first method was applying linear regression analysis on measured TP and TN concentration together with chlorophyll-a and Secchi depth. The data was collected from sampling programs stretching back to the 1970s and 80s, studying the summer period May to September for all sites but one, were the period April to October was studied. The second method calculated the TN/TP ratio during the summer period and compared it to the Redfield ratio. Thirdly, basic mass-balance calculations were carried out, with empirical data on the external loads and calibrated with the yearly average concentration in the surface water (0–10 m). From the calculations, both the annual external and internal load of TP and TN was obtained. The different TP and TN loads were likewise tested for a correlation with the measured summer chlorophyll-a concentration and Secchi depth. The results of using linear regression analysis on measured concentrations were mostly inconclusive, as the TP and TN concentrations for all sites and most years were related to each other. Consequently both nutrients often gave equal correlation coefficients. Similarly the TP and TN loads also matched each other for most sites and years, inherently obtaining the same inconclusive, but also contradictory results, as when using the measured concentrations. The TN/TP ratio indicated, for one site that it was limited by phosphorus and another site possibly nitrogen limitation. The ratio in the other sites periodically dropped between nitrogen and phosphorus limitation over the years. Thus it was difficult to draw an overall conclusion as to what nutrient was the limiting one for all the sites. However analysing the results from the individual sites showed that three of the five sites had signs of phosphorus limitation. Two factors were deemed as being the main reasons as to why the methods did not achieve more conclusive results. The first factor was the empirical data, which varied in frequency and extent over the studied time periods and between sites, making representative concentrations difficult to calculate and evaluate. The second was the matching trends between both the concentrations and the loads of TP and TN. To achieve a better result one nutrient could be increased or decreased while one remains relatively constant. The problem with such an experiment would be controlling the inflow of nutrients from the adjacent sea. / Syftet med detta projekt var att bestämma det begränsande näringsämnet för ett antal kustområden i Östersjön. Frågan huruvida fosfor eller kväve är det begränsande näringsämnet i kustområden runt Östersjön har varit omdiskuterad under flera år och undersökts vid ett flertal tillfällen. I denna studie testades tre metoder, i fem olika kustområden, med syftet att fastställa det begränsande näringsämnet. Först användes linjär regressionsanalys med uppmätta värden på TP och TN koncentrationer tillsammans med klorofyll-a och Secchidjup. Empirisk data insamlades från övervakningsprogram där prover tagits sedan 1970- och 80-talet. Medelvärden beräknades för perioden maj till september för alla områden förutom ett, där undersöktes perioden april till oktober. Sommarmedelvärdena för TN/TP kvoten analyserades också för alla områden med avseende på Redfield kvoten. Slutligen genomfördes massbalansberäkningar med data för de externa belastningarna av TP och TN, dessa beräkningar kalibrerades sedan med uppmätta värden på koncentrationen i ytvattnet (0–10 m). Utifrån beräkningarna erhölls värden på den externa och den interna belastningen. Dessa belastningar testades med linjär regression för ett samband med de uppmätta värdena på Secchidjup och klorofyll-a. Metoden att använda linjär regressionsanalys med empiriskt uppmätta koncentrationer och djup, gav generellt ett oklart resultat. Detta var en följd av att halterna av både TP och TN i regel följdes åt, vilket fick konsekvensen att korrelationskoefficienterna för TP och TN ofta var lika stora. Samma problem uppstod för regressionsanalysen med belastningarna, då även dessa ofta följde varandra, men även motsade resultatet med koncentrationerna. Analysen av TN/TP kvoten tydde på att ett område var fosforbegränsat och ett område möjligen var kvävebegränsat. För de övriga tre områdena skiftade TN/TP kvoten under åren mellan kväve- och fosforbegränsning. De oklara resultaten gjorde det svårt att dra en övergripande slutsats. Däremot vid analysen av de enskilda områdena uppvisade tre av de fem områdena tecken på fosforbegränsning, även om detta inte kunde med säkerhet fastställas. Det var huvudsakligen två faktorer vars inverkan anses ha haft stor betydelse för det oklara resultatet. Den första faktorn var uppmätt data, vars frekvens och omfattning skiljde sig avsevärt mellan år och platser. Följaktligen försvårades beräkningarna av koncentrationerna och tillförlitligheten i hur representativa värdena var. Den andra och avgörande faktorn var de matchande trenderna hos både koncentrationerna och belastningarna. För att förbättra resultatet kan ett näringsämne ändras, medan det andra näringsämnet hålls relativt konstant. Problemet med att genomföra ett sådant experiment skulle vara att kontrollera inflödet av näringsämnen från närliggande hav.

Diese Dissertation befasst sich mit Modellrechnungen zur Dynamik vom photoinduzierten Elektrontransfer und Exzitontransfer in Systemen mit vielen Freiheitsgraden. Außerdem trägt diese Arbeit zu einigen theoretischen und numerischen Aspekten der Redfield-Theorie bei. Betrachtet werden der ultraschnelle Elektrontransfer im Farbstoff Betain-30, die Elektroninjektion von einem Chromophormolekül ins Leitungsband von einem Halbleiter, sowie die Exziton-Ausbreitung in einem modellhaften ringförmigen System mit 18 Lokalisierungszentren. Zuerst wird der Einfluss der elektronischen Kopplung auf die Dissipationsterme der Redfield-Gleichung untersucht. Es wird gezeigt, dass bei bestimmten Potenzialkonfigurationen die Vernachlässigung der elektronischen Kopplung (die soganannte diabatische Dämpfungsnäherung) dazu führt, dass das System nicht in das thermische Gleichgewicht mit dem Wärmebad relaxiert. Jedoch verliert diese Näherung ihre Gültigkeit nicht für kleine elektronische Kopplung in einer ganzen Reihe von Fällen, z.B. im Marcus-invertierten Bereich. Die Transfermechanismen, welche jenseits dieser Näherung auftreten, werden mit Hilfe der Störungstheorie erster Ordnung in der elektronischen Kopplung detailliert untersucht. Weiterhin werden direkte Verfahren zur genauen numerischen Lösung zeitlokaler Mastergleichungen implementiert und getestet. Die Effizienz dieser Methoden wird am Beispiel von einem eindimensionalen Elektrontransfer-Modell bestimmt. Desweiteren wird noch ein neues stochastisches Verfahren zur Propagation von Dichtematrizen entwickelt und in den Simulationen verwendet. Der ultraschnelle photoinduzierte Elektrontransfer in Betain-30 wird sowohl mit einer einzelnen Reaktionsmode als auch mit zwei Reaktionsmoden modelliert. Anhand der reduzierten Dichtematrix lässt sich die Gesamtpolarisation berechnen und somit ist es möglich, ein Pump-Probe-Experiment zu simulieren. Die Rechenergebnisse werden mit experimentellen Daten verglichen.

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