Este trabalho tem por objetivo analisar as concepções a respeito de átomos e elementos químicos, expressas pelo químico russo Dmitri I. Mendeleev (1834 - 1907). Para isso, recorremos a textos desse autor que foram traduzidos para o inglês, principalmente seu livro didático Principles of Chemistry, mas também os artigos \"An Attempt to Apply to Chemistry one of The Principles of Newton\"s Natural Philosophy\", (1889) e \"An Attempt towards a chemical conception of the ether\", (1902). No decorrer do século XIX, a hipótese atômica formulada por Dalton, e elaborada por outros químicos, ainda não era unanimemente aceita pela comunidade científica. Nesse contexto, observamos que Mendeleev se alinhava aos autores que tinham reservas quanto à realidade física dos átomos. Para compreender seus motivos, é importante considerar que os primeiros estudos científicos de Mendeleev se referiam aos chamados \"compostos indefinidos\", isto é, compostos que não obedeciam à lei das proporções múltiplas de Dalton. Mendeleev reconhecia que a hipótese atômica explicava uma série de fenômenos químicos, mas não era capaz de explicar os compostos indefinidos. Por outro lado, Mendeleev propôs uma clara distinção entre os conceitos de elemento químico e de corpo simples. Os elementos seriam entidades abstratas, portadoras da individualidade química, ideia considerada fundamental por Mendeleev e que o conduziu à formulação da lei periódica. Nos escritos analisados, Mendeleev fez uso, de maneira recorrente, da analogia entre o sistema solar e as moléculas. Nessa analogia, os planetas remetem à ideia de individualidade, essencial para a compreensão do conceito de elemento químico; mas, ao mesmo tempo, não remetem à ideia de indivisibilidade física - o ponto da hipótese atômica que por muito tempo foi objeto de descrença por parte de Mendeleev. / This work aims to analyze the concepts of atom and chemical elements, as presented in the writings of the Russian chemist Dmitri I. Mendeleev, (1834 - 1907). In this analysis were used English translations of Mendeleev\"s texts, especially his textbook Principles of Chemistry, but also the papers \"An Attempt to Apply to Chemistry one of The Principles of Newton\'s Natural Philosophy\", (1889) and \"An Attempt Towards a chemical conception of the ether\", (1902). During the nineteenth century, the atomic hypothesis, proposed by Dalton and elaborated by other chemists, was not unanimously accepted by the scientific community. In this context, one may note that Mendeleev was aligned with authors who had reservations about the physical reality of atoms. To understand his reasons, it is important to consider that the first scientific researches performed by Mendeleev were on the so-called indefinite compounds, that is, compounds that did not obey Dalton\"s law of multiple proportions. Mendeleev recognized that the atomic hypothesis explained a series of chemical phenomena, but it could not explain indefinite compounds. Moreover, Mendeleev proposed a clear distinction between the concepts of chemical elements and simple bodies. Elements were abstract entities with chemical individuality, a crucial idea to Mendeleev that led him to the proposition of the periodic law. In the analyzed texts, Mendeleev used, on a recurring basis, the analogy between the solar system and molecules. In this analogy, planets relate to the idea of individuality, which is essential to understand the concept of chemical element, but at the same time does not refer to the idea of physical indivisibility - the point of the atomic hypothesis that has long been subject to disbelief by Mendeleev.

Neste trabalho, apresentamos uma montagem experimental capaz de aprisionar átomos de K e Rb simultaneamente em uma armadilha óptica de dipolo cruzada. Contudo, para atingir este feito foi necessário, durante o desenvolvimento do projeto, estudar cada espécie atômica de forma isolada. Assim, desenvolvemos uma técnica inédita para carregar uma armadilha de dipolo diretamente de uma armadilha magneto-óptica de 39K, constatando a importância da estrutura hiperfina do estado 4P3/2 no processo de carregamento. Também observamos e explicamos o processo de fotoassociação de Rb devido ao laser em 1071 nm, inclusive a diferença isotópica entre 85Rb e 87Rb. Na sequência, realizamos o carregamento duplo com K e Rb na armadilha e observamos a fotoassociação de KRb. Um modelo teórico simples prevê a distribuição dos níveis vibracionais obtidos. O mesmo modelo abre caminho para um possível bombeamento óptico para a produção e acúmulo de moléculas de KRb no estado X1Σ+ (v = 0). / In this paper, we present an experimental setup to simultaneously trap atoms of K and Rb in a crossed optical dipole trap. This accomplishment required the individual study of each atomic species during the project development. This allowed the development of a new technique for loading a dipole trap directly from a magneto-optical trap of 39K, providing evidence of the importance of the hyperfine structure of the state 4P3/2 in the loading process. We also observe and explain the photoassociation process of Rb2 due to the dipole trap laser at 1071 nm, including an isotopic difference between 85Rb and 87Rb. In the sequence, we performed dual species loading of K and Rb and observed the photoassociation process of KRb. A simple theoretical model predicts the final distribution of vibrational levels obtained. The same model provides a potential path to optically pump the molecules and accumulate them in the molecular ground state X1Σ+ (v = 0) for KRb.

Nesta dissertação, apresentamos estudos preliminares envolvendo a Termodinâmica de átomos aprisionados por potenciais inomogêneos. Estes estudos foram realizados em dois sistemas experimentais distintos, a partir da definição de novas variáveis globais propostas por V. Romero-Rochín, as quais, chamaremos parâmetro de volume e parâmetro de pressão. O primeiro sistema, consiste no experimento desenvolvido em nosso laboratório e envolve a Condensação de Bose-Einstein em átomos de 87Rb aprisionados em uma armadilha óptico-magnética. Este sistema é descrito em detalhe ao longo da dissertação. O segundo sistema consiste no sistema experimental do professor R. G. Hulet, na Universidade Rice, e envolve a Condensação de Bose-Einstein em átomos7 Li aprisionados em uma armadilha óptica. Neste segundo experimento é possível variar a interação da amostra atômica estudando a sua influência no parâmetro de pressão. Ainda, nesta dissertação, realizamos uma análise teórica da transição de fase para a fase condensada a partir dessas novas variáveis. / In this master thesis we present some preliminary studies on the Thermodynamics of ultracold gases in inhomogeneous potentials. These studies were performed in two different experimental setups using the new global variables proposed by V. Romero-Rochín, which we call volume parameter and pressure parameter. The first system is the experiment built in our laboratory in which we produce a Bose-Einstein Condensation (BEC) of 87Rb atoms in a hybrid trap. The second experiment is the Prof. Hulet´s setup at Rice University in which a BEC of 7Li is produced in an optical trap. At this second experiment it was possible to vary the interaction between the atoms in such a way that we were able to characterize its influence on the pressure parameter. In addition we present a theoretical analysis of the BEC phase transition in terms of these new variables.

This thesis considers two types of applications of quantum feedback control; feedback creation of nonclassical states of light, and controlling nonclassical properties of an ensemble of atoms. An electro-optical feedback loop will create an in-loop field with nonclassical photon statistics similar to squeezed light, resulting in fluorescence line-narrowing of a two-level atom coupled to such light. We extend this theory to study a three-level atom coupled to broadband squashed light, and confirm the two-level atom line-narrowing using a more realistic non-Markovian description of the feedback loop. The second type of application utilizes continuous QND measurement of atomic ensembles. If we measure the collective spin, then the system experiences conditional spin squeezing dependent on the measurement results. We show that feedback based on these results can continuously drive the system into the same conditioned state, resulting in deterministically reproducible spin squeezing. If we measure the atom number fluctuations of a BEC, then, due to the nonlinearity of atomic self interactions, this is also information about phase fluctuations. We show that feedback based on this information can greatly reduce the collisional broadening of the linewidth of an atom laser out-coupled from the condensate.

Ziel der Arbeit war die systematische theoretische Behandlung von Gemischen aus bosonischen und fermionischen Atomen in einem Parameterbereich, der sich zur Beschreibung von aktuellen Experimenten mit ultra-kalten atomaren Gasen eignet.<br /> <br /> Zuerst wurde der Formalismus der Quantenfeldtheorie auf homogene, atomare Boson-Fermion Gemische erweitert, um grundlegende Größen wie Quasiteilchenspektren, die Grundzustandsenergie und daraus abgeleitete Größen über die Molekularfeldtheorie hinaus zu berechnen.<br /> <br /> Unter Zuhilfenahme der dieser Resultate System wurde ein Boson-Fermion Gemisch in einem Fallenpotential im Rahmen der Dichtefunktionaltheorie beschrieben. Daraus konnten die Dichteprofile ermittelt werden und es ließen sich drei Bereiche im Phasendiagramm identifizieren: <br /> (i) ein Bereich eines stabilen Gemisches,<br /> (ii) ein Bereich, in dem die Spezies entmischt sind und <br /> (iii) ein Bereich, in dem das System kollabiert.<br /> <br /> Im letzten dieser drei Fällen waren Austausch--Korrelationseffekte signifikant. Weiterhin wurde die Änderung der kritischen Temperatur der Bose-Einstein-Kondensation aufgrund der Boson-Fermion-Wechselwirkung berechnet. Verursacht wird dieser Effekt von Dichtumverteilungen aufgrund der Wechselwirkung.<br /> <br /> Dann wurden Boson-Fermion Gemische in optischen Gittern betrachtet. Ein Stabilitätskriterium gegen Phasenentmischung wurde gefunden und es ließen sich Bedingungen für einen supraflüssig zu Mott-isolations Phasenübergang angeben. Diese wurden sowohl mittels einer Molekularfeldrechnung als auch numerisch im Rahmen eines Gutzwilleransatzes gefunden. Es wurden weiterhin neuartige frustrierte Grundzustände im Fall von sehr großen Gitterstärken gefunden. / The theory of atomic Boson-Fermion mixtures in the dilute limit beyond mean-field is considered in this thesis.<br /> Extending the formalism of quantum field theory we derived expressions for the quasi-particle excitation spectra, the ground state energy, and related quantities for a homogenous system to first order in the dilute gas parameter.<br /> <br /> In the framework of density functional theory we could carry over the previous results to inhomogeneous systems. We then determined to density distributions for various parameter values and identified three different phase regions: <br /> (i) a stable mixed regime, <br /> (ii) a phase separated regime, and <br /> (iii) a collapsed regime. <br /> <br /> We found a significant contribution of exchange-correlation effects in the latter case. Next, we determined the shift of the Bose-Einstein condensation temperature caused by Boson-Fermion interactions in a harmonic trap due to redistribution of the density profiles.<br /> <br /> We then considered Boson-Fermion mixtures in optical lattices. We calculated the criterion for stability against phase separation, identified the Mott-insulating and superfluid regimes both, analytically within a mean-field calculation, and numerically by virtue of a Gutzwiller Ansatz. We also found new frustrated ground states in the limit of very strong lattices. <br><br>----<br>Anmerkung:<br> Der Autor ist Träger des durch die Physikalische Gesellschaft zu Berlin vergebenen Carl-Ramsauer-Preises 2004 für die jeweils beste Dissertation der vier Universitäten Freie Universität Berlin, Humboldt-Universität zu Berlin, Technische Universität Berlin und Universität Potsdam.

I have been one of the main responsible for building a new laboratory for Bose-Einstein condensation with 87Rb. In particular, the experimental setup has been designed for performing experiments with Bose-Einstein condensates load into optical lattices of variable geometries. All parts essential for Bose-Einstein condensation are in place. Atoms are collected in a magneto-optical trap, transferred to another vacuum chamber, with better vacuum, and trapped in another magneto-optical trap. Atoms are successfully transferred to a dark magnetic trap, and system for diagnostics with absorption imaging has been realized. We have not yet been able to form a Bose-Einstein condensate, due to a range of technical difficulties. Equipment for alignment of optical lattices with flexible geometry has been designed, built, and tested. This tool has been proven to work as desired, and there is a great potential for a range of unique experiments with Bose-Einstein condensates in optical lattices of various geometries, including superlattices and quasi-periodic lattices. Numerical studies have been made on anisotropic optical lattices, and the existence of a transition between a 2D superfluid phase and a 1D Mott-insulating phase has been confirmed. We have shown that the transition is of Berezinskii-Kosterlitz-Thouless type. In another numerical study it has been shown that using stimulated Raman transitions is a practical method for transferring atoms between states in a double optical lattice. Thus, it will be possible to transfer populations between the lattices, with further applications in qubit read/write operations.

High angular momentum Rydberg wave packets are studied. Application of carefully tailored electric fields to low angular momentum, high- n ( n ∼ 300) Rydberg atoms creates coherent superpositions of Stark states with near extreme values of angular momentum, [cursive l]. Wave packet components orbit the parent nucleus at rates that depend on their energy, leading to periods of localization and delocalization as the components come into and go out of phase with each other. Monitoring survival probability signals in the presence of position dependent probing leads to observation of characteristic oscillations based on the composition of the wave packet. The discrete nature of electron energy levels is observed through the measurement of quantum revivals in the wave packet localization signal. Time-domain spectroscopy of these signals allows determination of both the population and phase of individual superposition components. Precise manipulation of wave packets is achieved through further application of pulsed electric fields. Decoherence effects due to background gas collisions and electrical noise are also detailed. Quantized classical trajectory Monte-Carlo simulations are introduced and agree remarkably well with experimental results.

The emergence of many-body physical phenomena from the quantum mechanical properties of atoms can be studied using ultracold alkali gases. The ability to manipulate both Bose-Einstein condensates (BECs) and degenerate Fermi gases (DFGs) with designer potential energy landscapes, variable interaction strengths and out-of-equilibrium initial conditions provides the opportunity to investigate collective behaviour under diverse conditions. With an appropriately chosen wavelength, optical standing waves provide a lattice potential for one target species while ignoring another spectator species. A “tune-in” scheme provides an especially strong potential for the target and works best for Li-Na, Li-K, and K-Na mixtures, while a “tune-out” scheme zeros the potential for the spectator, and is pre- ferred for Li-Cs, K-Rb, Rb-Cs, K-Cs, and 39K-40K mixtures. Species-selective lattices provide unique environments for studying many-body behaviour by allowing for a phonon-like background, providing for effective mass tuning, and presenting opportunities for increasing the phase-space density of one species. Ferromagnetism is manifest in a two-component DFG when the energetically preferred many-body configuration segregates components. Within the local density approximation (LDA), the characteristic energies and the three-body loss rate of the system all give an observable signature of the crossover to this ferromagnetic state in a trapped DFG when interactions are increased beyond kF a(0) = 1.84. Numerical simulations of an extension to the LDA that account for magnetization gradients show that a hedgehog spin texture emerges as the lowest energy configuration in the ferromagnetic regime. Explorations of strong interactions in 40K constitute the first steps towards the realization of ferromagnetism in a trapped 40K gas. The many-body dynamics of a 87Rb BEC in a double well potential are driven by spatial phase gradients and depend on the character of the junction. The amplitude and frequency characteristics of the transport across a tunable barrier show a crossover between two paradigms of superfluidity: Josephson plasma oscillations emerge for high barriers, where transport is via tunnelling, while hydrodynamic behaviour dominates for lower barriers. The phase dependence of the many-body dynamics is also evident in the observation of macroscopic quantum self trapping. Gross-Pitaevskii calculations facilitate the interpretation of system dynamics, but do not describe the observed damping.

The emergence of many-body physical phenomena from the quantum mechanical properties of atoms can be studied using ultracold alkali gases. The ability to manipulate both Bose-Einstein condensates (BECs) and degenerate Fermi gases (DFGs) with designer potential energy landscapes, variable interaction strengths and out-of-equilibrium initial conditions provides the opportunity to investigate collective behaviour under diverse conditions. With an appropriately chosen wavelength, optical standing waves provide a lattice potential for one target species while ignoring another spectator species. A “tune-in” scheme provides an especially strong potential for the target and works best for Li-Na, Li-K, and K-Na mixtures, while a “tune-out” scheme zeros the potential for the spectator, and is pre- ferred for Li-Cs, K-Rb, Rb-Cs, K-Cs, and 39K-40K mixtures. Species-selective lattices provide unique environments for studying many-body behaviour by allowing for a phonon-like background, providing for effective mass tuning, and presenting opportunities for increasing the phase-space density of one species. Ferromagnetism is manifest in a two-component DFG when the energetically preferred many-body configuration segregates components. Within the local density approximation (LDA), the characteristic energies and the three-body loss rate of the system all give an observable signature of the crossover to this ferromagnetic state in a trapped DFG when interactions are increased beyond kF a(0) = 1.84. Numerical simulations of an extension to the LDA that account for magnetization gradients show that a hedgehog spin texture emerges as the lowest energy configuration in the ferromagnetic regime. Explorations of strong interactions in 40K constitute the first steps towards the realization of ferromagnetism in a trapped 40K gas. The many-body dynamics of a 87Rb BEC in a double well potential are driven by spatial phase gradients and depend on the character of the junction. The amplitude and frequency characteristics of the transport across a tunable barrier show a crossover between two paradigms of superfluidity: Josephson plasma oscillations emerge for high barriers, where transport is via tunnelling, while hydrodynamic behaviour dominates for lower barriers. The phase dependence of the many-body dynamics is also evident in the observation of macroscopic quantum self trapping. Gross-Pitaevskii calculations facilitate the interpretation of system dynamics, but do not describe the observed damping.

En aquesta tesi presentem un conjunt d'estudis sobre sistemes atòmics on els efectes quàntics són especialment destacats. Aquests estudis s'han dut a terme aplicant diverses tècniques de la física teòrica de molts cossos.En primer lloc hem estudiat la possible existència d'una transició de fase superfluida en un gas ultrafred d'àtoms fermiònics, mitjançant una generalització de la teoria BCS de la superconductivitat que dóna especial rellevància al paper jugat per l'asimetria de densitat entre les dues espècies, i permet que l'estat fonamental presenti un trencament espontani de simetria.En una segona part, hem estudiat la dinàmica d'un condensat de Bose-Einstein el grau de llibertat d'espí del qual pot evolucionar dins d'una trampa òptica quasi-unidimensional, tant a temperatura zero com finita, mitjançant una formulació de camp mitjà.Finalment, hem dut a terme un estudi detallat de l'estat fonamental i la tensió lineal de sistemes bidimensional d'heli-4, primerament mitjançant les tècniques de Monte Carlo, i posteriorment amb un funcional de la densitat construit amb aquest objectiu. / EN CASTELLÀ: En esta tesis se presenta un conjunto de estudios sobre sistemas atómicos donde los efectos cuánticos son especialmente destacados. Dichos estudios se han llevado a cabo aplicando varias técnicas de la física teórica de muchos cuerpos.En primer lugar, se ha estudiado la posible existencia de una transición superfluida en un gas ultrafrío de átomos fermiónicos mediante una generalización de la teoría BCS de la superconductividad que presta especial atención al papel jugado por la asimetría de densidad entre las dos especies, y permite que el estado fundamental presente una rotura espontánea de simetría.En una segunda parte, se ha estudiado la dinámica de un condensado de Bose-Einstein cuyo grado de libertad de espín puede evolucionar en una trampa óptica cuasi-unidimensional, tanto a temperatura cero como finita, mediante una formulación de campo medio.Finalmente, se ha llevado a cabo un estudio detallado del estado fundamental y la tensión lineal de sistemas bidimensionales de helio-4, primeramente mediante las técnicas de Monte Carlo, y posteriormente con un funcional de la densidad construido al efecto.PALABRAS CLAVE: Átomos fríos, Aparejamiento, Condensado espinorial, Helio, Dos dimensiones / SUMMARY: This thesis presents a set of studies on atomic systems where quantum efects are particularly relevant. These studies have been developed by applying a variety of tools from many-body physics.First of all, we have studied the prospects for the existance of a superfluid transition in an ultracold gas of fermionic atoms, by generalizing the BCS theory of superconductivity to the case when the two species that pair have different densities and the ground state may spontaneously break one or more symmetries.In a second part, we have studied the dynamics of a Bose-Einstein condensate whose spin degree of freedom is free to evolve inside a quasi-onedimensional optical trap. We have used a mean-field formulation to address both the zero temperature case and the finite temperature one.Finally, we have performed a careful study of the ground state and the line tension of two-dimensional systems of helium-4. First, we have used Monte Carlo techniques, then with a Density Functional built on-purpose.KEYWORDS: Cold gases, Pairing, Spinor condensate, Helium, Two dimensions