In this thesis, both experimental studies and numerical simulations of ultracold atoms in optical potentials are presented in a collection of nine scientific papers. In particular, noise-induced transport in dissipative optical lattices and superfluid properties of Bose-Einstein condensates have been studied. Noise is usually regarded as a complication to most systems and as something that needs to be minimized. However, in a series of experiments at Umeå University, noise has been shown to play a key role for laser-cooled cesium atoms trapped in dissipative optical lattices. By using a combination of two dissipative optical lattices, where the relative spatial phase between them can be controlled, a so-called Brownian motor can be realized, where energy can be extracted from the inherent noise. In the experiment, this energy is used to control the transport of the laser-cooled atoms in real time and along pre-designed paths. This thesis also presents a way to characterize this system in terms of energy conversion efficiency and coherence of the transport, which may allow for a more straightforward comparison with other systems that rely on noise rectification. In the studies, it is also shown that the noise triggers a downward drift due to gravity, even though the optical potential should support the atoms. Further investigation of this might help to understand the underlying principles of laser cooling, as well as showing that the system might be suitable as a flexible test bed for statistical physics. In close relation to the experimental system, two numerical simulations are also presented, one in which different ways to induce asymmetries between two periodic potentials are investigated, and one in which a proposal for detecting quantum walks is explored. In the second part of the thesis, a work from the Joint Quantum Institute is presented, where a long-lived persistent current in a toroidal Bose-Einstein condensate, held in an all-optical trap, is created. The critical velocity of the superflow is measured in the presence of a tunable barrier. The system can be seen as a first realization of an elementary closed-loop atom circuit. Finally a theoretical study of the crossover between one- and two-dimensional systems is presented, in particular the transition between a two-dimensional superfluid to a one-dimensional Mott insulator is investigated. / Medelst nio vetenskapliga artiklar presenteras i denna avhandling experimentella och teoretiska studier av ultrakalla atomer fångade i optiska potentialer. Framförallt har brusinducerade transporter och supraytande egenskaper hos Bose-Einstein-kondensat studerats.     För de flesta system betraktas brus som något negativt som bör minimeras, men i en serie experiment som redovisas i denna avhandling spelar bruset istället en avgörande positiv roll. I ett system där laserkylda atomer genom växelverkan med laserstrålar fångas i två individuella optiska kristallgitter, kan atomernas kollektiva rörelse styras genom att energi utvinns ur det inneboende bruset. I denna avhandling, genom att kontrollera de optiska potentialerna i realtid, visas att atomernas kollektiva rörelse kan styras längs förutbestämda banor med en så kallade Brownska motor. I ett annat experiment mäts verkningsgraden i omvandligen mellan brus och arbete, samt koherensen i atomtransporten. En sådan karakterisering gör att systemet blir enklare att jämföra med andra system som bygger på liknande principer. I avhandlingen presenteras också en studie där det visas att det inneboende bruset i systemet, tillsammans med en svag kraft, i detta fall från gravitation, kan skapa drifter trots att de optiska potentialerna borde vara tillräckligt djupa för att atomerna ska vara fångade. Denna upptäckt kan leda till ökad grundläggande kundskap om laserkylning. Dessutom visar det att systemet kan beskrivas med modeller från statistisk fysik. I relation till det experimentella systemet i Umeå redovisas även två teoretiska studier, en för två symmetriska periodiska potentialer och deras sätt att möjliggöra inducerade drifter med olika typ av asymmetrier, samt en annan för möjligheten att genomföra och detektera kvantvandringar.     I avhandlingen presenteras också ett experimentellt arbete utfört vid Joint Quantum Institute, där en långlivad ihållande ström i ett torusformat Bose-Einstein-kondensat har skapats i en optisk fälla. Den kritiska hastigheten på strömmen har mätts i närvaron av en ställbar optisk barriär. Detta system kan ses som en första realisation av en grundläggande atomkrets. Slutligen presenteras även en teoretisk studie av övergången mellan en- och tvådimensionella system, där fasövergången mellan superytande och Mottisolation studeras.

This thesis investigates thermalisation, correlations and entanglement in Bose-Einstein condensates. Bose-Einstein condensates are ultra-cold collections of identical bosonic atoms which accumulate in a single quantum state, forming a mesoscopic quantum object. They are clean and controllable quantum many-body systems that permit an unprecedented degree of experimental flexibility compared to other physical systems. Further, a tractable microscopic theory exists which allows a direct and powerful comparison between theory and experiment, propelling the field of quantum atom optics forward at an incredible pace. Here we explore some of the fundamental frontiers of the field, examining how non-classical correlations and entanglement can be created and measured, as well as how non-classical effects can lead to the rapid heating of atom clouds. We first investigate correlations between two weakly coupled condensates, a system analogous to a superconducting Josephson junction. The ground state of this system contains non-classical number correlation arising from the repulsion between the atoms. Such states are of interest because they may lead to more precise measurement devices such as atomic gyroscopes. Unfortunately thermal fluctuations can destroy these correlations, and great care is needed to experimentally observe non-classical effects. We show that adiabatic evolution can drive the isolated quantum system out of thermal equilibrium and decrease thermal noise, in agreement with a recent experiment [Esteve et al. Nature 455, 1216 (2008)]. This technique may be valuable for observing and using quantum correlated states in the future. Next, we analyse the rapid heating that occurs when a condensate is placed in a moving periodic potential. The dynamical instability responsible for the heating was the subject of much uncertainty, which we suggest was due to the inability of the mean-field approximation to account for important spontaneous scattering processes. We show that a model including non-classical spontaneous scattering can describe dynamical instabilities correctly in each of the regimes where they have been observed, and in particular we compare our simulations to an experiment performed at the University of Otago deep inside the spontaneous scattering regime. Finally, we proposed a method to create and detect entangled atomic wave-packets. Entangled atoms are interesting from a fundamental perspective, and may prove useful in future quantum information and precision measurement technologies. Entanglement is generated by interactions, such as atomic collisions in Bose-Einstein condensates. We analyse the type of entanglement generated via atomic collisions and introduce an abstract scheme for detecting entanglement and demonstrating the Einstein-Podolsky-Rosen paradox with ultra-cold atoms. We further this result by proposing an experiment where entangled wave-packets are created and detected. The entanglement is generated by the pairwise scattering that causes the instabilities in moving periodic potentials mentioned above. By careful arrangement, the instability process can be controlled to to produce two well-defined atomic wave-packets. The presence of entanglement can be proven by applying a series of laser pulses to interfere the wave-packets and then measuring the output populations. Realising this experiment is feasible with current technology.

This thesis investigates thermalisation, correlations and entanglement in Bose-Einstein condensates. Bose-Einstein condensates are ultra-cold collections of identical bosonic atoms which accumulate in a single quantum state, forming a mesoscopic quantum object. They are clean and controllable quantum many-body systems that permit an unprecedented degree of experimental flexibility compared to other physical systems. Further, a tractable microscopic theory exists which allows a direct and powerful comparison between theory and experiment, propelling the field of quantum atom optics forward at an incredible pace. Here we explore some of the fundamental frontiers of the field, examining how non-classical correlations and entanglement can be created and measured, as well as how non-classical effects can lead to the rapid heating of atom clouds. We first investigate correlations between two weakly coupled condensates, a system analogous to a superconducting Josephson junction. The ground state of this system contains non-classical number correlation arising from the repulsion between the atoms. Such states are of interest because they may lead to more precise measurement devices such as atomic gyroscopes. Unfortunately thermal fluctuations can destroy these correlations, and great care is needed to experimentally observe non-classical effects. We show that adiabatic evolution can drive the isolated quantum system out of thermal equilibrium and decrease thermal noise, in agreement with a recent experiment [Esteve et al. Nature 455, 1216 (2008)]. This technique may be valuable for observing and using quantum correlated states in the future. Next, we analyse the rapid heating that occurs when a condensate is placed in a moving periodic potential. The dynamical instability responsible for the heating was the subject of much uncertainty, which we suggest was due to the inability of the mean-field approximation to account for important spontaneous scattering processes. We show that a model including non-classical spontaneous scattering can describe dynamical instabilities correctly in each of the regimes where they have been observed, and in particular we compare our simulations to an experiment performed at the University of Otago deep inside the spontaneous scattering regime. Finally, we proposed a method to create and detect entangled atomic wave-packets. Entangled atoms are interesting from a fundamental perspective, and may prove useful in future quantum information and precision measurement technologies. Entanglement is generated by interactions, such as atomic collisions in Bose-Einstein condensates. We analyse the type of entanglement generated via atomic collisions and introduce an abstract scheme for detecting entanglement and demonstrating the Einstein-Podolsky-Rosen paradox with ultra-cold atoms. We further this result by proposing an experiment where entangled wave-packets are created and detected. The entanglement is generated by the pairwise scattering that causes the instabilities in moving periodic potentials mentioned above. By careful arrangement, the instability process can be controlled to to produce two well-defined atomic wave-packets. The presence of entanglement can be proven by applying a series of laser pulses to interfere the wave-packets and then measuring the output populations. Realising this experiment is feasible with current technology.

Les neutrons peuvent rebondir sur un miroir horizontal parfait, et se comporter comme des objets quantiques à quelques dizaines de microns de la surface. Le spectromètre GRANIT, situé à l'Institut Laue-Langevin (Grenoble, France), a pour but d'étudier les états quantiques du neutron dans le champ de pesanteur terrestre. L'énergie d'un neutron, que l'on pourrait calculer de façon analogue à celle d'une balle de tennis de table à l'échelle macrosopique, est dans ce cas visiblement contrainte à prendre des valeurs discrètes. L'étude de ces états quantiques peut permettre la découverte d'une déviation par rapport aux prédictions des modèles actuels. La production de neutrons ultra-froids, pouvant rebondir sur un miroir avec n'importe quel angle d'incidence, est primordiale pour le spectromètre. Le cryostat de la source de neutrons ultra-froids SUN1 a été amélioré, et les différentes étapes de production des neutrons ultra-froids dans la source et leur extraction vers le spectromètre ont été caractérisées par des mesures et modélisées avec des simulations. De plus, un nouveau type de détecteur de neutrons ultra-froids sensible à la position est en conception spécialement pour le spectromètre. / Neutrons can bounce upon a perfect horizontal mirror and become quantum objects at dozens of micrometers over its surface. The GRANIT spectrometer, located at the Laue-Langevin Institute (Grenoble, France) aims at studying the neutron quantum states in the Earth's gravitational field. The energy of a neutron, that could be calculated the same as the one of a ball at a macroscopic scale, is then forced to take discrete values. The study of such quantum states can lead to the observation of deviations from the predictions of nowadays models. The production of ultracold nuetrons, able tobounce on a mirror at any incidence angle, is essential for the spectrometer. The cryostat of the ultracold neutrons source has been improved, and the steps of the ultracold neutrons production in the source, as well as the extraction toward the spectrometer have been characterized by measurements and modeled with simulations. Moreover, a new type of position-sensitive detector of ultracold neutrons for the spectrometer is designed.

No description available.

Condensados de Bose-Einstein (BEC) são sistemas macroscópicos excelentes para a observação do comportamento quântico da matéria. Desde sua obtenção experimental em gases atômicos alcalinos diluídos aprisionados por campos magnéticos, há importantes aspectos relacionados a esse sistema que foram intensamente explorados, como os modos coletivos do BEC harmonicamente aprisionado, seu tunelamento através de barreiras de potencial e os estados excitados desse sistema, incluindo vórtice e sóliton. O último consiste de pacote de onda localizado, que propaga sem mudança de forma. Nesse trabalho, investigamos os novos aspectos que surgem da dinâmica de um sistema composto (condensado aprisionado contendo um sóliton). Há muitos estudos tratando cada parte separadamente: estado fundamental do BEC ou um sóliton em um BEC infinito uniforme estacionário. Estamos nos baseando nessas análises prévias, além da simulação numérica de campo médio do nosso sistema submetido a diferentes condições iniciais (BEC aprisionado no mínimo do potencial harmônico ou BEC deslocado na armadilha contendo um sóliton, além de uma deformação no potencial) para caracterizar a dinâmica desse sistema. Alguns dos nossos resultados puderam ser explicados por meio de predições analítica da chamada aproximação de Thomas-Fermi. Ao final, comparamos as simulações de campo médio (equação de Gross-Pitaevskii) com as advindas da teoria de múltiplos orbitais a fim de justificar o regime de validade da nossa teoria. / Bose-Einstein Condensates (BEC) are excellent macroscopic systems to observe the quantum behavior of matter. Since it experimental production in dilute atomic alkali gases trapped by magnetic fields, there are important aspects related to this system that have been intensely explored, like the collective modes of the harmonically trapped BEC, its tunneling through a potential barrier and the excited states of this system, that include the vortex and soliton. The latter consist of localized disturbances, which propagate without change of form. In this work, we investigate the singular aspects that coming from the dynamics of a composite system (trapped BEC containing a soliton). There are many studies that treat each part separately, that include a fundamental state BEC or a soliton inside a uniform infinite extent stationary BEC. We are basing on these previous analyses, besides mean-field numeric simulating our particular system submitted to diferent initial conditions (minimum harmonic potential trapped BEC or dislocated trapped BEC plus a soliton, in addition to a deformation in the potential) to characterize the tunneling dynamics. Some of our results could be explained using analytical predictions of the so called Thomas-Fermi approximation. At the end, we compar the meanfield simulations (Gross-Pitavskii equation) with the simulations from the multiple orbitals theory to justify the validity regime of our theory.

Les systèmes composés d'atomes et d'ions ultrafroids ont étés un sujet d'intérêt pour les physiciens atomiques et, plus récemment, pour la communauté des ions froids (simulation et calcul quantique avec des ions piégés). Ils sont considéré la possibilité d'utiliser un gaz d'atomes ultrafroids pour refroidir sympathiquement les ions car la modulation intrinsèque du mouvement, le micromouvement, représente une source de décohérence dans les applications des ions froids. L'intérêt envers ce système mixte est aussi motivé par l'étude de la physique d'impuretés et par une meilleure compréhension des réactions entre espèces ioniques et neutre ayant pour but la création d'ions moléculaires. Cette thèse a pour objectif d'étudier les effets du micromouvement dans les collisions atome-ion. Nous traitons au préalable les collisions à 1D d'une particule dans un piège harmonique (un ion) et d'un particule libre (une atome) en utilisant différentes approches numériques. Ce système est intéressant en soi en raison de la dimensionnalité mixte 0D-1D. Le potentiel atome-ion est modélisé par une interaction à portée nulle tout au cours de ce travail. Par la suite, nous traitons un problème similaire mais dans le cas d'une particule dans un piège harmonique décrivant un piège de Paul. Enfin, nous généralisons l'étude du micromouvement à un système modèle 3D avec un ion dans un piège de Paul sphérique 3D et un atome lourd au centre du piège. Nous discutons de l'influence du micromouvement en vue d'applications potentielles de ce système telle que la porte logique de phase. / Ultracold atom-ion systems have been a topic of interest for atomic physicists studying chemical reactions and since recently, the cold ion community (ion trap quantum computation and simulation). They have been looking at the possibility of using an ultracold atom gas to sympathetically cool ions since intrinsic motional modulation i.e micromotion is an inherent cause of decoherence in coherent applications of cold ions. Interest is also piqued by the possibility of using this hybrid system for studying impurity physics and to better understand ion-neutral reactions aimed at creation of molecular ions. In this thesis, we aim to study the effect of ion micromotion in atom-ion collision. As a prelude, we treat the 1D collision of a particle in a harmonic trap (ion) and a free particle (atom) using different numerical schemes. This system is of interest in its own right due to the mixed 0D-1D dimensionality. Atom-ion potential is simplified to a zero range potential all through out the work. Next we deal with a similar problem but with the trapped particle in a time dependent harmonic trap identical to an ion Paul trap. Finally we extend the study of micromotion to a model system in 3D with an ion in a 3D spherical Paul trap and a heavy atom at the trap centre. We discuss the effect micromotion has on potential applications of such a system, like a quantum phase gate.

This thesis deals with systems of interacting particles with very low energy in the limit where the particle-particle scattering is much larger than the range of the interactions. We use a quantum-field-theory approach which allows us to study both few-body and dense-matter systems in a unified framework. This allows to introduce composite fields of two and three particles (when appropriate). The quantum corrections are calculated nonperturbatively with the Functional RenormalisationGroup. We deal with three types of systems. First we study systems with three and four scalar particles. For three-particle systems our framework describes the Efimov effect. During the FRG flow in the scaling limit, the four-particle system has an infinite sequence of (unphysical) four-particle states on top of each Efimov trimer. This is a case of super Efimov behaviour. Three of these four-particle states survive to the physical limit. Two of these three states have been found in exact quantum-mechanical calculations, and have also been observed in gases of ultracold atoms. Next, this thesis studies systems of three and four spin-1/2 particles. In the scaling limit, we find attractive fixed points for the three- and four-particle systems. Out of the scaling limit, we study atom-molecule scattering and molecule-molecule scattering, in particular their scattering length. Finally, we study dense-matter systems of spin-1/2 particles. This calculation includes all the two-, three-, and four-particle interactions. These systems show spontaneous symmetry breaking: the two-particle field has a finite classical value. We find the value of the atom gap in units of the chemical potential.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21550号 / 理博第4457号 / 新制||理||1640(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 戸塚 圭介, 教授 川上 則雄, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Au sein de physique des systèmes quantiques désordonnés, le domaine des atomes ultra-froids est en pleine croissance. En l’occurrence, l'étude de la relation entre la localisation et les interactions a permis de découvrir la richesse de la physique de la localisation à N-corps. Ce phénomène remarquable fournit un mécanisme pour la brisure de l'ergodicité dans les systèmes quantiques isolés et désordonnés. Plusieurs questions ont été évoquées après cette découverte, comme la possibilité d'une transition fluide-isolant à température finie. Dans cette thèse, j'étudie la localisation à N-corps dans le contexte de bosons désordonnés à deux dimensions. Dans la première partie, je présente l'étude d'un gaz interactif de Bose bidimensionnel dans un potentiel aléatoire à température finie. Le système présente deux transitions à température finie: la transition de localisation à N-corps entre fluide et isolant, et la transition de Berezinskii-Kosterlitz-Thouless entre superfluide algébrique et fluide. J'examine ensuite l'influence de la troncature de la distribution d'énergie dû au piégeage, un phénomène générique dans le cadre du refroidissement d'atomes ultra-froids. Finalement, je conclus en discutant la stabilité de la phase isolante dans des systèmes définis sur un continuum. / The study of the interplay between localization and interactions in disordered quantum systems led to the discovery of the interesting physics of many-body localization (MBL). This remarkable phenomenon provides a generic mechanism for the breaking of ergodicity in quantum isolated systems, and has stimulated several questions such as the possibility of a finite-temperature fluid-insulator transition. At the same time, the domain of ultracold interacting atoms is a rapidly growing field in the physics of disordered quantum systems. In this thesis, we study many-body localization in the context of two-dimensional disordered ultracold bosons. After reviewing some importance concepts, we present a study of the phase diagram of a two-dimensional weakly interacting Bose gas in a random potential at finite temperatures. The system undergoes two finite-temperature transitions: the MBL transition from normal fluid to insulator and the Berezinskii-Kosterlitz-Thouless transition from algebraic superfluid to normal fluid. At T=0, we show the existence of a tricritical point where the three phases coexist. We also discuss the influence of the truncation of the energy distribution function at the trap barrier, a generic phenomenon for ultracold atoms. The truncation limits the growth of the localization length with energy and, in contrast to the thermodynamic limit, the insulator phase is present at any temperature. Finally, we conclude by discussing the stability of the insulating phase with respect to highly energetic particles in systems defined on a continuum.