Dissertations / Theses on the topic 'Bose-Einstein condensates'
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Henkel, Nils. "Rydberg-dressed Bose-Einstein condensates." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-130499.
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My dissertation treats the physics of ultracold gases, in particular of Bose-Einstein condensates with long-ranged interactions induced by admixing a small fraction of a Rydberg state to the atomic ground state. The resulting interaction leads to the emergence of supersolid states and to the self-trapping of a Bose-Einstein condensate.
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Söhn, Matthias. "Solitons in Bose-Einstein Condensates." [S.l. : s.n.], 2002. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB10047894.
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Carr, Lincoln D. "Solitons in Bose-Einstein condensates /." Thesis, Connect to this title online; UW restricted, 2001. http://hdl.handle.net/1773/9702.
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Hallwood, David William. "Macroscopic superpositions using Bose-Einstein condensates." Thesis, University of Oxford, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491506.
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The differences between classical and quantum mechanics were highlighted early in the development of quantum mechanics when Schrodinger proposed the thought experiment of a cat in a superposition of alive and dead. In this thesis I try to understand these differences by considering superpositions of large objects at a single particle level. Research in the field of superconductors has provided evidence for macroscopic quantum superpositions (or cat states) of currents in superconducting loops. Bose-Einstein condensates of ultracold atoms provide another promising system for experimentally producing similar results. I begin by describing two straightforward schemes that make macroscopic superpositions of superfluid flow states of Bose-Einstein condensates trapped in optical lattice rings. The first scheme achieves a superposition of three flow states by nonadiabatically evolving the barrier heights between the sites. The second scheme produces a superposition of two flow states by applying a 7f phase around the ring. This could be experimentally achieved by physically rotating the sites or imparting angular momentum from two co-propagating lasers. The next part of the thesis investigates why it is difficult to produce macroscopic superpositions. By treating the interaction strength between the atoms as a perturbation I show three reasons, other than decoherence, why macroscopic superpositions are hard to make. Firstly, the energy of the two distinct flow states must be sufficiently close. Secondly, coupling between the two states must be sufficiently strong, and thirdly, other states must be well separated from those two flow states. To make larger superpositions I look at a Josephson junction coupled to a superfluid loop. This shows that making superpositions depends on the number of atoms in the junction rather than the whole system. Finally I propose ways of developing the work. This concentrates on how the systems could experimentally create macroscopic superpositions and how we could measure signatures of these states. I then suggest ways of using the systems, such as quantum information and precision measurement schemes.
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Murray, Douglas R. "Vector potentials in bose-einstein condensates." Thesis, University of Strathclyde, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.501825.
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Dunningham, Jacob Andrew. "Quantum phase of Bose-Einstein condensates." Thesis, University of Oxford, 2001. http://ora.ox.ac.uk/objects/uuid:b6cc8b74-753c-4b3e-ad5e-68bd7e32b652.
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The quantum phase of a Bose-Einstein condensate has long been a subject fraught with misunderstanding and confusion. In this thesis we provide a consis- tent description of this phenomenon and, in particular, discuss how phase may be defined, created, manipulated, and controlled. We begin by describing how it is possible to set up a reference condensate against which the phase of other condensates can be compared. This allows us to think of relative phases as if they were absolute and gives a clear and precise definition to 'the phase of a condensate'. A relative phase may also be established by coupling condensates and we show how this can be controlled. We then extend this model to explain how the phase along a chain of coupled condensates can lock naturally without the need for any measurements. The second part of the thesis deals primarily with the link between entangle- ment and phase. We show that, in general, the more entangled a state is, the better its phase resolution. This leads us to consider schemes by which maximally entangled states may be able to be created since these should give the best prac- tical advantages over their classical counterparts. We consider two such states: a number correlated pair of condensates and a Schrodinger cat state. Both schemes are shown to be remarkably robust to loss. A comparison of the merits of these two states, as the inputs to an interferom- eter, reveals very different behaviours. In particular, the number correlated state performs significantly better than the cat state in the presence of loss, which means that it might be useful in interferometry and frequency standard schemes where phase resolution is of the utmost importance. Finally, we propose a scheme for concentrating the entanglement between con- densates, which is an important step in quantum communication protocols. This, along with the ability to manipulate phase and entanglement, suggests that the future for condensates holds not only academic interest but great potential for practical applications.
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Proud, Harry. "Soliton structures in Bose-Einstein condensates." Thesis, University of Birmingham, 2018. http://etheses.bham.ac.uk//id/eprint/8156/.
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The generation of dark solitons in Bose-Einstein condensates has been an area of interest since the first experimental condensates were produced. The ubiquity of solitons in the natural world makes them an important phenomenon to understand. Despite excellent theoretical work in two dimensional dark solitons, few experiments have had the opportunity to investigate this regime. The work presented investigates the generation of dark solitons in a Rb-87 Bose-Einstein condensate. The evolution and decay of these topological excitations are investigated. The decay of the dark solitons is found to vary with the phase-step used to generate them. Dark solitons created with a phase-step width of 0.60 ±0.15 μm are found to decay into vortices after 10 ms. Dark solitons generated with larger phase-steps are found not to exhibit this vortex decay, instead dissipating over 10-15 ms back into the condensate. The first experimental generation of two dimensional Jones-Roberts solitons is reported in this work. These dark solitons differ from the standard planar dark soliton in that they are finite in extent and are found to be more dynamically stable. The Jones-Roberts solitons are observed for 40 ms with no observed change in energy.
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Cragg, George E. (George Edwin) 1972. "Coherent decay of Bose-Einstein condensates." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/35304.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
"June 2006."
Includes bibliographical references (p. 205-209).
As the coldest form of matter known to exist, atomic Bose-Einstein condensates are unique forms of matter where the constituent atoms lose their individual identities, becoming absorbed into the cloud as a whole. Effectively, these gases become a single macroscopic object that inherits its properties directly from the quantum world. In this work, I describe the quantum properties of a zero temperature condensate where the atoms have a propensity to pair, thereby leading to a molecular character that coexists with the atoms. Remarkably, the addition of this molecular component is found to induce a quantum instability that manifests itself as a collective decay of the assembly as a whole. As a signature of this phenomenon, there arises a complex chemical potential in which the imaginary part quantifies a coherent decay into collective phonon excitations of a collapsing ground state. The unique decay rate dependencies on both the scattering length and the density can be experimentally tested by tuning near a Feshbach resonance. Being a purely quantum mechanical effect, there exists no mechanical picture corresponding to this coherent many-body process. The results presented can serve as a model for other systems with similar underlying physics.
by George E. Cragg.
Ph.D.
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Pasquini, Thomas A. Jr. "Quantum reflection of Bose-Einstein Condensates." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/45442.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (p. 133-147).
Recent developments in atom optics have brought Bose-Einstein condensates within 1 pm of solid surfaces where the atom-surface interactions can no longer be ignored. At long- range, the atom-surface interaction is described by the weakly attractive Casimir-Polder potential which is classically predicted to accelerate an incident atom toward the surface where it will interact strongly with the internal modes of the surface, lose energy, and land in a bound state of the surface. When the incident atom is very cold, on the order of a few nanokelvin, however, the acceleration of the atomic wavefunction is so abrupt that the atom may partially reflect from the attractive tail in a process known as quantum reflection. This work presents experimental evidence for quantum reflection from a solid surface at normal incidence. Using atoms from a 23Na BEC, cooled to a few nanokelvin in a recently demonstrated single-coil trap, controlled collisions were induced between atoms and solid silicon surface. A maximum reflection probability of - 12% was observed for an incident velocity of 1 mm/s. Atoms confined against the surface at low density exhibited an enhanced lifetime due to quantum reflection. A surprising aspect of quantum reflection is that nano-structured surfaces are predicted to exhibit enhanced quantum reflection due to the reduction of the atom-surface interaction from reduced density surfaces. Using a pillared surface with an density reduced to 1% of bulk density, we observe an enhancement of the reflection probability to ' 60%. At velocities from 2-25 mm/s, predicted threshold dependence of the reflection probability was observed. At velocities below 2 mm/s, the reflection probability was observed to saturate. We develop a simple model which predicts the saturation as a result of mean-field interactions between atoms in the incident Bose-Einstein condensate.
by Thomas A. Pasquini.
Ph.D.
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Moulder, Stuart. "Persistent currents in Bose-Einstein condensates." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648095.
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Jackson, Brian. "Vortices in trapped Bose-Einstein condensates." Thesis, Durham University, 2000. http://etheses.dur.ac.uk/4241/.
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In this thesis we solve the Gross-Pitaevskii equation numerically in order to model the response of trapped Bose-Einstein condensed gases to perturbations by electromagnetic fields. First, we simulate output coupling of pulses from the condensate and compare our results to experiments. The excitation and separation of eigen-modes on flow through a constriction is also studied. We then move on to the main theme of this thesis: the important subject of quantised vortices in Bose condensates, and the relation between Bose-Einstein condensation and superfluidity. We propose methods of producing vortex pairs and rings by controlled motion of objects. Full three-dimensional simulations under realistic experimental conditions are performed in order to test the validity of these ideas. We link vortex formation to drag forces on the object, which in turn is connected with energy transfer to the condensate. We therefore argue that vortex formation by moving objects is intimately related to the onset of dissipation in superfluids. We discuss this idea in the context of a recent experiment, using simulations to provide evidence of vortex formation in the experimental scenario. Superfluidity is also manifest in the property of persistent currents, which is linked to vortex stability and dynamics. We simulate vortex line and ring motion, and find in both cases precessional motion and thermodynamic instability to dissipation. Strictly speaking, the Gross-Pitaevskii equation is valid only for temperatures far below the BEG transition. We end the thesis by describing a simple finite- temperature model to describe mean-field coupling between condensed and non- condensed components of the gas. We show that our hybrid Monte-Carlo/FFT technique can describe damping of the lowest energy excitations of the system. Extensions to this model and future research directions are discussed in the conclusion.
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Cooper, Jessica Jane. "Quantum metrology with Bose-Einstein condensates." Thesis, University of Leeds, 2011. http://etheses.whiterose.ac.uk/1690/.
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The ability to make ultra-precise measurements is fundamentally important to science as it allows theories to be tested and refined. Interferometers offer unrivalled measurement precision and therefore form the basis of many metrology schemes. Research has shown that by using quantum states as inputs to interferometers, precisions better than anything possible classically can be achieved. Nevertheless, these states are difficult to produce and fragile to particle losses. Consequently, classical inputs, which are extremely robust, are used in experiments. Here, however, we propose experimentally accessible schemes to make quantum-limited measurements, in particular rotation measurements using Bose-Einstein condensates, that are robust to losses. We begin by describing how, by loading a Bose-Einstein condensate into an optical ring lattice, multiport beam splitters are created through a simple raising and lowering of potential barriers between sites. We then use these ‘splitters’ to create an atomic gyroscope. We demonstrate how to create several quantum states in the gyroscope, all capable of making rotation measurements. Whilst NOON states afford best precision in idealised set-ups, we find they are outperformed by ‘bat’ states for modest loss rates. However, bat states are not ideal as they are outperformed by classical states for large losses. A second gyroscope scheme is therefore developed. Using multiple momentum modes, rather than just two, we show quantum-limited precisions can be reached using states that have similar robustness to classical states. The final section focuses on the precision of linear interferometers. Recent work[1, 2] has calculated the theoretical optimum initial states for two-mode lossy interferometers. Here we present an experimental way to produce initial states that afford similar precisions to this optimum. We also consider lossy multimode interferometry and demonstrate a potential advantage over two-mode systems. It is thought with further investigation other advantages will be found.
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Melé, Messeguer Marina. "Josephson effect in multicomponent Bose-Einstein condensates." Doctoral thesis, Universitat de Barcelona, 2013. http://hdl.handle.net/10803/119605.
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In this thesis we study and characterize the behavior of Bose-Einstein condensates in a BJJ, using two different theoretical formalisms: the mean-field approximation (with the Gross-Pitaevskii equation) and many-body calculations (based on Bose-Hubbard models).
With single-component Bose-Einstein condensates, we have focused on the study of the structure of the ground state as a function of the system parameters. We have looked for strongly correlated states, that cannot be described with mean-field theories, and we have proposed a variational wave function that captures the structure of the ground state for a broad interval of the system parameters. We have also studied the nonlinear effects of the Gross-Pitaevskii equation, visible when atom-atom interactions are strong.
In the case of binary mixtures of Bose-Einstein condensates we have performed an intensive study of the different regimes that can arise and in which conditions. The standard two-mode approximation is one of the most used in the study of the Josephson effect, as it gives simple analytic equations that capture, to a great extend, the behavior of the system. When the link between condensates is not weak enough, one has to consider a correction to this approximation, namely, the improved two-mode approximation. In this thesis, we have derived this last approximation for the binary mixture and we have checked its validity comparing it with numerical simulations of the three-dimensional Gross-Pitaevskii equation. Moreover, as the Josephson dynamics is almost one-dimensional, we have considered the two most common reductions of the dimensionality of the Gross-Pitaevskii equation. In this case, we have also compared, using simulations, these one-dimensional reductions with the three-dimensional equation.
We have also studied spinor condensates in an external BJJ. We have focused on condensates formed by atoms with spin $F=1$, which can be in any of the three internal states $m_F = 0, \pm 1$. Furthermore, in contrast to the binary mixture, spin interchange is allowed, so that the number of particles of each component becomes a dynamic variable. First, we have studied this system within the mean-field framework, using the Gross-Pitaevskii equation. We have derived the two-mode approximation equations and we have focused in studying the decoupling of the Josephson effect and the population transfer dynamics. In this case, we also compare the results with numerical simulations of the three dimensional Gross-Pitaevskii equation. Second, we have studied the spinor BJJ using the Bose-Hubbard formalism, because some features of quantum fluctuations are better captured than with the mean-field. We have characterized the ground state, paying special attention to the regions where it is strongly correlated. We have seen how the spin singlet formation (strongly correlated state between two particles) affects the structure of the ground state.
Finally, we have studied finite temperature effects on spinor Bose-Einstein condensates in the presence of a magnetic field, for two different cases. First, we have analyzed a condensate formed by $F=1$ atoms with contact interactions. We have considered that the condensate was formed by atoms in the internal state $m_F=0$ and we have studied the dependence of the fluctuations of the other two components $m_F = \pm 1$ as a function of temperature. We have used the Bogoliubov formalism applied to an homogeneous system, and then, we have generalized the result to an harmonic trap by using the local density approximation. Second, we have studied a condensate formed by particles with $F=3$ with contact interactions and moreover, dipolar interactions. In a similar way as the previous case, we consider that the condensate is formed by particles with $m_F =-3$ and we study the fluctuations in $m_F = -2$ and $m_F = -3$.En aquesta Tesi s'estudia i es caracteritza el comportament dels condensats de Bose-Einstein en una junció bosònica de Josephson (BJJ), tot utilitzant dos formalismes teòrics diferents: l'aproximació de camp mig (amb l'equació de Gross-Pitaevskii) i càlculs de molts cossos (basats en models de Bose-Hubbard). En condensats d'una sola component, ens hem centrat en l'estudi de l'estructura de l'estat fonamental en funció dels paràmetres del sistema. Hem identificat estats altament correlacionats que no es poden descriure amb teories de camp mig, i hem proposat una funció d'ona variacional que captura l'estructura de l'estat fonamental en un ampli ventall de valors d'aquests paràmetres. També hem estudiat els efectes no lineals de l'equació de Gross-Pitaevskii, visibles quan les interaccions entre àtoms són fortes. Per condensats formats per dues components hem fet un estudi intensiu dels diferents règims que es poden formar i en quines condicions. Hem utiltizat el formalisme de camp mig i hem derivat l'aproximació bimodal estàndard millorada (I2M) tot comprovant-ne la seva validesa, comparant-la amb simulacions numèriques de l'equació de Gross-Pitaevskii tridimensional. També hem estudiat condensats espinorials en una BJJ externa. Ens hem centrat en condensats formats per àtoms amb spin $F=1$, que poden estar en qualsevol dels tres estats interns $m_F=0,\pm 1$. Primer, hem estudiat aquest sistema dins la teoria de camp mig, tot utilitzant l'equació de Gross-Pitaevskii. Hem derivat les equacions de l'aproximació bimodal, i ens hem centrat en estudiar com es desacobla l'efecte Josephson de la dinàmica d'intercanvi de partícules. Segon, hem utilitzant el formalisme de Bose-Hubbard i hem caracteritzat l'estat fonamental, tot fixant-nos en els effectes de la creació de singlets. Finalment, hem estudiat l'efecte de temperatura finita en condensats de Bose-Einstein espinorials en presència d'un camp magnètic, per dos casos ben diferenciats: 1) un condensat amb $F=1$ i interaccions de contacte i 2) un condensat amb $F=3$ i interaccions de contacte i dipolars. Per a tots dos cassos, proposem un mètode per fer termometria a molt baixes temperatures, i un mètode per refredar el sistema tot variant el camp magnètic extern.
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Charalambous, Christos. "Quantum Brownian motion in Bose-Einstein condensates." Doctoral thesis, Universitat Politècnica de Catalunya, 2020. http://hdl.handle.net/10803/668822.
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Quantum Brownian motion is one of the most prominent examples of an open quantum system, a system which cannot be treated in isolation from its environment. The simplest method to study the dynamics of a system undergoing such a type of motion, that satisfies Heisenberg Uncertainty principle is the approach of Quantum Generalized Langevin Equations (QGLE), which was used throughout this thesis. A Quantum Brownian motion approach is used in this work to study the Bose polaron problem. In this case, one transforms the original problem into one where the impurities are treated as quantum Brownian particles interacting with a bath composed of the Bogoliubov modes of the condensate. Then by deriving the relevant QGLE, it was shown that the dynamics of the Bose polaron exhibit memory effects. This was studied for both a free Bose-Einstein condensate (BEC) and a harmonically trapped one, in both cases for experimentally relevant parameters. Taking advantage of this recent theoretical development, we study a number of phenomena that can be examined under this prism and show how various microdevices can be constructed and controlled. In the first project, we study the creation of entanglement and squeezing of two uncoupled impurities that are immersed in a single common (BEC) bath. We treat these impurities as two quantum Brownian particles. We study two scenarios:(i) In the absence of an external potential, we observe sudden death of entanglement;(ii) In the presence of an external harmonic potential, where entanglement survives even at the asymptotic time limit. In our second work, we studied the diffusive behavior of a Bose Polaron immersed in a coherently coupled two-component BEC. The particle superdiffuses if it couples in the same manner to both components, i.e. if it couples either attractively or repulsively to both of them. This is the same behavior of an impurity immersed in a single BEC. Conversely, we find that it exhibits a transient nontrivial subdiffusive behavior if it couples attractively to one of the components and repulsively with the other. We show how the magnitude of the anomalous exponent reached and the duration of the subdiffusive interval can be controlled with the Rabi frequency of the coherent coupling between the two components and the coupling strength of the impurity to the BEC. Then we proceeded with the construction of two microdevices, a quantum sub-nk thermometer and a heat diode. In the first project, we introduced a novel minimally disturbing method for sub-nK thermometry in a BEC. In this case, the impurity acted as a thermometer, where one detects temperature fluctuations from measurements of the position and momentum of the impurity. Crucially, these cause minimal backaction on the BEC and hence, realize a nondemolition temperature measurement. Following the paradigm of the emerging field of quantum thermometry, we combine tools from quantum parameter estimation and the theory of open quantum systems to solve the problem in full generality. We thus avoid any simplification, such as demanding thermalization of the impurity atoms. In our final work, we investigated the heat transport and the control of heat current among two spatially separated trapped BECs, each of them at a different temperature. To allow for heat transport among the two independent BECs we consider a link made of two dipole-dipole interacting harmonically trapped impurities, each of them interacting with one of the BECs. We address the dependence of heat current and current-current correlations on the physical parameters of the system. Interestingly, we show that heat rectification, can occur in our system, when a periodic driving on the trapping frequencies of the impurities is considered. Therefore, our system is a possible setup for the implementation of a phononic circuit, and hence contributes in the general framework of using BECs as platforms for quantum information processing.El movimiento Browniano, es un ejemplo de un sistema abierto, es decir un sistema que no se puede tratar en aislamiento. El método más simple para estudiar la dinámica de dicho sistema, que cumple el principio de la incertidumbre de Heisenberg es el de Quantum Generalized Langevin Equations (QGLE), que es el método que se usa en esta tesis. La perspectiva de Quantum Brownian motion se ha usado para estudiar muchos sistemas, entre ellos el problema de Bose polaron. En este caso, uno pasa el problema original a uno donde las impurezas se tratan como partículas Brownianas quanticas interactuando con un baño compuesto de modos de Bogoliubov del condensado. Después de derivar la QGLE relevante, se puede demostrar que la dinámica del Bose polaron muestra efectos de memoria. Esto se ha estudiado tanto en un Bose Einstein Condensate (BEC) libre como en uno atrapado en un trapo harmónico, para parámetros relevantes en experimentos. Aprovechando de este reciente desarrollo, estudiamos muchos fenómenos que se pueden investigar bajo este prisma y mostramos cómo se pueden construir y controlar varios microdispositivos. En el primer proyecto, estudiamos la creación de enlazamiento y squeezing de dos impurezas no acopladas, inmersas en un único BEC baño común. Estudiamos dos senarios: (i) en la ausencia de un potencial externo, donde observamos la muerte repentina del enlazamiento (ii) en la presencia de un trapo externo harmónico, donde el enlazamiento sobrevive incluso en el límite asintótico de largos tiempos. En nuestro segundo trabajo, estudiamos el comportamiento difusivo de un Bose polaron inmerso en un BEC de dos componentes que están acopladas coherentemente. La partícula es superdffusa si se acopla en la misma manera a los dos componentes, i.e. atractivamente o repulsivamente, como en el caso de un unico BEC. En el caso contrario, encontramos que la partícula muestra un comportamiento transitorio non-trivial. Mostramos como la magnitud del exponente anómalo y la duración del periodo transitorio se pueden controlar a través de la frecuencia Rabi del acoplamiento coherente entre los dos componentes y la fuerza del acoplamiento de la impureza a los dos componentes del BEC. En seguida, procedemos con la construcción de dos microdispositivos, un termómetro quántico y un diodo térmico. En el primer proyecto, hemos introducido un nuevo método de mínimo disturbio, que sirve para termometría en temperaturas sub-nK en un BEC. Nuestra técnica está basada otra vez en el modelo de Bose polaron, donde esta vez la impureza inmersa en un BEC sirve como un termómetro. La propuesta es detectar fluctuaciones de la temperatura de las medidas de la posición y el impulso de la impureza. Crucialmente, estas causan una reacción mínima en el BEC y, por lo tanto, realizan una medida de la temperatura no demoledora. En nuestro trabajo, evitamos cualquiera simplificación, como la imposición de la termalización de la impureza, o del acoplamiento débil de la impureza con el BEC. En el último trabajo, investigamos el transporte de calor y el control de corrientes de calor entre dos BECs espacialmente separados y atrapados harmónicamente, en temperaturas distintas. El flujo de calor entre los dos BECs, esta facilitado a través de dos impurezas harmónicamente atrapadas, cada una interactuando con su propio BEC. Las impurezas estan acopladas a traves de interacciones de dipolo-dipolo. Examinamos la dependencia del corriente de calor y sus correlaciones en los parámetros físicos del sistema. Mostramos que la rectificación del corriente del calor, i.e. el flujo de calor unidireccional puede ocurrir en el sistema, cuando aplicamos una conducción periódica en las frecuencias de los trapos de las impurezas. Por lo tanto, nuestro sistema es una posible configuración para la implementación de un circuito fononico.
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Karhu, Robin. "Targeted Energy Transfer in Bose-Einstein Condensates." Thesis, Linköpings universitet, Teoretisk Fysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-98279.
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Targeted Energy Transfer is a resonance phenomenon in coupled anharmonic oscillators. In this thesis we investigate if the concept of Targeted Energy Transfer is applicable to Bose-Einsteain condensates in optical lattices. The model used to describe Bose-Einstein condensates in optical lattices is based on the Gross-Pitaevskii equation. Targeted Energy Transfer in these systems would correspond to energy being transferred from one lattice site to another. We also try to expand the concept of Targeted Energy Transfer to a system consisting of three sites, where one of the sites are considered a perturbation to the system. We have concluded that it is possible to achieve Targeted Energy Transfer in a three-site system. The set-up of the system will in some of the cases studied lead to interesting properties, such as more energy being transferred to the acceptor site than what was initially localized on the donor site.
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Lee, Mark David. "Interactions in low-dimensional Bose-Einstein condensates." Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270676.
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Xiong, Bo. "Interference and transport of Bose-Einstein condensates." Thesis, University of Nottingham, 2009. http://eprints.nottingham.ac.uk/14333/.
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This dissertation studies the dynamics of atomic Bose-Einstein condensates (nEes) and Bose gases in a suddenly modified potential. Firstly, we investigate the correlation between vortex formation and interference in merging Bose-Einstein condensates. This inherent correlation can explain some experiments in which vortices are formed in interfering condensates. Furthermore, we show the interference properties of merging condensates, particularly the relation of interference among colliding, expanding, and merging condensates, which can explain some complex interference phenomena in recent experiments. Secondly, using the truncated Wigner approximation, we investigate the role of quantum fluctuations in different forms on the transport properties of bosonic atoms in a ID optical lattice. The dynamics of transport with respect to quantum fluctuations in the plane-wave modes is distinct from that in the single-harmonic-oscillator modes. The discrepancies are demonstrated in detail. Quantum fluctuations in Bogoliubov modes lead to stronger damping behaviour of the centre-of-mass motion than quantum fluctuations in the plane-wave and single-harmonic-oscillator modes, which is in agreement with the experiment. Thirdly, the role of the relative phase variation and velocity of two low-density condensates, and quantum noise on interference properties are discussed. In particular, the incoherent atoms have significant effect on the interference visibility and microscopic dynamics. Although the interference pattern is not broken by quantum fluctuations, indicating the robust character of this interference, the process of inner correlations and dynamics is very complex and cannot he understood purely with mean-field theory. Finally, we investigate the elementary excitation spectrum and mode functions of a trapped Bose gas by numerically solving the Bogoliubov-De Gennes equation. The characteristic form of the Bogoliubov matrix, determined by the interatomic interactions, and the interaction between atoms and confining potential, specifies excitation spectra and mode functions. The role of these interactions on the properties of spectra and mode functions are shown.
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Madarassy, E. J. M. "Vortex motion in trapped Bose-Einstein condensates." Thesis, University of Newcastle Upon Tyne, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485606.
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We have performed numerical simulations of various vortex configurations in a trapped BoseEinstein condensate by solving the two-dimensional Gross-Pitaevskii equation in the presence of a simple model of interaction between the condensate and the finite temperature thermal cloud that surrounds it. In that interaction the non-condensed thermal cloud acts as a source of dissipation with a damping effect of excitations. In the case ora single vortex and a vortex - anti vortex pair, we have found that the path of the vortices depends on the initial position, the initial separation distance if the case of two vortices and dissipation. This motion is periodic and it was found that sound waves are created by vortex motion; the intensity was stronger when the initial vortex separation distance was smaller. We have calculated the sound energy as the difference between the kinetic energy and the vortex energy. With no dissipation the vortices followed the same path with a slight oscillation due to the sound waves. We found that the smaller the initial vortex separation distance do is, the larger the sound production. The period, frequency, translation speed, sound energy and vortex energy were measured for different initial separation distances do and for different dissipation parameters 'Y. In the case of motion of one vortex, the connection between the dissipation 'Y and the friction coefficients, a and a' was studied as well. To create a simple turbulent state, we put eight pairs of vortex - anti vortex at random positions in the condensate with initial separation distance do =1.8 between them. 'Ve have studied the decay rate of the total energy, kinetic energy, quantum energy, trap energy and the z - component of the angular momentum together with the increase rate of the internal energy. Finally, we finished our investigation by putting randomly vortex - anti vortex pairs and studied the decrease of the number of vortices with time t. We found that the decrease is exponential.
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Bruderer, Martin Ulrich. "Neutral impurities immersed in Bose-Einstein condensates." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489414.
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Ultra-cold atomic gases are formidable systems to investigate fundamental phenomena occurring in many-body quantum physics. In combination with scattering resonances and optical potentials, degenerate quantum gases provide new insight into strongly correlated systems and their characteristic properties. Notably, the admixture of impurity atoms to a Bose-Einstein condensate (BEC) has recently extended the physics of ultra-cold atoms into the area of quantum impurity problems.
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Hooley, Samantha. "Instability and excitations in Bose-Einstein condensates." Thesis, University of Nottingham, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.437088.
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Xu, Kaiwen. "Effects of interaction in Bose-Einstein condensates." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/37214.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.Includes bibliographical references (p. 150-167).
This thesis discusses a series of studies that investigate the effects of interaction - essentially the s-wave scattering - in the various properties of Bose-Einstein condensates (BEC). The phonon wavefunction in a BEC was measured using Bragg spectroscopy and compared with the well-known Bogoliubov theory. Phonons were first excited in a BEC of 3 x 107 condensed 23Na atoms via small-angle two-photon Bragg scattering. Large angle Bragg scattering was then used to probe the momentum distribution. We found reasonable agreement with the theory. With the same technique of Bragg diffraction, we studied the four-wave mixing process for matter waves. The BEC was split into two strong source waves and a weak seed wave. The s-wave scattering coherently mixed pairs of atoms from the sources into the seed and its conjugate wave, creating a pair-correlated atomic beams with "squeezed" number difference. A Feshbach resonance was used to produce ultracold Na2 molecules with initial phase-space density in excess of 20. Starting from an atomic BEC, a magnetic field ramp shifted a bound state from above the threshold of the unbound continuum to below, creating a molecular population with almost zero center-of-mass motion.
(cont.) A reverse field ramp dissociated the cold molecules into free atom pairs carrying kinetic energy dependent on the ramp speed. This dependence provided a measure of the coupling strength between the bound state and the continuum. Condensates were loaded into optical lattices formed with retro-reflected single frequency lasers. Quantum phase transition from the superfluid state to Mott-insulator state was observed in a three dimensional lattice. The increased interaction and flattened dispersion relation led to strongly enhanced quantum depletion in the superfluid state.
by Kaiwen Xu.
Ph.D.
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Leanhardt, Aaron E. (Aaron Edward) 1977. "Microtraps and waveguides for Bose-Einstein condensates." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/17650.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.Includes bibliographical references (leaves 126-149).
Gaseous Bose-Einstein condensates containing up to 3 x 10⁶ ²³Na atoms were loaded into magnetic microtraps and waveguides on a microfabricated "atom chip" using optical tweezers. Single-mode propagation was observed along the waveguide. Closer to the microfabricated surface, perturbations to the waveguide potential spatially modulated the condensate density. The condensate lifetime was >[or equal to] 20 s and independent of the atom-surface separation, for separations >[or equal to] 70 [mu]m. Condensates were coherently split by deforming an optical single-well potential into a double-well potential. The relative phase between the two resulting condensates was determined from the matter wave interference pattern formed upon releasing the atoms from the separated potential wells. Coherent phase evolution was observed for condensates held separated by 13 [mu]m for <[or equal to ] 5 ms and was controlled by applying AC Stark shifts to either condensate. This demonstrated a trapped-atom interferometer. Vortices and spin textures were imprinted in spinor condensates using topological phases. The order parameter of condensates held in a Ioffe-Pritchard magnetic trap was manipulated by adiabatically varying the magnetic bias field along the trap axis. Fully inverting the axial bias field imprinted vortices in F = 1 and F = 2 condensates with 2h and 4h of angular momentum per particle, respectively. Reducing the axial bias field to zero distributed the condensate population across its 2F + 1 spin states, each with a different phase winding, and created a spin texture.
(cont.) Partially condensed atomic vapors were confined by a combination of gravitational and magnetic forces. They were adiabatically decompressed, by weakening the gravito-magnetic trap to a mean frequency of 1 Hz, then evaporatively reduced in size to 2500 atoms. This lowered the peak condensate density to 5 x 10¹⁰ atoms/cm³ and cooled the entire cloud in all three dimensions to a kinetic temperature of 450±80 pK.
by Aaron E. Leanhardt.
Ph.D.
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Durfee, Dallin S. (Dallin Shane) 1970. "Dynamic properties of dilute Bose-Einstein condensates." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/84775.
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Inouye, Shin 1971. "Manipulating Bose-Einstein condensates with laser light." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/8274.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2001.Includes bibliographical references (p. 126-136).
A dilute gas Bose-Einstein condensate was probed and manipulated by off-resonant laser beams. Spontaneous and stimulated off-resonant light scatterings were studied experimentally. Stimulated, two-photon Bragg scattering was used for spectroscopic measurement of the mean-field energy and of the intrinsic momentum uncertainty of the condensate. The high momentum and energy resolution of this method allowed the determination of the coherence length of the condensate, which was shown to be equal to its size. Spontaneous, off-resonant Rayleigh scattering was studied by exposing an elongated condensate to a single off-resonant laser beam. Highly directional scattering of light and atoms was observed. This collective light scattering is shown to be directly analogous to Dicke superradiance, where the electronic coherence is replaced by the coherent center-of-mass motion of the atoms in the condensate. Superradiant Rayleigh scattering was used to amplify atomic matter waves. The active medium was a Bose-Einstein condensate, pumped by off-resonant laser light ("Dressed condensate"). An atomic wave packet was amplified with a gain of 10 to 100. Phase-coherence of the amplifier was verified by observing the interference of the output wave with a reference wave packet. Optical properties of the dressed condensate were also characterized, focusing on the key role of long-lived matter wave gratings produced by interference between the condensate at rest and the recoiling atoms. The narrow bandwidth for the optical gain gave rise to an extremely slow group velocity of an amplified light pulse ([approx.] 1 m/s).
(cont.) The role of quantum statistics in these enhanced scatterings was studied. It was shown that the macroscopic occupation of a single quantum state is not necessary. These processes are in principle possible for fermionic or non-degenerate samples, provided the atomic ensemble has a sufficiently long coherence time. By moving a focused, far off-resonant laser beam through a condensate, vortex excitations were created in a Bose-Einstein condensate. They were observed as dislocations in the interference fringes formed by the stirred condensate and a second unperturbed condensate. The technique was shown to be a powerful tool to study turbulent superfluid flow.
by Shin Inouye.
Ph.D.
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Campbell, Gretchen K. (Gretchen Kathleen). "⁸⁷Rubidium Bose-Einstein condensates in optical lattices." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/39295.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2007.Includes bibliographical references (p. 130-142).
Bose-Einstein condensates in optical lattices have proven to be a powerful tool for studying a wide variety of physics. In this thesis a series of experiments using optical lattices to manipulate 87Rb Bose-Einstein condensates are described. A systematic shift of the photon recoil momentum due to the index of refraction of a dilute gas of atoms has been observed. The recoil frequency was measured interferometrically using a two-pulse Ramsey interferometer. The two pulses were created using a one dimensional optical lattice. By measuring the resulting frequency as a function of the lattice detuning from the atomic resonance, we found a distinctive dispersive shape for the recoil frequency that fit the recoil momentum as n,.hk. A one-dimensional optical lattice was used to modify the dispersion relation of the condensate in order to demonstrate the matter-wave analogue of Optical Parametric Generation (OPG) and Amplification (OPA) of photons. A condensate was loaded into a moving optical lattice with adjustable quasimomentum k0. As the value for k0o was varied, we observed elastic scattering into two distinct final momentum states k1 and k2.
(cont.) When a small fraction of atoms was first transferred to k1 before ramping on the lattice, we observed the amplification of scattered atoms into k1 and k2. The superfluid-Mott Insulator transition was studied using microwave spectroscopy in a deep three-dimensional optical lattice. Using the density dependent clock shift we were able to spectroscopically distinguish sites with different occupation numbers, and to directly image sites with occupation number from 1 to 5, revealing the shell structure of the Mott Insulator phase.
by Gretchen K. Campbell.
Ph.D.
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Campbell, Daniel L. "Engineered potentials in ultracold Bose-Einstein condensates." Thesis, University of Maryland, College Park, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3725451.
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Bose-Einstein condensates (BECs) are a recent addition to the portfolio of quantum materials some of which have profound commercial and military applications e.g., superconductors, superfluids and light emitting diodes. BECs exist in the lowest motional modes of a trap and have the lowest temperatures achieved by mankind. With full control over the shape of the trap the experimentalist may explore an extremely diverse set of Hamiltonians which may be altered mid-experiment. These properties are particularly suited for realizing novel quantum systems.
This thesis explores interaction-driven domain formation and the subsequent domain coarsening for two immiscible BEC components. Because quantum coherences associated with interactions in BECs can be derived from low energy scattering theory we compare our experimental results to both a careful simulation (performed by Brandon Anderson) and an analytical prediction. This result very carefully explores the question of how a metastable system relaxes at the extreme limit of low temperature.
We also explore spin-orbit coupling (SOC) of a BEC which links the linear and discrete momentum transferable by two counterpropagating ''Raman'' lasers that resonantly couple the ground electronic states of our BECs. SOC is used similarly in condensed matter systems to describe coupling between charge carrier spin and crystal momentum and is a necessary component of the quantum spin Hall effect and topological insulators.
SOC links the linear and discrete momentum transferable by two counterpropagating ''Raman'' lasers and a subset of the ground electronic states of our BEC. The phases of an effective 2-spin component spin-orbit coupling (SOC) in a spin-1 BEC are described in Lin et al. (2011). We measure the phase transition between two phases of a spin-1 BEC with SOC which cannot be mimicked by a spin-1/2 system. The order parameter that describes transitions between these two phases is insensitive to magnetic field fluctuations.
I also describe a realistic implementation of Rashba SOC. This type of SOC is expected to exhibit novel many-body phases [Stanescu et al. 2008, Sedrakyan et al. 2012, Hu et al. 2011].
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Welch, Nathan. "Non-equilibrium dynamics of Bose-Einstein condensates." Thesis, University of Nottingham, 2015. http://eprints.nottingham.ac.uk/29244/.
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In the following work we investigate the dynamics of Bose-Einstein condensates(BECs) under the influence of time-dependent potentials. The response of such a sensitive system to external perturbations is a matter of increasing interest. This is because of the enormous growth in understanding the physics and emerging applications of BECs in many areas of physics such as sensing, microscopy and quantum information.
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Martikainen, Jani-Petri. "Dynamics and excitations of Bose-Einstein condensates." Helsinki : University of Helsinki, 2001. http://ethesis.helsinki.fi/julkaisut/mat/fysii/vk/martikainen/.
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Grzeschik, Christoph. "Experiments with Bose-Einstein Condensates in Microgravity." Doctoral thesis, Humboldt-Universität zu Berlin, 2017. http://dx.doi.org/10.18452/18037.
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Atominterferometer erlauben es, Beschleunigungen mit bisher nicht erreichter Präzision zu messen. Anwendungen in der Grundlagenforschung beinhalten Gravitationswellendetektoren, die Bestimmung von Naturkonstanten oder Tests des schwachen Äquivalenzprinzips. Die Sensitivität eines Sensors für Tests des schwachen Äquivalenzprinzips skaliert quadratisch mit der Zeit der freien Entwicklung der Atome während der Interferometersequenz. Durch die Verwendung von Bose-Einstein-Kondensaten mit stark reduzierter Ausdehnungsgeschwindigkeit sowie dem Betrieb in Schwerelosigkeit kann die Sensitivität um Größenordnungen verbessert werden.
Das QUANTUS-2 Experiment stellt die zweite Generation eines mobilen Atominterferometers dar, welches am Fallturm in Bremen zum Einsatz kommt und dient als Wegbereiter für zukünftige Experimente mit kalten Atomen auf Satelliten. Durch differentielle Messung der Beschleunigung von Rubidium und Kalium mit Hilfe der Atominterferometrie soll das schwache Äquivalenzprinzip getestet werden.
Im Rahmen dieser Arbeit wurde das auf mikro-integrierten Diodenlasern sowie einer kompakten Elektronik basierende Rubidiumlasersystem aufgebaut und qualifiziert. Nach erfolgter Integration in die QUANTUS-2 Kapsel, wurden über 200 Abwürfe und Katapultflüge am Fallturm absolviert. Diese demonstrieren die Robustheit des Experimentes unter Beschleunigungen von bis zu 43 g während eines Katapultfluges. Die Dynamik des Kondensates wurde in Schwerelosigkeit untersucht und die Ausbreitungsgeschwindigkeit in allen drei Raumrichtungen mit Hilfe einer magnetischen Linse verringert. Die dabei erreichten Ausbreitungsgeschwindigkeiten entsprechen effektiven Temperaturen von unter 120 pK eines thermischen Ensembles. Dieser stellt den niedrigsten in allen drei Raumrichtungen erreichten Wert dar. Die gezeigten Ergebnisse demonstrieren somit die Verfügbarkeit wichtiger Schlüsselkonzepte zukünftiger hochpräziser Quantensensoren auf Satelliten.Atom interferometers offer the possibility to measure accelerations with unprecedented precision. Applications in fundamental research include gravitational wave detectors, the determination of physical constants, or tests of the weak equivalence principle. The sensitivity of an atom interferometer testing the weak equivalence principle scales quadratically with the time of free evolution of the atoms during the interferometer sequence. By using Bose-Einstein condensates with ultra-low expansion rates as test masses and operating the experiment in microgravity, one can enhance the sensitivity by orders of magnitude. QUANTUS-2 is the second generation mobile atom interferometer to be operated at the drop tower in Bremen and serves as a pathfinder for future cold atom experiments in space. It is envisaged to test the weak equivalence principle by a differential measurement of the acceleration of rubidium and potassium by means of atom interferometry. Within this thesis, the rubidium laser system was set up and qualified. It is based on micro-integrated laser modules and compact electronics. After integration into the QUANTUS-2 capsule, 200 drops and catapult flights were conducted at the drop tower. These are demonstrating the robustness of the complete experiment when being subjected to accelerations of up to 43 g during a catapult flight. The dynamics of the condensate were analyzed and the mean kinetic energy was reduced in all three dimensions by means of a magnetic lens. Expansion rates equivalent to a thermal ensemble having a temperature below 120 pK have been reached and represent the lowest value ever achieved in all three dimensions. The results prove the availability of relevant key concepts for future high-precision quantum sensors on a satellite platform.
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Grimm, Douglas. "The dynamics of collapsing Bose-Einstein condensates." View electronic text, 2002. http://eprints.anu.edu.au/documents/disk0/00/00/07/64/index.html.
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Thesis (BSc. (Hons))--Australian National University, 2002.Available via the Australian National University Library Electronic Pre and Post Print Repository. Title from title screen (viewed Mar. 28, 2003). "A thesis submitted for the degree of Bachelor of Science with Honours of The Australian National University" Bibliography: p. 45.
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Yi, Su. "Properties of trapped dipolar condensates." Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/27356.
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Sanz, Sánchez Julio. "Two-component Bose-Einstein condensates with competing interactions." Doctoral thesis, Universitat Politècnica de Catalunya, 2020. http://hdl.handle.net/10803/668865.
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This thesis reports the experimental study of two-component Bose-Einstein condensates with tunable interactions, which are exploited as a platform to perform quantum simulation of many-body quantum systems.
To perform this experiments, we have implemented an atomic source consisting on a glass cell 2D MOT vacuum chamber and a high resolution optical system to image and manipulate the atoms. Furthermore, we develop and characterize a polarization phase contrast technique which is able to probe optically dense atomic mixtures at intermediate and high magnetic fields in open transitions. This technique has been used to either probe the total column density of a two-component atomic cloud or the difference in column density between both components.
We report on the first observation of composite quantum liquid droplets in an incoherent mixture with residual mean field attraction. Strikingly, this novel phase is stabilized due to the repulsive beyond mean field corrections in a weakly interacting system. Moreover, we have characterized the liquid to gas phase transition which occurs for small atom numbers.
Additionally, we have compared two different self-bound states in a quasi-1D geometry with incoherent mixtures: quantum droplets and bright solitons. Depending on the atom number and interaction strengths both states can be smoothly connected through a crossover or be distinct entities separated by a transition. We have measured its composition, its phase diagram and mapped out the soliton to droplet transition.
Finally, we report on a technique to modify the elastic and inelastic interactions in a two-component Bose-Einstein condensate with very unequal and competing interactions under the presence of strong coherent coupling. This technique provides a wide flexibility and has allowed us to observe bright solitons in quasi-1D in a coherently coupled dressed state. We exploit the fast temporal control of the effective interactions to quench them into the attractive regime and study the resulting modulational instability which develops into a bright soliton train.Aquesta tesi descriu l'estudi experimental d'una mescla de dos condensats de Bose-Einstein amb interaccions ajustables. Aquest sistema és utilitzat com una plataforma per a estudiar sistemes quàntics formats per moltes partícules a partir de la simulació quàntica. Per a fer aquests experiments, he construït una font atòmica formada per una trampa magneto-òptica en 2D que s'implementa en una cambra de buit feta de vidre. A més a més, he desenvolupat i caracteritzat una tècnica d'imatge de contrast de fase basada en la rotació de la polarització de la llum. Aquesta tècnica està preparada per fer imatges de mescles atòmiques a camps magnètics intermedis i alts amb una gran densitat òptica i amb transicions òptiques obertes. Hem utilitzat la tècnica per a mesurar la densitat integrada total en l'eix òptic així com la diferència entre ambdues components. Es descriu la primera observació de gotes líquides quàntiques compostes per dues components incoherents amb una atracció residual en l'aproximació de camp mitjà. Sorprenentment, aquesta nova fase està estabilitzada a causa de la repulsió generada per les correccions de l'energia més enllà de l'aproximació de camp mitjà en un sistema amb interaccions dèbils. També hem caracteritzat la transició de fase líquid-gas que succeeix quan el sistema té un nombre d'àtoms reduït. A més a més, hem comparat dos estats autoconfinats de diferent natura en una geometria quasi-1D amb una mescla d'àtoms incoherents: les gotes quàntiques i els solitons brillants. Segons el nombre d'àtoms i la força de les interaccions aquests estats poden estar connectats o bé suaument o bé per una transició de fase. Hem mesurat la seva composició, el diagrama de fases i hem traçat el mapa de transició entre solitons i gotes en funció del camp magnètic i nombre d'àtoms. Finalment, es descriu una tècnica per a modificar les interaccions elàstiques i inelàstiques en un condensat de Bose-Einstein format per dues components, amb interaccions diferents i en competició, coherentment acoblades. Aquesta tècnica ens proveeix d'una gran flexibilitat per a modificar les interaccions i ens ha permès observar solitons brillants en quasi-1D en un estat vestit per l'acoblament coherent. Hem utilitzat l'habilitat per a modificar temporalment les interaccions per a canviar-les bruscament cap al règim atractiu i estudiar la inestabilitat dels modes que es manifesta amb la formació d'un tren de solitons brillants.
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Geursen, Reece Wim, and n/a. "Experiments with Bose-Einstein condensates in optical potentials." University of Otago. Department of Physics, 2005. http://adt.otago.ac.nz./public/adt-NZDU20070131.162251.
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We present a detailed experimental investigation into Bose-Einstein condensates loaded into a one-dimensional optical standing wave at the Bragg condition.
The main emphasis of this thesis is the experimental and theoretical investigation into Bragg spectroscopy performed on circularly accelerating Bose-Einstein condensates. The condensate undergoes circular micromotion in a magnetic time-averaged orbiting potential trap and the effect of this motion on the Bragg spectrum is analysed. A simple frequency modulation model is used to interpret the observed complex structure, and broadening effects are considered using numerical solutions to the Gross-Pitaevskii equation.
The second part of this thesis is an experimental investigation into the effect of nonlinearity on the non-adiabatic loading of a condensate into a optical lattice at the Brillouin zone boundary. Results of using a phase shifting technique to load a single Bloch band in the presence of strong interactions are presented. We observe a depletion of the condensed component, and we propose possible mechanisms for this result.
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Braun-Munzinger, Karen Anna Clara. "Dynamics of Bose-Einstein condensates in optical lattices." Thesis, University of Oxford, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.420450.
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Smirne, Giuseppe. "Experiments with Bose-Einstein condensates in optical traps." Thesis, University of Oxford, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.414271.
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Schumm, Thorsten. "Bose-Einstein condensates in magnetic double well potentials." Phd thesis, Université Paris Sud - Paris XI, 2006. http://tel.archives-ouvertes.fr/tel-00129501.
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Ce manuscrit présente deux réalisations d'un double puit magnétique pour des condensats de Bose-Einstein (CBE) sure une puce atomique. Une approche utilise des pièges statiques, crées par des micro fils (en amènent ?) manipulant les atomes proche a la surface de la puce. Comme dans toute manipes, on observe une fragmentation du nuage atomique, quand on approche les atomes vers la structure piégeant. Cet effet était expliqué par une déviation du courant dans le fil à cause d'une rugosité des bords. Pour éviter la fragmentation, une nouvelle technique de fabrication (lithographie a faisceaux a électrons, évaporation d'or) a été utilisé pour créer des fils d'un section de 700nm et une qualité amélioré. Un CBE a été crée et chargé dans le double puit généré par la nano structure. On a testé le double puit comme séparatrice avec des atomes thermiques. Des nombreuses problèmes techniques nous empêchent pour le moment d'effectuer la manip avec un CBE.La deuxième approche poursuit dans cette thèse combine des pièges magnétique statique avec un champ (RF) magnétique alternant et génère un double puit dans le potentiel habillé. Car ce schéma peut être réalisé loin de la surface de la puce, la fragmentation n'apparaisse pas et on a pu séparer un CBE en deux. Une interféromètre d'ondes a matière est réalisé en recombinant les deux nuages en expansion libre. La figure d'interférence permet de mesurer la phase relative, on trouve une distribution étroite de cette phase et donc la séparation est cohérente. L'évolution de la phase relative est mesurée pendant et après la séparation et contrôlé par déséquilibrant le double puit.
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Rusch, Martin. "Excitations of Bose-Einstein condensates at finite temperatures." Thesis, University of Oxford, 2000. http://ora.ox.ac.uk/objects/uuid:cb07f727-3d56-4b18-b32f-4971116bd7fd.
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Recent experimental observations of collective excitations of Bose condensed atomic vapours have stimulated interest in the microscopic description of the dynamics of a Bose-Einstein condensate confined in an external potential. We present a finite temperature field theory for collective excitations of trapped Bose-Einstein condensates and use a finite-temperature linear response formalism, which goes beyond the simple mean-field approximation of the Gross-Pitaevskii equation. The effect of the non-condensed thermal atoms we include using perturbation theory in a quasiparticle basis. This presents a simple scheme to understand the interaction between condensate and non-condensed atoms and enables us to include the effect the condensate has on collision dynamics. At first we limit our treatment to the case of a spatially homogeneous Bose gas. We include the effect of pair and triplet anomalous averages and thus obtain a gapless theory for the excitations of a weakly interacting system, which we can link to well known results for Landau and Beliaev damping rates. A gapless theory for trapped systems with a static thermal component follows straightforwardly. We then investigate finite temperature excitations of a condensate in a spherically symmetric harmonic trap. We avoid approximations to the density of states and thus emphasise finite size aspects of the problem. We show that excitations couple strongly to a restricted number of modes, giving rise to resonance structure in their frequency spectra. Where possible we derive energy shifts and lifetimes of excitations. For one particular mode, the breathing mode, the effects of the discreteness of the system are sufficiently pronounced that the simple picture of an energy shift and width fails. Experiments in spherical traps have recently become feasible and should be able to test our detailed quantitative predictions.
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Campbell, Russell. "Localisation of Bose-Einstein condensates in optical lattices." Thesis, University of Strathclyde, 2018. http://digitool.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=30137.
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The properties of Bose-Einstein condensates can be studied and controlled effectively when trapped in optical lattices formed by two counter-propagating laser beams. The dynamics of Bose-Einstein condensates in optical lattices are well-described by a continuous model using the Gross-Pitaevskii equation in a modulated potential or, in the case of deep potentials, a discrete model using the Discrete Nonlinear Schrodinger equation. Spatially localised modes, known as lattice solitons in the continuous model, or discrete breathers in the discrete model, can occur and are the focus of this thesis. Theoretical and computational studies of these localised modes are investigated in three different situations. Firstly, a model of a Bose-Einstein condensate in a ring optical lattice with atomic dissipations applied at a stationary or at a moving location on the ring is presented in the continuous model. The localised dissipation is shown to generate and stabilise both stationary and traveling lattice solitons. The solutions generated include spatially stationary quasiperiodic lattice solitons and a family of traveling lattice solitons with two intensity peaks per potential well with no counterpart in the discrete case. Collisions between traveling and stationary lattice solitons as well as between two traveling lattice solitons display a dependence on the lattice depth. Then, collisions with a potential barrier of either travelling lattice solitons or travelling discrete breathers are investigated along with their dependence on the height of the barrier. Regions of complete reection or of partial reflection where the incoming soliton/breather is split in two, are observed and understood interms of the soliton properties. Partial trapping of the atoms in the barrier is observed for positive barrier heights due to the negative effective mass of the solitons/breathers. Finally, two coupled discrete nonlinear Schrodinger equations can describe the interaction and collisions of breathers in two-species Bose-Einstein condensates in deep optical lattices. This is done for two cases of experimental relevance: a mixture of two ytterbium isotopes and a mixture of Rubidium (87Rb) and Potassium(41K) atoms. Depending on their initial separation, interaction between stationary breathers of different species can lead to the formation of symbiotic localised structures or transform one of the breathers from a stationary one into a travelling one. Collisions between travelling and stationary discrete breathers composed of different species are separated in four distinct regimes ranging from totally elastic when the interspecies interaction is highly attractive to mutual destruction when the interaction is suffciently large and repulsive.
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Boyd, Micah (Micah Scott). "Novel trapping techniques for shaping Bose-Einstein condensates." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/39296.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2007.Includes bibliographical references (p. 125-133).
A combination of radio frequency radiation and magnetic field gradients was used to trap atoms in dressed states. In a magnetic field with a quadrupole minimum. RF fields resonant with the (I F. m)) 11. -1) -- 1, 0) transition trapped the atoms on the surface of a sphere, and gravity caused the atoms to pool at the bottom of the sphere. BECs were transferred into this dressed Zeeman trap with 100% efficiency, with lifetimes of up to 30 s, and trapping frequencies of up to 250 Hz were measured. A hard disk platter with a specially written magnetic pattern was used to generate magnetic fields to confine atoms tightly. Detrimental interactions with the surface were avoided by using an extremely thin film with a large magnetic remnant. BECs of up to 5 x 10" atoms were produced in cigar shaped traps -40 pin above the surface, and trap frequencies up to 5 kHz were measured. After evaporation, condensed clouds moved(] closer to the surface to probe imperfections in the magnetic potential, revealing defects at distances closer than 35 prn. Finally, BECs were dropped from a height of 350 pm in an attempt to achieve specular reflection. but a large amount of dispersion was observed.
(cont.) Finally, BECs were loaded into a three-dimensional optical lattice, and a quantum phase transition from a superfluid to a Mott insulator was observed. Using microwave spectroscopy. the density dependent "clock shift" was was found to depend on the occupation number of the wells. The singly occupied lattice sites were then investigated as an atomic clock system with no density shift. Linewidths as small as 1 Hz FWHMI out of 6.8 GHz are comparable to current atomic frequency standards.
by Micah Boyd.
Ph.D.
40
Mason, Peter. "Travelling waves in two-dimensional Bose-Einstein condensates." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611373.
Full text
41
Lindberg, Martin. "Dark Matter Galactic Halos as Bose-Einstein Condensates." Thesis, KTH, Teoretisk fysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-189576.
Full text
42
McKinney, Sarah. "Dynamics of Bose-Einstein condensates in optical lattices /." Thesis, Connect to this title online; UW restricted, 2004. http://hdl.handle.net/1773/9805.
Full text
43
Zhang, Bo. "Quantum turbulence in two dimensional Bose-Einstein condensates." W&M ScholarWorks, 2011. https://scholarworks.wm.edu/etd/1539623584.
Full textAbstract:
We examine the energy cascades and quantum vortex structures in two-dimensional quantum turbulence through a special unitary time evolution algorithm. An early attempt at using the Lattice Boltzmann Method proved successful in correctly representing some features of the Nonlinear Schrodinger System (NLS), such as the phase shift following the one-dimensional soliton-soliton collision, as well as the two-dimentional modulation instability. However, to accurately evaluate NLS, the implicit Euler method is required to resolve the time evolution, which is computationally expensive. A more accurate and efficient method, the Quantum Lattice Gas model is employed to simulate the quantum turbulence governed by the Gross-Pitaevskii equation, an equaiton that describes the evolution of the ground state wave function for a Bose-Einstein condensate (BEC). It is discovered that when the ratio of the internal energy to the kinetic energy is below 0.05, an unexpected short Poincare recurrence occurs independent of the initial profile of the wave function. It is demonstrated that this short recurrence is destroyed as the internal energy is strengthened. to compare the two-dimensional quantum turbulence with its classical counterpart, the incompressible energy spectra of quantum turbulence is analyzed. However, the result reveals no sign of dual cascades which is a hallmark of the classical incompressible two-dimensional fluid (inverse energy cascade to large scales with a direct cascade of enstrophy to small scales). It is the spectra of the compressible energy that can exhibits multiple cascades, but this is strongly dependent on the initial condition.
44
Farolfi, Arturo. "Spin dynamics in two-component Bose-Einstein condensates." Doctoral thesis, Università degli studi di Trento, 2021. http://hdl.handle.net/11572/299835.
Full textAbstract:
Bose-Einstein condensates (BECs) of ultra-cold atoms have been subjects of a large research effort, that started a century ago as a purely theoretical subject and is now, since the invention of evaporative cooling thirty years ago, a rich research topic with many experimental apparatuses around the world. A deep knowledge of its underlying physics has been now acquired, for example on the thermodynamics of the gas, superfluidity, topological excitations and many-body physics. However, many topics are still open for investigation, thanks to the flexibility and the high degree of control of these systems. During the course of my PhD, I developed and realized a new experimental apparatus for the realization of coherently-coupled mixtures of sodium BECs. The highly stable and low-noise magnetic environment of this apparatus enables the experimental investigation of a previously inaccessible regime, where the energy of the coupling becomes comparable to the energy of spin excitations of the mixture. With this apparatus, I concluded two experimental investigations: I produced and investigated non-dispersive spin-waves in an two-component BEC and I experimentally observed the quantum spin-torque effect on a elongated bosonic Josephson junction.The research activity in multi-component BECs of alkali atoms begun shortly after the first realization of a condensate, thanks to the low energy splitting between the internal sub-states of the electronic ground state. These internal states can be coherently coupled with an external electromagnetic field and can interact via mutual mean-field interaction, generating interestinc effects such as ground states with different magnetic ordering depending on their interaction constants, density as well as spin dynamics and internal Josephson effects. The research interest on mixtures of sodium atoms sparks from the peculiar characteristic of the system: in the $ket{F = 1, m_F = pm 1}$ states, the interaction constants are such that the ground state has anti-ferromagnetic ordering and the system is perfectly symmetric for exchanges of the two species. In these peculiar system, density- and spin-excitations have very different energetic cost, with the latter being much less energetic, and can be completely decoupled. Moreover, spin-excitations, that are connected to excitations in the relative-phase between the components, change drastically in nature when a coupling of comparable energy is added between the states. The presence of the coupling effectively locks the relative-phase in the bulk and spin excitations become localized. While extensive theoretical predictions on the spin dynamics of this system has been already performed, experimental confirmation was still lacking because of the high sensitivity to external forces (due to the very low energy of the spin excitations) and the impossibility of realizing a low-energy coupling between these states in the presence of environmental magnetic noise. During my PhD, I realized an experimental apparatus where magnetic noises are suppressed by five orders of magnitude using a multi-layer magnetic shield made of an high-permeability metal alloy (μ-metal), that encases the science chamber. In this apparatus, I developed a protocol, compatible with the technical limitations of the magnetic shield, to produce BECs in a spin-insensitive optical trapping potential. I then characterized the residual magnetic noise and found it compatible with the requirements for observing spin-dynamics effects. Finally, I realized a system and a set of protocols for the manipulation of the internal state of the sample allowing arbitrary preparation of the sample while maintaining the long coherence times necessary to observe the spin dynamics, that have been used in the subsequent experimental observations. The first experimental result discussed in this thesis, is the production of magnetic solitons and the observation of their dynamic in a trapped sample. Waves in general spread during their propagation in a medium, however this tendency can be counterbalanced by a self-focusing effect if dispersion of the wave is non-linear, generating non-dispersive and long-lived wavepackets commonly named solitons. These have been found in many fields of physics, such as fluid dynamics, plasma physics, non-linear optics and cold-atoms BECs, attracting interest because of their ability to transport information or energy unaltered over long distances, as they are robust against the interaction with in-homogeneities in the medium. Of these systems, cold-atoms can be widely manipulated to generated different kinds of solitons, both in single and in multi-components systems. A new kind of them, named magnetic solitons, has been predicted in a balanced mixture of BECs of sodium in $ket{F = 1, m_F = pm 1}$, however experimental observation was still lacking. I deterministically produced magnetic solitons via phase engineering of the condensate using a spin-sensitive optical potential. I then developed a tomographic imaging technique to semi-concurrently measure the densities of both components and the discontinuities in their relative phase, allowing for the reconstruction of all the relevant quantities of the spinor wavefunction. This allowed to observe the dispersionless dynamics of the solitons as they perform multiple oscillation in the trapped sample in a timescale of the order of the second. Moreover, I engineered collisions between different kinds of magnetic solitons and observed their robustness to mutual interaction. The second experimental results presented in this thesis is the observation of the breaking of magnetic hetero-structures in BECs due to the quantum spin torque effect, an effect with strong analogies with electronic spins traveling through magnetic devices.
Spins in magnetic material precess around the axis of the effective magnetic field, and their dynamics must take into account the external field as well as non-linear magnetization and the inhomogeneity of the material. These effects are commonly described by the Landau-Lifshitz equation and have been mainly studied for electronic spins in magnetic hetero-structures, where the inhomogeneity in the material at the interfaces enhances the exchange effects between spins. For homogeneous materials, this description reduces to the Josephson system, a closely related effect that is more known in cold-atoms systems. The Josephson effect arises when a macroscopic number of interacting bosonic particles are distributed in two possible states, weakly tunnel-coupled together, with the average energy of particles occupying each of the states depending on the occupation number itself. In these conditions, the dynamics of the system depends on the difference in occupation numbers, the relative phase between the states and the self-interaction to tunneling ratio, giving raise to macroscopic quantum effects such as oscillating AC and DC Josephson currents and self-trapping. While these phenomena has been historically studied in junctions between superconducting systems, they can be also realized with cold-atoms systems, allowing the study of Josephson junctions with finite dimensions and in regimes that are hard to reach for superconducting systems. In this thesis, I realized a magnetic hetero-structure in a two-component elongated BECs thanks to the simultaneous presence of self-trapped (ferromagnetic) and oscillating (paramagnetic) regions in the sample. While the dynamics at short times is correctly described by the Josephson effects, at the interface between the regions the particle nature of the gas creates a strong exchange effect, named the quantum spin torque, that produces magnetic excitations that spread trough the sample and break the local Josephson behaviour. I experimentally studied the spread and nature of these magnetic excitations, while numerical simulations confirmed the dominant role played by the quantum spin torque effect. The structure of this thesis is the following: in the first chapter is given a review of theoretical concepts and existing literature. In the second chapter is described the experimental apparatus and the protocols developed to prepare the ultra-cold atoms sample. In the third chapter is presented the experimental observation of magnetic solitons. In the fourth chapter is presented the experimental investigation of the quantum spin torque effect in magnetic heterostructures. The last chapter is devoted to conclusions and outlook of this work.
45
Farolfi, Arturo. "Spin dynamics in two-component Bose-Einstein condensates." Doctoral thesis, Università degli studi di Trento, 2021. http://hdl.handle.net/11572/299835.
Full textAbstract:
Bose-Einstein condensates (BECs) of ultra-cold atoms have been subjects of a large research effort, that started a century ago as a purely theoretical subject and is now, since the invention of evaporative cooling thirty years ago, a rich research topic with many experimental apparatuses around the world. A deep knowledge of its underlying physics has been now acquired, for example on the thermodynamics of the gas, superfluidity, topological excitations and many-body physics. However, many topics are still open for investigation, thanks to the flexibility and the high degree of control of these systems. During the course of my PhD, I developed and realized a new experimental apparatus for the realization of coherently-coupled mixtures of sodium BECs. The highly stable and low-noise magnetic environment of this apparatus enables the experimental investigation of a previously inaccessible regime, where the energy of the coupling becomes comparable to the energy of spin excitations of the mixture. With this apparatus, I concluded two experimental investigations: I produced and investigated non-dispersive spin-waves in an two-component BEC and I experimentally observed the quantum spin-torque effect on a elongated bosonic Josephson junction.The research activity in multi-component BECs of alkali atoms begun shortly after the first realization of a condensate, thanks to the low energy splitting between the internal sub-states of the electronic ground state. These internal states can be coherently coupled with an external electromagnetic field and can interact via mutual mean-field interaction, generating interestinc effects such as ground states with different magnetic ordering depending on their interaction constants, density as well as spin dynamics and internal Josephson effects. The research interest on mixtures of sodium atoms sparks from the peculiar characteristic of the system: in the $ket{F = 1, m_F = pm 1}$ states, the interaction constants are such that the ground state has anti-ferromagnetic ordering and the system is perfectly symmetric for exchanges of the two species. In these peculiar system, density- and spin-excitations have very different energetic cost, with the latter being much less energetic, and can be completely decoupled. Moreover, spin-excitations, that are connected to excitations in the relative-phase between the components, change drastically in nature when a coupling of comparable energy is added between the states. The presence of the coupling effectively locks the relative-phase in the bulk and spin excitations become localized. While extensive theoretical predictions on the spin dynamics of this system has been already performed, experimental confirmation was still lacking because of the high sensitivity to external forces (due to the very low energy of the spin excitations) and the impossibility of realizing a low-energy coupling between these states in the presence of environmental magnetic noise. During my PhD, I realized an experimental apparatus where magnetic noises are suppressed by five orders of magnitude using a multi-layer magnetic shield made of an high-permeability metal alloy (μ-metal), that encases the science chamber. In this apparatus, I developed a protocol, compatible with the technical limitations of the magnetic shield, to produce BECs in a spin-insensitive optical trapping potential. I then characterized the residual magnetic noise and found it compatible with the requirements for observing spin-dynamics effects. Finally, I realized a system and a set of protocols for the manipulation of the internal state of the sample allowing arbitrary preparation of the sample while maintaining the long coherence times necessary to observe the spin dynamics, that have been used in the subsequent experimental observations. The first experimental result discussed in this thesis, is the production of magnetic solitons and the observation of their dynamic in a trapped sample. Waves in general spread during their propagation in a medium, however this tendency can be counterbalanced by a self-focusing effect if dispersion of the wave is non-linear, generating non-dispersive and long-lived wavepackets commonly named solitons. These have been found in many fields of physics, such as fluid dynamics, plasma physics, non-linear optics and cold-atoms BECs, attracting interest because of their ability to transport information or energy unaltered over long distances, as they are robust against the interaction with in-homogeneities in the medium. Of these systems, cold-atoms can be widely manipulated to generated different kinds of solitons, both in single and in multi-components systems. A new kind of them, named magnetic solitons, has been predicted in a balanced mixture of BECs of sodium in $ket{F = 1, m_F = pm 1}$, however experimental observation was still lacking. I deterministically produced magnetic solitons via phase engineering of the condensate using a spin-sensitive optical potential. I then developed a tomographic imaging technique to semi-concurrently measure the densities of both components and the discontinuities in their relative phase, allowing for the reconstruction of all the relevant quantities of the spinor wavefunction. This allowed to observe the dispersionless dynamics of the solitons as they perform multiple oscillation in the trapped sample in a timescale of the order of the second. Moreover, I engineered collisions between different kinds of magnetic solitons and observed their robustness to mutual interaction. The second experimental results presented in this thesis is the observation of the breaking of magnetic hetero-structures in BECs due to the quantum spin torque effect, an effect with strong analogies with electronic spins traveling through magnetic devices.
Spins in magnetic material precess around the axis of the effective magnetic field, and their dynamics must take into account the external field as well as non-linear magnetization and the inhomogeneity of the material. These effects are commonly described by the Landau-Lifshitz equation and have been mainly studied for electronic spins in magnetic hetero-structures, where the inhomogeneity in the material at the interfaces enhances the exchange effects between spins. For homogeneous materials, this description reduces to the Josephson system, a closely related effect that is more known in cold-atoms systems. The Josephson effect arises when a macroscopic number of interacting bosonic particles are distributed in two possible states, weakly tunnel-coupled together, with the average energy of particles occupying each of the states depending on the occupation number itself. In these conditions, the dynamics of the system depends on the difference in occupation numbers, the relative phase between the states and the self-interaction to tunneling ratio, giving raise to macroscopic quantum effects such as oscillating AC and DC Josephson currents and self-trapping. While these phenomena has been historically studied in junctions between superconducting systems, they can be also realized with cold-atoms systems, allowing the study of Josephson junctions with finite dimensions and in regimes that are hard to reach for superconducting systems. In this thesis, I realized a magnetic hetero-structure in a two-component elongated BECs thanks to the simultaneous presence of self-trapped (ferromagnetic) and oscillating (paramagnetic) regions in the sample. While the dynamics at short times is correctly described by the Josephson effects, at the interface between the regions the particle nature of the gas creates a strong exchange effect, named the quantum spin torque, that produces magnetic excitations that spread trough the sample and break the local Josephson behaviour. I experimentally studied the spread and nature of these magnetic excitations, while numerical simulations confirmed the dominant role played by the quantum spin torque effect. The structure of this thesis is the following: in the first chapter is given a review of theoretical concepts and existing literature. In the second chapter is described the experimental apparatus and the protocols developed to prepare the ultra-cold atoms sample. In the third chapter is presented the experimental observation of magnetic solitons. In the fourth chapter is presented the experimental investigation of the quantum spin torque effect in magnetic heterostructures. The last chapter is devoted to conclusions and outlook of this work.
46
Ferreira, Henrique Fabrelli. "Estabilidade de vórtices em condensados de Bose-Einstein." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-12052016-171955/.
Full textAbstract:
Neste trabalho de mestrado é estudada a estabilidade de vórtices em condensados de Bose-Einstein com interação atrativa entre os átomos através da solução numérica da equação de Gross-Pitaevskii. Inicialmente são reproduzidos resultados da literatura, nos quais são estudados vórtices em condensados bidimensionais atrativos com potencial interatômico homogêneo em todo o condensado. A estabilidade de tais sistemas é inferida através da solução numérica das equações de Bogoliubov-de Gennes e da evolução temporal dos vórtices. Demonstra-se que esses vórtices são estáveis, até um certo número crítico de átomos, apenas para valores de vorticidade S=1. Em seguida foi proposto um modelo no qual a interação entre os átomos é espacialmente modulada. Neste caso é possível demonstrar que vórtices com valores de vorticidade de até S=6, pelo menos, são estáveis. Finalmente é estudada a estabilidade de vórtices em condensados tridimensionais atrativos, novamente com potencial interatômico homogêneo em todo o condensado. Assim como no caso bidimensional mostra-se que tais vórtices são estáveis para valores de vorticidade de S=1. Espera-se em breve estudar a estabilidade de vórtices em condesados tridimensionais com potencial de interação espacialmente modulado.In this work we study the stability of vortices in attractive Bose-Einstein condensates by solving numerically the Gross-Pitaevskii equation. Initially we reproduce some results from the literature, in which vortices in two-dimensional attractive Bose-Einstein condensates with homogeneous interatomic potential are studied. The stability of these systems is determined by solving numerically the Bogoliubov-de Gennes equations and by studying the time evolution of these vortices. We demonstrate that these vortices are stable, up to a certain critical number of atoms, just for the value of vorticity S=1. After we propose a model in which the interatomic interaction are spatially modulated. In this case it is possible to verify that vortices with values of vorticity up to S=6 , at least, are stable. Finally, we study the stability of vortices in three-dimensional attractive condensates, again with a homogeneous interatomic potential. As in the two-dimensional case, we show that vortices in these systems are stable to values of vorticity S=1. The next step in this work is study the stability of vortices in three-dimensional condensates with spatially modulated interatomic interaction.
47
Corre, Vincent. "Magnetism in spin-1 Bose-Einstein condensates with antiferromagnetic interactions." Thesis, Paris, Ecole normale supérieure, 2014. http://www.theses.fr/2014ENSU0020/document.
Full textAbstract:
Dans cette thèse nous étudions expérimentalement les propriétés magnétiques de condensats de sodium de spin 1 à l'équilibre. Dans ce système les atomes peuvent occuper chacun des trois états Zeeman caractérisés par la projection de leur spin sur l'axe de quantification m=+1,0,-1. Nous mesurons l'état de spin à N particules du système en fonction du champ magnétique appliqué et et de la magnétisation (différence entre les populations des états m=+1 et m=-1) du nuage atomique. Nos mesures sont en très bon accord avec la prédiction de la théorie de champ moyen, et nous identifions deux phases magnétiques résultant de la compétition entre les interactions de spin antiferromagnétiques et l'effet du champ magnétique. Nous décrivons ces deux phases en terme d'un ordre nématique de spin caractérisant la symétrie de l'état de spin à N particules. Dans une seconde partie nous nous concentrons sur les propriétés du condensat à très faible magnétisation et soumis à un faible champ magnétique. Dans ces conditions, la symétrie du système se manifeste à travers de très grandes fluctuations de spin. Ce phénomène n'est pas explicable par une théorie de champs moyen naïve, et nous développons une approche statistique plus élaborée pour décrire l'état de spin du condensat. Nous mesurons les fluctuations de spin et nous sommes capables de déduire de leur analyse la température caractérisant le degré de liberté de spin du condensat. Nous trouvons que cette température diffère de celle décrivant les atomes thermiques entourant le condensat. Nous interprétons cette différence comme une conséquence du faible couplage entre ces deux systèmesIn this thesis we study experimentally the magnetic properties of spin-1 Bose-Einstein condensate of Sodium at equilibrium. In this system the atoms can occupy any of the three Zeeman states characterized by their spin projection on the quantization axis m=+1,0,-1. We measure the many-body spin state of the system as a function of the applied magnetic field and of the magnetization (difference between the populations of the spin states m=+1 and m=-1) of the atomic sample. We find that our measurements reproduce very well the mean-field prediction, and we identify two magnetic phases expressing the competition between the antiferromagnetic inter-particle interactions and the effect of the magnetic field. We describe these phases in terms of a spin nematic order characterizing the symmetry of the many-body spin state. In a second part we focus on the properties of condensates of very low magnetization under a weak magnetic field. In these conditions, the symmetry of the system manifests itself in huge spin fluctuations. This phenomenon is not explainable by a naive mean-field theory and we develop a more elaborate statistical approach to describe the spin state of the condensate. We measure the spin fluctuations and are able from their analysis to infer the temperature characterizing the spin degree of freedom of the condensate. We find that this temperature differs from the temperature of the thermal fraction surrounding the condensate. We interpret this difference as a consequence of the weak coupling between these two systems
48
Silva, Luis Ever Young [UNESP]. "Exotic Bose-Einstein condensates: binary mixtures and dipolar gases." Universidade Estadual Paulista (UNESP), 2013. http://hdl.handle.net/11449/102535.
Full textAbstract:
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Nesta tese estudamos conceitos básicos de um condensado de Bose-Einstein (BEC) e sua extensão para sistemas com propriedades mais exóticas, incluindo misturas de dois componentes, com algumas características interessantes encontradas devido à interação entre espécies, e condensados de átomos com forte momento (magnético) dipolar, nos quais a interação dipolo-dipolo (anisotrópica e de longo-alcance), abre novas possibilidades de pesquisa na procura por desconhecidas e fascinantes características para gases atômicos ultra-frios. Mostramos o modelo de campo-médio para misturas de dois BECs interagindo através do potencial de contato e da interação dipolar de longo-alcance empregando termos não lineares de inter e intra-espécies. Aplicamos este modelo em sistemas binários com diferentes armadilhas em que um deles ou ambos podem ser dipolares. Especificamente, estudamos as características físicas de uma mistura de dois BECs - com e sem interação dipolar -, a formação (e dinâmica) de bright solitons para um BEC dipolar, algumas propriedades interessantes para um BEC dipolar no limite de interação forte, e as características de um BEC dipolar quase-livre vinculado à um outro BEC não dipolar confinado numa armadilha magnética. Apresentamos nossos resultados numéricos usando gráficos de densidade, diagramas de fase, de formação de estruturas nas densidades ou a dinâmica dos sistemas, entre outros. Sempre que possível, nossos resultados serão associados com quantidades usadas em técnicas experimentais através de um tipo específico de átomo, o número de partículas, os valores dos parâmetros de interação, a anisotropia da armadilha ou outras quantidades relacionadas com observáveis experimentais
We described the basic ideas of Bose-Einstein condensation (BEC), and then we focused our study on extensions to more exotic condensates including mixtures of two components, where interesting characteristics are found due to the interspecies interaction, and magnetic dipolar gases, which with their ansiotropic long-range dipolar interaction have opened up new avenues of research into cold atoms, in a quest for novel and fascinating features. In this thesis, we present the mean-field model for the binary BEC interacting two-componente mixtures of dipolar and nondipolar BECs, the formation and dynamics of bright solitons, the strong coupling domain for dipolar BECS, and the features of an untrapped bound dipolar droplet in a trapped nondipolar condensate. Our numerical results are presented in density plots, stability, phase plots, structure formartion in densities, breathing oscillation, and more. However, these solutions, whenever possible, are associated with quantities widely handled in experimental techniques, theough a specific types of atoms, number of particles, values of parameters of interaction or the anisotropy of trap, and others quantities related to experimental observables
49
Whitlock, Shannon, and n/a. "Bose-Einstein condensates on a magnetic film atom chip." Swinburne University of Technology, 2007. http://adt.lib.swin.edu.au./public/adt-VSWT20070613.172308.
Full textAbstract:
Atom chips are devices used to magnetically trap and manipulate ultracold atoms
and Bose-Einstein condensates near a surface. In particular, permanent magnetic film
atom chips can allow very tight confinement and intricate magnetic field designs while
circumventing technical current noise. Research described in this thesis is focused
on the development of a magnetic film atom chip, the production of Bose-Einstein
condensates near the film surface, the characterisation of the associated magnetic
potentials using rf spectroscopy of ultracold atoms and the realisation of a precision
sensor based on splitting Bose-Einstein condensates in a double-well potential.
The atom chip itself combines the edge of a perpendicularly magnetised GdTbFeCo
film with a machined silver wire structure. A mirror magneto-optical trap collects
up to 5 x 108 87Rb atoms beneath the chip surface. The current-carrying wires
are then used to transfer the cloud of atoms to the magnetic film microtrap and
radio frequency evaporative cooling is applied to produce Bose-Einstein condensates
consisting of 1 x 105 atoms.
We have identified small spatial magnetic field variations near the film surface that
fragment the ultracold atom cloud. These variations originate from inhomogeneity in
the film magnetisation and are characterised using a novel technique based on spatially
resolved radio frequency spectroscopy of the atoms to map the magnetic field landscape
over a large area. The observations agree with an analytic model for the spatial decay
of random magnetic fields from the film surface.
Bose-Einstein condensates in our unique potential landscape have been used as a
precision sensor for potential gradients. We transfer the atoms to the central region
of the chip which produces a double-well potential. A single BEC is formed far from
the surface and is then dynamically split in two by moving the trap closer to the
surface. After splitting, the population of atoms in each well is extremely sensitive to
the asymmetry of the potential and can be used to sense tiny magnetic field gradients
or changes in gravity on a small spatial scale.
50
Wuester, Sebastian, and sebastian wuester@gmx net. "Classical and Quantum Field Theory of Bose-Einstein Condensates." The Australian National University. Faculty of Science, 2007. http://thesis.anu.edu.au./public/adt-ANU20070802.161045.
Full textAbstract:
We study the application of Bose-Einstein condensates (BECs) to simulations of phenomena across a number of disciplines in physics, using theoretical and computational methods.
¶
Collapsing condensates as created by E. Donley et al. [Nature 415, 39 (2002)]
exhibit potentially useful parallels to an inflationary universe. To enable the exploitation of this analogy, we check if current quantum field theories describe collapsing condensates quantitatively, by targeting the discrepancy between experimental and theoretical values for the time to collapse. To this end, we couple the lowest order quantum field correlation functions to the condensate wavefunction, and solve the resulting Hartree-Fock-Bogoliubov equations numerically. Complementarily, we perform stochastic truncated Wigner simulations of the collapse. Both methods also allow us to study finite temperature effects.
¶
We find with neither method that quantum corrections lead to a faster collapse than is predicted by Gross-Pitaevskii theory. We conclude that the discrepancy between the experimental and theoretical values of the collapse time cannot be explained by Gaussian quantum fluctuations or finite temperature effects.
Further studies are thus required before the full analogue cosmology potential of collapsing condensates can be utilised.
¶
As the next project, we find experimental parameter regimes in which stable three-dimensional Skyrmions can exist in a condensate. We show that their stability in a harmonic trap depends critically on scattering lengths, atom numbers, trap rotation and trap anisotropy. In particular, for the Rb87 |F=1,m_f=-1>, |F=2,m_f=1> hyperfine states, stability is sensitive to the scattering lengths at the 2% level. We find stable Skyrmions with slightly more than 2*10^6 atoms, which can be stabilised against drifting out of the trap by laser pinning.
¶
As a stepping stone towards Skyrmions, we propose a method for the stabilisation of a stack of parallel vortex rings in a Bose-Einstein condensate. The method makes use of a ``hollow'' laser beam containing an optical vortex, which realises an optical tunnel for the condensate. Using realistic experimental parameters, we demonstrate numerically that our method can stabilise up to 9 vortex rings.
¶
Finally, we focus on analogue gravity, further exploiting the analogy between flowing condensates and general relativistic curved space time. We compare several realistic setups, investigating their suitability for the observation of analogue Hawking radiation. We link our proposal of stable ring flows to analogue gravity, by studying supersonic flows in the optical tunnel. We show that long-living immobile condensate solitons generated in the tunnel exhibit sonic horizons, and discuss whether these could be employed to study extreme cases in analogue gravity.
¶
Beyond these, our survey indicates that for conventional analogue Hawking radiation,
simple outflow from a condensate reservoir, in effectively one dimension, has the best properties. We show with three dimensional simulations that stable sonic horizons exist under realistic conditions. However, we highlight that three-body losses impose limitations on the achievable analogue Hawking temperatures. These limitations vary between the atomic species and favour light atoms.
¶
Our results indicate that Bose-Einstein condensates will soon be useful for interdisciplinary studies by analogy, but also show that the experiments will be challenging.