Novo sistema experimental para a estudo de vórtices acoplados em um condensado de Bose-Einstein de duas espécies atômicas 23Na-41K com interação variável
Two-component fluids can be miscible (if they overlap in space) or immiscible (if they remain phase-separated). In the context of trapped two-species Bose-Einstein condensates (BECs), these miscibility regions can only be fully characterize if one considers the interspecies interaction, the mass ratio and the number of atoms in each species. The dynamics of coupled vortices is different for each miscibility region and exotic vortices configurations (such as, square vortex lattices, \"vortex sheets\", skyrmions, etc.) are expected to occur. In this thesis, we present the construction of a new experimental system able to produce a two-species Bose-Einstein condensate of 23Na-41K atoms with tunable interspecies interactions and study the dynamics of coupled vortices in the different miscibility regimes. The BEC of sodium atoms obtained first in a Plug trap and later, in a crossed optical dipole trap, is fully characterized as well as the cold atomic cloud of potassium atoms produced by means of a Gray molasses cooling procedure. In the crossed optical dipole trap, the vortices will be nucleated with the use of a stirring beam. Therefore, in the end of this thesis, we present the stirring beam setup and its characterization prior aligning it into the 23Na BEC.
Two-component fluids can be miscible (if they overlap in space) or immiscible (if they remain phase-separated). In the context of trapped two-species Bose-Einstein condensates (BECs), these miscibility regions can only be fully characterize if one considers the interspecies interaction, the mass ratio and the number of atoms in each species. The dynamics of coupled vortices is different for each miscibility region and exotic vortices configurations (such as, square vortex lattices, "vortex sheets", skyrmions, etc.) are expected to occur. In this thesis, we present the construction of a new experimental system able to produce a two-species Bose-Einstein condensate of 23Na-41K atoms with tunable interspecies interactions and study the dynamics of coupled vortices in the different miscibility regimes. The BEC of sodium atoms obtained first in a Plug trap and later, in a crossed optical dipole trap, is fully characterized as well as the cold atomic cloud of potassium atoms produced by means of a Gray molasses cooling procedure. In the crossed optical dipole trap, the vortices will be nucleated with the use of a stirring beam. Therefore, in the end of this thesis, we present the stirring beam setup and its characterization prior aligning it into the 23Na BEC.
Two-component fluids can be miscible (if they overlap in space) or immiscible (if they remain phase-separated). In the context of trapped two-species Bose-Einstein condensates (BECs), these miscibility regions can only be fully characterize if one considers the interspecies interaction, the mass ratio and the number of atoms in each species. The dynamics of coupled vortices is different for each miscibility region and exotic vortices configurations (such as, square vortex lattices, \\"vortex sheets\\", skyrmions, etc.) are expected to occur. In this thesis, we present the construction of a new experimental system able to produce a two-species Bose-Einstein condensate of 23Na-41K atoms with tunable interspecies interactions and study the dynamics of coupled vortices in the different miscibility regimes. The BEC of sodium atoms obtained first in a Plug trap and later, in a crossed optical dipole trap, is fully characterized as well as the cold atomic cloud of potassium atoms produced by means of a Gray molasses cooling procedure. In the crossed optical dipole trap, the vortices will be nucleated with the use of a stirring beam. Therefore, in the end of this thesis, we present the stirring beam setup and its characterization prior aligning it into the 23Na BEC.
In this thesis, we produce a Bose-Einstein condensate of sodium atoms in a magnetic plugged trap and in an optical dipole trap. First, we investigate the critical velocity by moving an obstacle through the condensate, comparing the result obtained with the the sound velocity. We make use of the critical velocity value to determine the rigth parameters to stir the condensate and nucleate quantized vortices, by introducing angular with the rotation of the srirring beam the number of vortices increases linearly until the rigid-body limit. In the second part we introduce potassium atom in the science chamber where from the begining of the MOT stage are observed ligth induced losses. In a single-specie operation we are able to cool down the potassium atoms by implementing the gray molasses technique, increasing its phase-space density in order have a good tranference efficienty to the magnetic trap. We also present the the sympathetic cooling of the K atoms due by elastic collision between the atomic species.
In this work we constructed an experimental system to realize a BEC of sodium atoms. In the first part of the work, we study two atomic sources in order to choose the most suitable for our system. The comparison between a Zeeman slower and a bidimensional magnetooptical trap (2D-MOT) was performed to evaluate the capacity of producing an appropiate flux of atoms in order to load a tridimensional magneto-optical trap (3D-MOT). The experimental results show that the 2D-MOT is as efficient as the Zeeman slower with the advantage of being more compact and easier to operate, and for these reasons we choose it as our source of cold atoms. After this, the experimental apparatus to produce a Bose-Einstein condensate was constructed and characterized. With this experimental system we realized all the required stages to achieve the Bose-Einstein condensation (BEC). Initially, we characterized and compared the performance between the Bright-MOT and Dark-SPOT MOT of sodium atoms, observing the great advantages this last configuration offers. Afterward, we implemented the experimental sequence for the achivement of the BEC of sodium atoms. After the optical molasses process, the atoms are tranferred to an optically plugged quadrupole trap (OPT) where the process of evaporative cooling is performed. With this setup, we achive a sodium BEC with ∼ 5×105 atoms and a critical temperature of ∼ 1.1 μK. Finally, with the constructed and characterized machine, we started to perform experiments of cooperative absorption of two photons by two trapped atoms. With the new system, we wanted to take advantage of the higher densities in the magnetic trap and BEC to explore some features of this phenomenon in the classical and quantum regimes. We were interested in exploring some features of this nonlinear light-matter interaction effect. The idea of having two or more photons interacting with two or more atoms is the beginning of a new possible class of phenomena that we could call many photons-many body intercation. In this new possibity, photons and atoms will be fully correlated, introducing new aspects of interactions.
In this thesis I present the construction of a new apparatus aimed at studying two-component Bose-Einstein condensates (BECs) in the presence of a Rabi coupling, where the two components correspond to internal states of sodium atoms. The coherent mixture, in the miscible regime, also exhibits a metastable excitation consisting in a domain wall of relative phase connecting vortices of different components. Due to the peculiar energy dependence of such a configuration, an attractive force, independent of the vortex distance, is expected, making this system a candidate for mimicking features of quark confinement in QCD. The surface tension of the domain wall structure can be experimentally controlled via the strength of the coupling, allowing to study the system dynamics in different regimes. These include a predicted regime in which, for sufficient high coupling strength, the domain wall breaks and new vortex couples nucleate, providing by itself an interesting experimental realization of spin counterflow dynamical instabilities in a superfluid system, as well as a phenomenon analogous to string breaking in QCD. The choice of sodium as atomic species is motivated by its collisional properties that allow to obtain a perfect spatial superposition between the two miscible components |F = ±1⟩ if trapped by a spin-independent potential, avoiding the known phenomenon of ’buoyancy’. Studying the dynamics of such systems for sufficiently long times, with a mixture subject to Zeeman differential energy shifts, requires a specific effort to remove magnetic field fluctuations: a rough estimate suggests that in order to maintain the system coherence for a sufficiently long time to study its dynamics, magnetic field fluctuations should be reduced by at least three orders of magnitude compared to typical values observed in laboratory environment. Such attenuations can be obtained by means of multiple layers of μ-metal, that is incompatible with the use of ordinary magnetic traps, characterized by large magnetic field gradients on the atoms, due to residual magnetization and saturation of the shielding material. To avoid such effects it is required to either evaporatively cool atoms into an optical dipole trap loaded from a molasses stage, or a hybrid approach by means of which atoms are transferred to a low-gradient quadrupole trap superimposed to the optical trap. Producing BECs with such protocols greatly benefits from an efficient optical molasses cooling stage to prepare the sample in the best conditions of temperature and density before loading atoms into the trap. With this regard, the main limitation of ordinary laser cooling techniques is their reliance on absorption and spontaneous emission cycles, which limits the lowest temperature and highest density that can be reached as a consequence of residual heating effects and photon-reabsorption. An important resource to cope with these limits are dark states. In a broader sense a dark state is a state which does not interact with the exciting light field, and an atom in such a state would be neither subject to the beneficial cooling effects nor to the detrimental effects of light scattering. It is possible, however, to exploit the phenomenon of electromagnetically induced transparency (EIT) to induce a velocity-selective cooling mechanism for which slower atoms, that do not need further cooling, are trapped in a dark state corresponding to a coherent superposition of atomic levels whose excitation probabilities interfere destructively, while the cooling mechanism still applies to the fastest atoms. Among these techniques, gray molasses cooling allows to reach temperatures as low as a few recoil temperatures, while retaining atomic densities useful to reach quantum degeneracy in the subsequent stages of the experiment. In order to exploit this technique, an additional laser source had to be implemented during my thesis. To realize a gray molasses on the sample only |F⟩ → |F − 1⟩ or |F⟩ → |F⟩ transitions can be chosen, requiring blue-detuned laser, in contrast to ordinary (sub)Doppler laser cooling techniques. Both these requirements rule out the \nuse of the D2 transition used for ordinary laser cooling techniques, due to its finely-spaced hyperfine structure. On the other hand, the D1 transition is characterized by a broader level spacing in the hyperfine structure and the absence of higher energy states on the blue side of the |F = 2⟩ → |F' = 2⟩ transition. As part of the work for this thesis, I successfully implemented and characterized gray molasses cooling on the D1 optical line of sodium. The buildup of the new apparatus includes the assembly of a new laser source for laser trapping and cooling on the D2 line, the assembly of the optical table devoted to the frequency and amplitude control of all the laser beams involved in the optical laser cooling procedures as well as the electronic control system. Design and assembly of the UHV and baking procedures for the stainless steel vacuum chamber are also described as well as the laser cooling techniques employed to load the atoms in a Dark-Spot MOT. Regarding the production of BEC, various strategies were attempted for different dipole beam configurations. Dipole traps typically suffer from the tradeoff between capture volume and trap depth at a given power, while hybrid traps usually take advantage of a magnetic trap stage that would not be compatible with the μ-metal shielding. Preliminary attempts to reach quantum degeneracy after directly loading the dipole trap from molasses were unsuccessful due to spurious effects. An alternative approach based on a magnetic-shield compatible hybrid trap protocol, in the absence of magnetic trap compression, was successfully implemented.
Abstract We present a compact and versatile experimental system for producing Bose–Bose superfluid mixtures composed of sodium and potassium atoms. The compact design combines the necessary ultra-high vacuum enviroment for ultracold atom experiments with efficient atomic fluxes by using two-dimensional magneto-optical traps as independent source of atoms. We demonstrate the performance of this new machine by producing a Bose–Einstein condensate of 23 Na with ∼ atoms. The tunability of Na–K bosonic mixtures is particularly interesting for studies regarding the nucleation of vortices and quantum turbulence. In this direction, the large optical access of the science chamber along the vertical direction provides the conditions to implement high resolution optical setups for imaging and rotating the condensate with a stirring beam. We show the nucleation of a vortex lattice with up to 14 vortices in the 23 Na BEC, attesting the efficiency of the experimental apparatus in studying the dynamics of vortices.
The effects of miscibility in interacting two-component classical fluids are relevant in a broad range of daily applications. When considering quantum systems, two-component Bose–Einstein condensates provide a well-controlled platform where the miscible–immiscible phase transition can be completely characterized. In homogeneous systems, this phase transition is governed only by the competition between intra- and inter-species interactions. However, in more conventional experiments dealing with trapped gases, the pressure of the confinement increases the role of the kinetic energy and makes the system more miscible. In the most general case, the miscibility phase diagram of unbalanced mixtures of different atomic species is strongly modified by the atom number ratio and the different gravitational sags. Here, we numerically investigate the ground-state of a 23Na–39K quantum mixture for different interaction strengths and atom number ratios considering realistic experimental parameters. Defining the spatial overlap between the resulting atomic clouds, we construct the phase diagram of the miscibility transition which could be directly measured in real experiments.