From superfluids to droplets: quantum phenomena with a tunable Bose-Bose mixture

Superfluids are one of the most intriguing among macroscopic quantum phenomena, where the large-scale properties of the system reveal the small-scale quantum properties of its constituent components: this happens because of the coherent interaction of this components, which interfere constructively, and therefore amplify their characteristics. For the specific case of superfluids, this is most famously manifest in their dissipationless flow, whose observation lead to their discovery, but also in a plethora of other peculiarities, which are, as said, ultimately due to coherence effects within the fluid particles. Apart from a low temperature laboratory, the presence of superfluids has been hypothesized in many systems, ranging from superconductors to neutron stars. A facet of superfluidity that has long been a fertile area of study is that of multicomponent superfluids, where two different specimens of such fluids interact, giving rise to novel and interesting phenomena, which would not be present in the single component case. While different from stricto sensu superfluidity, Bose-Einstein condensation is intimately linked to it, as it is, ultimately, its cause. In this thesis, we focused on the physics of superfluids, investigated with a mixture of Bose-Einstein condensates realized with ultracold 41 K and 87 Rb. These atoms have the capability, which is of particular interest for our stated purpose, of tuning the interactions between the K and the Rb atoms, enabling the exploration of the whole phase diagram of the mixture. The work has been done in three phases. In the first phase, we have developed a procedure to obtain the double condensate in the correct hyperfine state, and has resulted in the production of Bose-Einstein condensates with tunable interspecies interactions, having a total number of atoms ranging between 5 × 10^4 and 3 × 10^5 , in a consistent and reliable way. In the second phase, we have investigated the physics of the dipole oscillation of the condensate mixture. Dipole oscillations are one of the possible collective excitations of a condensates, and those where the presence of interspecies interactions more dramatically changes the features of the modes, with respect to the single component case. We have experimentally measured their behaviour for various interspecies interaction strengths, and compared it with the theoretical models, finding which are more amenable to the description of the observations in various cases. In the third phase, we have designed and built a new, high-resolution imaging system. This system is capable of resolution of approximately 1.5 μm, in contrast to the 5 μm previously attainable, widening the capabilities of the apparatus. The system needed a compensation of the objective, which is on the path of the magneto-optical trap laser beam, and a new sequence of imaging pulses, to adapt to the different orientation of the probe beam with respect to the magnetic fields, and to the different strength of the latter when the image is done on a trapped condensate, as opposed to one that is free-falling. We used this apparatus to obtain some preliminary in-situ images of a quantum droplet. Quantum droplets are very peculiar self-bound states of a mixture, where the collapse of the atomic cloud, in a regime of very strong interspecies attraction, is impeded by the quantum fluctuations, and instead an hydrodynamic equilibrium is reached. Such states have peculiar features not normally found in quantum gases. With the work done in this thesis, we have built the foundations for further investigations of multicomponent superfluidity, with the eventual aim of contributing to the exploration of this wide and fascinating subject.