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Topological physics in tunable optical lattices

Periodic Reporting for period 1 - TOP-DOL (Topological physics in tunable optical lattices)

Période du rapport: 2015-06-01 au 2017-05-31

The goal of the project Topological physics with disorder in tunable optical lattices (TOPDOL) was to develop a new experimental apparatus for exploring topological systems, exploiting ultracold atoms. We aimed to: (i) produce interacting Bose and Fermi systems (and mixtures of them), in order to get access to interacting topological phases; (ii) study different schemes for producing optical lattices of non-trivial topology; (iii) obtain a setup well suited for demonstrating topological protection, i.e. the robustness of topological transport properties against external perturbations (such as a disordered potential).
During the two years of the project we have designed, constructed and validated an experimental apparatus well adapted to these goals. It gives access to quantum degenerate mixtures of potassium, and is designed to work with the three isotopes (two bosons, 39K and 41K, and one fermion, 40K). Exploiting a combination of gray molasses sub-Doppler cooling and a hybrid magnetic/optical trap, it routinely produces large Bose-Einstein condensates of 41K that can be used to sympathetically cool the other two potassium isotopes. This approach has allowed us to observe for the first time dual Bose-Einstein condensation of 39K and 41K, and to demonstrate an alternative approach for cooling 39K to quantum degeneracy.
In a first series of experiments we have performed an extensive study of the scattering properties of potassium Bose-Bose mixtures. We have located 27 new Feshbach resonances, including 39K-41K mixtures and spin mixtures of 39K and 41K. In collaboration with M. Tomza (ICFO and University of Warsaw) and A. Simoni (University of Rennes), we are currently exploiting these results to further constrain the model potentials available for potassium scattering.
In parallel, we have carried out preparatory work for the implementation of a tunable optical lattice of non-trivial topology in our apparatus, developing the required phase stabilization schemes, and designed, characterized and validated a high numerical aperture imaging system. The latter has already been implemented in the experimental apparatus, and should allow for the generation of the disordered external potentials required to demonstrate topological protection.
Finally, in a series of experiments not planned in the original proposal we have studied spin mixtures of 39K with repulsive interspin interactions and intraspin attractive interactions. In this system, quantum fluctuations stabilize the system against collapse and lead to the formation of quantum droplets: macroscopic self-bound states with liquid-like properties. We have experimentally studied quantum droplets in a quasi-one dimensional geometry, where they compete with more conventional bright solitons. We have determined the droplet properties (density and spin composition) as a function of atom number and interaction strength, and observed a transition between dilute bright solitons and liquid-like droplets, in good agreement with a simple theoretical model.
The work carried out in this project has laid solid foundations for the study of topological systems in the apparatus, which will be pursued by the group in the future. It has already lead to original scientific results: the production of a new degenerate quantum mixture, the characterization of previously unobserved Feshbach resonances, and the observation of a novel quantum droplet phase stabilized by quantum fluctuations in mixtures of Bose-Einstein condensates. The latter result considerably extends the state-of-the-art in the field: although quantum droplets in Bose-Bose mixtures were theoretically predicted in 2015 by D. Petrov (Université Paris Sud), until our work (and independent experiments carried out at LENS during the same period), they had only been observed in dipolar quantum gases, which have notable differences (results obtained by the Pfau and Ferlaino groups during 2016). Thus, our experiments constitute the first step towards the systematic study of weakly interacting ultra-dilute quantum liquids, with universal properties uniquely determined by contact interactions.
Three publications are currently in preparation concerning the results described above. I have presented them in 6 international conferences, workshops or meetings (two invited talks, one contributed talk and three poster presentations), and given as well two invited seminars. I have also visited several industrial R&D laboratories during the project, and since the end of the project I work as research engineer in industry. There, I develop inertial navigation systems based on cold atoms and directly apply the technical knowledge acquired in TOPDOL to a more applied setting.
During the project, I have participated in the formation of two undergraduate, four Master and two PhD students, and personally supervised the project of a visiting Master student, thus contributing to the transfer of knowledge in Europe. Our results have also been disseminated to a more general audience: the first observation of Bose-Einstein condensation in Spain was featured in the national press (El País), as well as in a general audience book (El frío absoluto by B. Juliá Díaz, RBA 2016). The progress of the project has been continuously reflected in the group website (www.qge.icfo.es) and in a YouTube channel which I personally set up.
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