We investigate experimentally the entropy transfer between two distinguishable atomic quantum gases at ultralow temperatures. Exploiting a species-selective trapping potential, we are able to control the entropy of one target gas in presence of a second auxiliary gas. With this method, we drive the target gas into the degenerate regime in conditions of controlled temperature by transferring entropy to the auxiliary gas. We envision that our method could be useful both to achieve the low entropies required to realize new quantum phases and to measure the temperature of atoms in deep optical lattices. We verified the thermalization of the two species in a 1D lattice.PACS numbers: 03.75. Hh, 67.85.Pq, 05.30.Jp In recent years an intense research of quantum phases common to condensed matter systems and atomic quantum gases has made remarkable progresses [1]. Some of these phases can only be reached provided that the temperature is suitably low. However, in strongly correlated quantum systems, even the temperature measurement can be a challenging task. If so, to ascertain whether a given quantum phase is accessible, it is convenient to focus on the critical value of entropy, rather than temperature. The advantage is especially clear when the strongly correlated regime is reached by sufficiently slow, entropy-preserving, transformations of the trapping potential, as it is often the case for atoms in deep optical lattices [2]. For these reasons, it is important to determine and grasp control of the entropy of degenerate quantum gases [3,4,5]. In this work, we demonstrate the reversible and controlled transfer of entropy between the two ultracold, harmonically trapped Bose gases, which is based on the use of a species-selective dipole potential (SSDP), i.e., an optical potential experienced exclusively by one species (Fig. 1) [6,7]. In particular, we drive the target gas across the threshold for Bose-Einstein condensation, by a reversible transfer of entropy to the auxiliary gas.The main idea can be understood from textbook thermodynamics. Let us consider two distinguishable gases filling an isolated box, exchanging neither particles nor energy with the outside, and imagine that only one gas (target) is compressed, e.g. through a piston permeable to the second gas (auxiliary). The temperature will increase and, in thermal equilibrium, heat, hence entropy, will transfer from the target to the auxiliary uncompressed gas. In the limit of the auxiliary gas containing a large number of particles, it stands as a thermal bath. In a more formal way, for an ideal gas of N particles, the entropy S is proportional to N log(Σ/N ), where the number of accessible single-particle states Σ increases with the energy density of states and with the average energy, i.e., the temperature. In an adiabatic compression of one single gas, the reduction of the energy density of states is compensated by a temperature raising such that Σ, hence S, remains constant. If we add the uncompressed auxiliary gas in thermal contact, the temperature rai...

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The Efimov effect represents a cornerstone in few-body physics. Building on the recent experimental observation with ultracold atoms, we report the first experimental signature of Efimov physics in a heteronuclear system. A mixture of 41 K and 87 Rb atoms was cooled to few hundred nanoKelvins and stored in an optical dipole trap. Exploiting a broad interspecies Feshbach resonance, the losses due to three-body collisions were studied as a function of the interspecies scattering length. We observe an enhancement of the three-body collisions for three distinct values of the interspecies scattering lengths, both positive and negative. We attribute the two features at negative scattering length to the existence of two kind of Efimov trimers, namely Sun-Earth-Moon, the Helium atom, the proton: at all length scales, three-body systems are ubiquitous in physics, yet they challenge our understanding in many ways. Their complexity conspicuously exceeds the two-body counterparts. A peculiar class of three-body systems defying our intuition arises when the constituents feature resonant pair-wise interactions, such that the scattering length is much larger than the effective range of the pair potential. In a few seminal papers, V. Efimov advanced our understanding of such three-body systems and demonstrated the existence of a large number of weakly bound three-body states, thereafter known as the Efimov effect [1,2]. What makes Efimov states truly remarkable is their universality, i.e., the fact that their main properties are independent from the details of the pair potential, be it the strong interaction between two nucleons or the van der Waals force between two neutral atoms.For over 35 years, the Efimov effect sparked an intense theoretical research [3], while eluding experimental observation. The first experimental evidence of Efimov states was only recently reached with ultracold 133 Cs [4] and 39 K [5] atoms, thanks to the possibility to adjust at will the scattering length by means of Feshbach resonances. In nuclear physics, the original context studied by V. Efimov, the Efimov effect is hampered by the strong long-range Coulomb interactions and therefore confined to triads where at least two constituents are neutral. Among these, halo nuclei, i.e., nuclei like 6 He, 11 Li,14 Be, 20 C composed of a smaller core nucleus plus two loosely bound neutrons, have been identified as possible examples of Efimov physics [6] and there is an ongoing debate about the prospects of observing nuclear Efimov states [7]. To this goal, it is crucial to study Efimov physics in systems composed of distinguishable particles with different masses.In this work, we report the first experimental evidence of Efimov physics with particles of different masses, i.e., Efimov resonances in the three-body collisions of a mixture of ultracold 41 K and 87 Rb atoms. Our experiment demonstrates that two resonantly interacting pairs are sufficient to grant the exis- tence of Efimov states [3] and, thanks to universality, suggests that they could be obs...

The route toward a Bose-Einstein condensate of dipolar molecules requires the ability to efficiently associate dimers of different chemical species and transfer them to the stable rovibrational ground state. Here, we report on recent spectroscopic measurements of two weakly bound molecular levels and newly observed narrow d-wave Feshbach resonances. The data are used to improve the collisional model for the Bose-Bose mixture 41 K 87 Rb, among the most promising candidates to create a molecular dipolar BEC.