We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows us to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.

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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...

We report on the creation of heterospecies bosonic molecules, associated from an ultracold BoseBose mixture of 41 K and 87 Rb, by using a resonantly modulated magnetic field close to two Feshbach resonances. We measure the binding energy of the weakly bound molecular states versus the Feshbach field and compare our results to theoretical predictions. We observe the broadening and asymmetry of the association spectrum due to thermal distribution of the atoms, and a frequency shift occurring when the binding energy depends nonlinearly on the Feshbach field. A simple model is developed to quantitatively describe the association process. Our work marks an important step forward in the experimental route towards Bose-Einstein condensates of dipolar molecules.PACS numbers: 34.20. Cf, Ultracold polar molecules hold the promise of a revolution in the domain of quantum degenerate gases and precision measurements. Degenerate molecules are sought primarily to produce a gas with strong long-range interactions, stemming from the coupling of electric dipole moments that heterospecies dimers feature. Such molecules would create strongly correlated systems with a wealth of quantum phases [1], provide candidate qubits [2,3], allow for a new generation of dipolar Bose-Einstein condensates (BECs) [4], and help in the search of the electron dipole moment [5]. Starting from ultracold atoms, molecules have been successfully created following two different approaches: photoassociation [6] and magnetoassociation [7], but few experiments have hitherto reported the production of Feshbach heteronuclear dimers. Two groups have reported the creation of fermionic KRb molecules [8,9], while the only bosonic dimer so far associated is 85 Rb 87 Rb [10], which can not be dipolar since the constituents share the same electronic configuration. Heterospecies bosonic dimers, i.e., the constituents of the dipolar BEC envisioned in Ref. [4], have instead eluded experimental realization so far.A Bose-Bose mixture is particularly suitable to associate such dimers due to the high phase-space densities achievable, while the atom-dimer relaxation, which limits the lifetime of the molecules, can be strongly suppressed in optical lattices with a single atom pair per lattice site [11]. Recent progresses in molecular stabilization schemes [12] make the Bose-Bose 41 K 87 Rb mixture truly promising for the experimental observation of BECs of dipolar molecules. Following a different route, other experiments have very recently obtained ultracold heterospecies Fermi-Fermi mixtures [13,14] that also can provide a way to compound bosonic dimers.In this Letter, we report on the production of heterospecies 41 K 87 Rb bosonic molecules starting from an ultracold mixture. In proximity of Feshbach resonances (FR's) at moderate magnetic fields, by adding a modulation to the Feshbach field [15], we have converted up to 12 000 41 K 87 Rb pairs into dimers, i.e., 40% of the minority 41 K atoms, at temperatures between 200 and 600 nK. We estimate the molecular lifetime to be a...