We report on the observation of weakly bound dimers of bosonic dysprosium with a strong universal $s$-wave halo character, associated with broad magnetic Feshbach resonances. These states surprisingly decouple from the chaotic background of narrow resonances, persisting across many such narrow resonances. In addition they show the highest reported magnetic moment $\mu \simeq 20{\mu }_{\mathrm{B}}$ of any ultracold molecule. We analyze our findings using a coupled-channel theory taking into account the short range van der Waals interaction and a correction due to the strong dipole moment of dysprosium. We are able to extract the scattering length as a function of magnetic field associated with these resonances and obtain a background scattering length $a_{\mathrm{bg}}=91\left(16\right)a_0$. These results offer prospects of a tunability of the interactions in dysprosium, which we illustrate by observing the saturation of three-body losses.

Two isotopic chemical reactions, Ne^* + NH₃, and Ne^* + ND₃, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely, Ne^* + NH₃ → Ne + ${\mathrm{NH}}_3^{+}$ + e⁻, and Ne^* + NH₃ → Ne + ${\mathrm{NH}}_2^{+}$ + H + e⁻, by detecting the ${\mathrm{NH}}_3^{+}$ and ${\mathrm{NH}}_2^{+}$ product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, E_coll, in the range 8 μeV < E_coll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(E_coll) plot from which it is concluded that the Langevin capture model does not properly describe the Ne^* + NH₃ reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, $\Gamma =\frac{\left[N{H}_2^{+}\right]}{\left[N{H}_2^{+}\right]+\left[N{H}_3^{+}\right]}$ , is relatively constant, Γ ≈ 0.3, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalized in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the Ne^* + NH₃ and Ne^* + ND₃ reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions.

We investigate transport dynamics of a single low-energy ionic impurity in a Bose-Einstein condensate. The impurity is implanted into the condensate starting from a single Rydberg excitation, which is ionized by a sequence of fast electric field pulses aiming to minimize the ion’s initial kinetic energy. Using a small electric bias field, we study the subsequent collisional dynamics of the impurity subject to an external force. The fast ion-atom collision rate, stemming from the dense degenerate host gas and the large ion-atom scattering cross section, allow us to study a regime of frequent collisions of the impurity within only tens of microseconds. Comparison of our measurements with stochastic trajectory simulations based on sequential Langevin collisions indicate diffusive transport properties of the impurity and allows us to measure its mobility. Our results open a novel path to study dynamics of charged quantum impurities in ultracold matter.

We develop a general quantum theory for reactive collisions involving power-law potentials ($-1/{r}^{n}$) valid from the ultracold up to the high-temperature limit. Our quantum defect framework extends the conventional capture models to include the nonuniversal case when the short-range reaction probability ${P}^{\mathrm{re}}<1$. We present explicit analytical formulas as well as numerical studies for the van der Waals ($n=6$) and polarization ($n=4$) potentials. Our model agrees well with recent merged beam experiments on Penning ionization, spanning collision energies from 10 mK to 30 K [Henson et al., Science 338, 234 (2012)].

Resonances are among the clearest quantum mechanical signatures of scattering processes. Previously, shape resonances and Feshbach resonances have been observed in inelastic and reactive collisions involving atoms or diatomic molecules. Structure in the integral cross section has been observed in a handful of elastic collisions involving polyatomic molecules. The present paper presents the observation of shape resonances in the reactive scattering of a polyatomic molecule, NH₃. A merged-beam study of the gas phase He(³S₁) + NH₃ Penning ionization reaction dynamics is described in the collision energy range 3.3 μeV < E_coll < 10 meV. In this energy range, the reaction rate is governed by long-range attraction. Peaks in the integral cross section are observed at collision energies of 1.8 meV and 7.3 meV and are assigned to ℓ = 15,16 and ℓ = 20,21 partial wave resonances, respectively. The experimental results are well reproduced by theoretical calculations with the short-range reaction probability P_sr = 0.035. No clear signature of the orbiting resonances is visible in the branching ratio between NH₃⁺ and NH₂⁺ formation.

We study a system of three photons in an atomic medium coupled to Rydberg states near the conditions of electromagnetically induced transparency. Based on the analytical analysis of the microscopic set of equations in the far-detuned regime, the effective three-body interaction for these Rydberg polaritons is derived. For slow light polaritons, we find a strong three-body repulsion with the remarkable property that three polaritons can become essentially noninteracting at short distances. This analysis allows us to derive the influence of the three-body repulsion on bound states and correlation functions of photons propagating through a one-dimensional atomic cloud.