We propose a novel experimental method to extend the investigation of ion-atom collisions from the so far studied cold, essentially classical regime to the ultracold, quantum regime. The key aspect of this method is the use of Rydberg molecules to initialize the ultracold ion-atom scattering event. We exemplify the proposed method with the lithium ion-atom system, for which we present simulations of how the initial Rydberg molecule wave function, freed by photoionization, evolves in the presence of the ion-atom scattering potential. We predict bounds for the ion-atom scattering length from ab initio calculations of the interaction potential. We demonstrate that, in the predicted bounds, the scattering length can be experimentally determined from the velocity of the scattered wave packet in the case of ^{6}Li^{+}-^{6}Li and from the molecular ion fraction in the case of ^{7}Li^{+}-^{7}Li. The proposed method to utilize Rydberg molecules for ultracold ion-atom scattering, here particularized for the lithium ion-atom system, is readily applicable to other ion-atom systems as well.

We study the long-range interaction of a single ion with a highly excited ultracold Rydberg atom and report on the direct observation of ion-induced Rydberg excitation blockade mediated over tens of micrometer distances. Our hybrid ion-atom system is directly produced from an ultracold atomic ensemble via near-threshold photo-ionization of a single Rydberg excitation, employing a two-photon scheme which is specifically suited for generating a very low-energy ion. The ion's motion is precisely controlled by small electric fields, which allows us to analyze the blockade mechanism for a range of principal quantum numbers. Finally, we explore the capability of the ion as a high-sensitivity single-atom-based electric field sensor. The observed ion -Rydberg-atom interaction is of current interest for entanglement generation or studies of ultracold chemistry in hybrid ion-atom systems.Ultracold Rydberg atoms have recently been shown to provide a highly flexible platform for quantum simulation of long-range interacting many-body systems [1-3], generation of nonclassical photonic states [4,5], or quantum information processing applications [6][7][8]. A central aspect in this context is the Rydberg blockade phenomenon, which inhibits the simultaneous excitation of two closeby atoms into Rydberg states as a consequence of strong Rydberg-Rydberg interaction [9]. A similar concept applies to hybrid systems of Rydberg atoms and ions, for which strong mutual interaction may lead to single chargeinduced blockade phenomena mediated over macroscopic distances [10]. Collisions of highly excited atoms with ions have been studied early on with atom-beam experiments [11]. More recently, their interaction affects quantum optics applications based on room temperature vapors [12].In the context of trapped ions and ultracold atoms [13][14][15][16], where motional degrees of freedom are exquisitely controlled at the single particle level, strong coupling of ions to Rydberg atoms has been proposed for generating ion-atom entanglement [10] and for controlling cold collisions, chemistry, or charge mobilities in ion-atom mixtures [17,18]. However, the observation of an ion-induced Rydberg blockade so far remained elusive. The major obstacle to probe ion -Rydberg-atom interaction in traditional hybrid settings, based on radio-frequency ion traps and Rydberg states excited from trapped neutral ensembles, are trap-induced lineshifts on the Rydberg state, which are challenging to distinguish from interaction effects. Moreover, systems of trapped Rydberg ions are essentially unamenable to ion -Rydberg-atom interaction due to the cancellation of Coulomb and trapping fields at the ions' equilibrium positions [19,20].In this Letter, we demonstrate excitation blockade of a single Rydberg atom by a single low-energy ion. The ion is produced from a single Rydberg excitation in an ultracold sample via a novel optical two-photon ionization scheme.Our approach provides precise spatial and motional control of the initially ultracold free ion, which constitut...

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