  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.
