A Rydberg impurity in an ideal Bose-Einstein condensate

The excitation of a Rydberg atom in an ideal Bose-Einstein condensate (BEC) leads to a generalization of the paradigmatic Bose polaron. While the interaction between ground-state atoms is short-ranged in nature and can be replaced by a contact pseudopotential, this cannot be done in general for a Rydberg impurity, whose interaction range typically surpasses the mean interparticle spacing in the bath. Furthermore, this interaction can support several deeply bound two-body molecular states. These features of the Rydberg impurity complicate its theoretical description and analysis and cause it to exhibit a host of new behaviors not seen in the simpler system of a ground-state impurity in an ultracold gas. By computing the absorption spectrum, we map out the phase diagram of the Rydberg impurity in a BEC. We see evidence for both attractive and repulsive polaron branches, where the Rydberg atom is dressed by excitations of the surrounding bath, in addition to a many-body bound state characterized by the attachment of many atoms to the impurity. The latter scenario has been experimentally observed already, and we propose regimes where the quasiparticle branches, or their effects, can be similarly observed.