Anisotropic and Nonadditive Interactions of a Rydberg Impurity in a Quantum Bath

We present a framework for treating mesoscopic anisotropic and nonadditive impurity–bath interactions, ubiquitous in realistic quantum impurity problems, which are often neglected in conventional approaches relying on additive, spherically symmetric pseudopotentials. To illustrate this general approach, we focus on the specific impurity problem of a Rydberg atom immersed in a Bose-Einstein condensate, where the internal-state degeneracy of the Rydberg impurity gives rise to configuration-dependent nonadditive potentials. With increasing interaction strength, anisotropy-induced partial-wave mixing generates distinct polaron and molaron resonances, allowing for radially and angularly excited bound states to become accessible. This approach captures the anisotropy and nonadditivity characteristic of a Rydberg impurity immersed in a quantum bath, and is broadly applicable to a host of quantum impurity problems beyond the Fröhlich paradigm.