We develop a microscopic theory describing a quantum impurity whose
rotational degree of freedom is coupled to a many-particle bath. We approach
the problem by introducing the concept of an 'angulon' - a quantum rotor
dressed by a quantum field - and reveal its quasiparticle properties using a
combination of variational and diagrammatic techniques. Our theory predicts
renormalisation of the impurity rotational structure, such as observed in
experiments with molecules in superfluid helium droplets, in terms of a
rotational Lamb shift induced by the many-particle environment. Furthermore, we
discover a rich many-body-induced fine structure, emerging in rotational
spectra due to a redistribution of angular momentum within the quantum
many-body system.Comment: 5 pages, 2 figures; revised version, supplementary adde
