Lamb shift of rotational energy levels in quantum gravity

One of the major challenges of modern foundational physics is the combination of quantum physics and general relativity into a theory of quantum gravity. A severe obstacle are the required extremely high energy scales for direct tests of quantum gravity effects. In recent years, attention has shifted towards near-future experimental predictions of effective or phenomenological models, such as gravitationally induced entanglement or interactions of linearized gravity with non-relativistic measurement devices. These developments motivate us to study the coupling between the quantized metric perturbation and an anisotropic quantum rigid rotor. The intuitive motivation for our study is that a sufficiently heavy rotor consists of a very large number of component systems. Although quantum gravitational effects are completely negligible at the scale of a single atom or molecule, the effects can get amplified by the sheer number of constituents. The resulting effective model describes the gravitational analogue of the angulon quasiparticle excitations discussed in condensed matter physics, suggesting a pathway for future analogue experiments. As a first concrete test of our model, we derive the gravitational Lamb shift for the energy eigenstates of the rotor, which become dressed by a cloud of virtual gravitons. Although the effect is very small, it could provide a concrete test for whether gravity is truly quantum.