The Lamb shift of a Rydberg atom in a cavity is shown to be enhanced with the resonance interaction of a virtual atomic transition and cavity modes. The dependence of the Lamb shift on quantum numbers and atomic number changes drastically. By shifting cavity walls and scanning the atomic beam one can vary the Lamb shift. The value of the Lamb shift in a cavity may exceed a typical magnitude of the fine-structure energy. For a rough resonance tuning the Coulomb multiplet is strongly mixed and a novel classification is necessary.
The Unruh effect predicts that accelerated observers perceive the inertial vacuum as populated by particles, providing a flat-spacetime analogue of Hawking radiation. Its direct observation, however, remains experimentally challenging, since an Unruh temperature of
1K requires accelerations of order
1020m/s2. Here, we show that the Lamb shift of a centripetally accelerated atom inside a high-
Q cavity provides a sensitive spectroscopic probe of the Unruh effect at dramatically lower accelerations. The cavity reshapes the electromagnetic density of states and converts otherwise tiny noninertial corrections into tunable level shifts. Depending on the atomic angular velocity and cavity detuning, the Lamb shift can be enhanced, strongly quenched, or completely screened. Remarkably, for experimentally realistic parameters, a rotation-induced shift of order
10Hz can arise already at accelerations as low as
0.5m/s2, more than twenty orders of magnitude below the acceleration scale conventionally associated with direct Unruh detection. These results identify cavity-controlled Lamb-shift spectroscopy as a viable route toward laboratory tests of the circular Unruh effect in the ultralow-acceleration regime.