This study revisits the Fulling-Davies-Unruh (FDU) effect [1–3] for single and entangled two-level atoms interacting with a massless scalar field in free space and cavity setups. In free space, the equivalence of upward and downward transition rates holds between uniformly accelerated atoms and static atoms in a thermal bath at the Unruh temperature but breaks down in a cavity. For entangled atoms, this equivalence persists only under specific conditions in free space and breaks down entirely in a cavity. This proceeding is based on the work [4].