Dark states from superradiant decay of multilevel atoms in a cavity*

We investigate the collective decay dynamics of atoms with a generic multilevel structure (F → F') coupled to two light modes of different polarization inside a cavity. Due to the multiple decay channels, the eigenstate structure and superradiant behavior is much richer and more complex than for two-level atoms. In particular, we find that, in contrast to the two-level case, multilevel atoms can harbour eigenstates that are perfectly dark to cavity decay even within the subspace of permutationally symmetric states (collective Dicke manifold). As a consequence, the superradiant decay of multilevel atoms can end up stuck in one of these dark states, where a macroscopic fraction of the atoms remains excited. These dark states should be readily observable in current optical cavity experiments with alkaline-earth atoms or Raman-dressed transitions. Their long-lived nature anticipates potential applications in quantum sensing and metrology, and quantum simulation.