Non-Hermitian optical atomic mirror*

Explorations of symmetry and topology have led to important breakthroughs in quantum optics, but much richer behaviour arise from the non-Hermitian nature of interactions of light with matter. Recently an optical mirror has been realized by using subwavelength arrays of atomic reflectors, which display the cooperative optical effects resulting from the photon-mediated dipole-dipole interactions. The loss processes associated with free space emission intrinsically endow the optical mirror with capability of carrying and creating a new non-Hermitian platform with unique symmetry and topology. Here we show that exceptional points and bulk Fermi arcs develop from a nondefective degeneracy by lowering the crystal symmetry of a two-dimensional square array of neutral atoms. We find, although the dipole-dipole interaction is reciprocal, the geometry-dependent non-Hermitian skin effect emerges. This phenomenon can be understood as resulting from the interplay of non-Hermiticity and low crystalline symmetry. Furthermore, long-range interactions lead to the anomalous size-dependent skin modes which are localized at a boundary. The long-rang interacting optical mirror provides a genuine paradigm that bridges two of the most active fields, non-Hermitian and topological physics, and opens the door to the observation of a wide range of outstanding phenomena that would be challenging in condensed matter.