A non-Hermitian optical atomic mirror

TL;DR

This paper demonstrates how a two-dimensional atomic array can function as a non-Hermitian optical mirror exhibiting exceptional points, Fermi arcs, and a geometry-dependent skin effect, revealing rich topological phenomena driven by long-range interactions.

Contribution

It introduces a novel non-Hermitian optical mirror based on atomic arrays, exploring the emergence of exceptional points, Fermi arcs, and skin effects due to symmetry breaking and long-range interactions.

Findings

Exceptional points arise from symmetry lowering in atomic lattices.

Dispersive Fermi arcs are truncated by the light cone.

Long-range interactions lead to a scale-free skin effect.

Abstract

Explorations of symmetry and topology have led to important breakthroughs in quantum optics, but much richer behaviors arise from the non-Hermitian nature of light-matter interactions. A high-reflectivity, non-Hermitian optical mirror can be realized by a two-dimensional subwavelength array of neutral atoms near the cooperative resonance associated with the collective dipole modes. Here we show that exceptional points develop from a nondefective degeneracy by lowering the crystal symmetry of a square atomic lattice, and dispersive bulk Fermi arcs that originate from exceptional points are truncated by the light cone. We also find, although the dipole-dipole interaction is reciprocal, the geometry-dependent non-Hermitian skin effect emerges. Furthermore, skin modes localized at a boundary show a scale-free behavior that stems from the long-range interaction and whose mechanism goes beyond the framework of non-Bloch band theory. Our work opens the door to the study of the interplay among non-Hermiticity, topology, and long-range interaction.