Non-Hermitian topological optical atomic mirror*

Recent studies of light-matter interactions in two-dimensional atomic arrays with subwavelength spacings have revealed intriguing phenomena in quantum optics, topology, and non-Hermitian physics [1,2,3]. It has been shown that the breaking of time-reversal symmetry (TRS) leads to non-trivial topological bands and edge states, while the breaking of rotational symmetry (RS) in the lattice geometry induces exceptional points (EPs) and a geometry-dependent non-Hermitian skin effect. However, the simultaneous breaking of both TRS and RS remains underexplored.

In this work, we investigate the non-Hermitian and topological properties of a two-dimensional atomic array with long-range dipole-dipole interactions influenced by lattice geometry and magnetic fields. By examining continuous deformations between honeycomb, square, and triangular lattices while maintaining atomic spacing, we demonstrate that exceptional points emerge within the light cone throughout the deformation process. Furthermore, we obtain a phase diagram as a function of deformation strength and magnetic field magnitude. Notably,  the emergence of topological phases labeled by Chern numbers can be separated by a distinct gapless phase characterized by EPs. Finally, we demonstrate the geometry-dependent skin-topological edge modes localized at the boundaries.

[1] Shahmoon, E., Wild, D. S., Lukin, M. D. & Yelin, S. F. Phys. Rev. Lett. 118, 113601 (2017).

[2] Perczel, J. et al. Phys. Rev. Lett. 119, 023603 (2017).

[3] Y.-C. Wang, J.-S. You, and H. H. Jen. Nat. Commun. 13, 4598 (2022).