Non-Hermitian skin effect (NHSE) has now become the paradigmatic example of the topologically nontrivial impact of loss or gain in optical and condensed matter systems. Here, I will present our latest theoretical results on an analog of NHSE in the platform of waveguide quantum electrodynamics —  the area of quantum optics studying interaction with propagating photons with atoms in a waveguide [1].

I will focus on the Non-Hermitian Skin effect for the bound states of interacting photons propagating in a waveguide and chirally coupled to an array of atoms. I will demonstrate the concentration of bound modes at the edge of the array that can be interpreted as a non-Hermitian skin effect. The mechanism behind this effect is rather unusual: contrary to the usual local loss or gain, the spectrum for the bound states in the infinite structure is lossless. Still, the non-Hermiticity for bound pairs is driven by their dissociation into scattering states in the finite structure, as shown in the Figure.

The coexistence of the continuum with the quasiparticle dispersion branch is a generic feature for the energy spectra of various many-body systems, for example, with plasmonic or magnonic excitations. Thus, we believe that our results could apply beyond one-dimensional systems and beyond setups with atom-photon coupling.

[1]  A. S. Sheremet, M. I. Petrov, I. V. Iorsh, A. V. Poshakinskiy, and A. N. Poddubny, Waveguide quantum electrodynamics: Collective radiance and photon-photon correlations, Rev. Mod. Phys. 95, 015002 (2023).