Fundamental constraints on the observability of non-Hermitian effects in passive systems

TL;DR

This paper uses scattering theory to analyze how fundamental physical constraints, like causality, limit the observability of non-Hermitian effects such as exceptional points and the skin effect in passive systems.

Contribution

It introduces an internal time-delay operator that quantifies the visibility of non-Hermitian phenomena under physical constraints in passive devices.

Findings

Causality imposes fundamental limits on non-Hermitian effect observability.

The internal time-delay operator measures the visibility of phenomena.

Extreme mode nonorthogonality can be effectively hidden.

Abstract

Utilizing scattering theory, we quantify the consequences of physical constraints that limit the visibility of non-Hermitian effects in passive devices. The constraints arise from the fundamental requirement that the system obeys causality, and can be captured concisely in terms of an internal time-delay operator, which furthermore provides a direct quantitative measure of the visibility of specific non-Hermitian phenomena in the density of states. We illustrate the implications by contrasting different symmetry classes and non-Hermitian effects, including exceptional points and the non-Hermitian skin effect, whose underlying extreme mode nonorthogonality turns out to be effectively disguised.