# Robust entanglement with 3D nonreciprocal photonic topological insulators

## TL;DR

This paper demonstrates that entanglement between two-level systems near a 3D topological insulator interface is highly robust due to nonreciprocal surface plasmons, with potential applications in quantum technologies.

## Contribution

It develops a master equation approach using the Green function to analyze entanglement in nonreciprocal, inhomogeneous environments, highlighting topological protection effects.

## Key findings

- Entanglement remains robust despite material defects.
- Nonreciprocal environment can prevent entanglement at close distances.
- Entanglement is insensitive to variations in topological material parameters.

## Abstract

We investigate spontaneous and pumped entanglement of two level systems in the vicinity of a photonic topological insulator interface, which supports a nonreciprocal (unidirectional), scattering-immune and topologically-protected surface plasmon polariton in the bandgap of the bulk material. To this end, we derive a master equation for qubit interactions in a general three-dimensional, nonreciprocal, inhomogeneous and lossy environment. The environment is represented exactly, via the photonic Green function. The resulting entanglement is shown to be extremely robust to defects occurring in the material system, such that strong entanglement is maintained even if the interface exhibits electrically-large and geometrically sharp discontinuities. Alternatively, depending on the initial excitation state, using a non-reciprocal environment allows two qubits to remain unentangled even for very close spacing. The topological nature of the material is manifest in the insensitivity of the entanglement to variations in the material parameters that preserve the gap Chern number. Our formulation and results should be useful for both fundamental investigations of quantum dynamics in nonreciprocal environments, and technological applications related to entanglement in two-level systems.