The One-Way Dark

A magnonic mirror array made reflectionless — but only from one direction.

An exceptional point in a non-Hermitian system is where eigenvalues and eigenvectors simultaneously coalesce. At a reflectionless exceptional point, two zero-reflection states merge, flattening the reflection spectrum from a pair of Lorentzian dips into a broad, quartic-order suppression. The system becomes invisible over a wide bandwidth.

Dinc et al. (arXiv:2603.11485) achieve this unidirectionally — reflectionless from one side, reflective from the other — in a magnonic system. Three magnetic mirrors (spin ensembles) are coupled to a microwave waveguide at spatially separated points, forming an anti-Bragg array. The spatial separation breaks inversion symmetry: waves traveling left see a different effective coupling than waves traveling right.

Tuning the magnon-photon coupling strengths drives two reflectionless states to coalesce into a unidirectional exceptional point. At this point, the reflection spectrum from one direction shows the characteristic quartic flattening — broadband suppression far wider than any individual resonance. From the other direction, normal reflection persists.

The exceptional point also exposes dark states — magnon modes that are normally decoupled from the waveguide and invisible to spectroscopy. At the EP, the merging of the bright and dark sectors redistributes spectral weight, and features that were hidden in conventional reflection measurements become accessible. The EP acts as a spectroscopic tool, not just a design target.

The unidirectionality converts a fundamental curiosity (exceptional points) into a functional device primitive: a broadband magnetic isolator whose operating bandwidth is set by topology (the order of the EP) rather than by the linewidth of any single resonance.

Dinc et al., “Unidirectional exceptional point of reflectionless states in a magnonic mirror array,” arXiv:2603.11485 (2026).