Bilayer Cuprate Antiferromagnets Enable Programmable Cavity Optomagnonics

Abstract

Hybrid platforms that couple microwave photons to collective spin excitations offer promising routes for coherent information processing, yet conventional magnets face inherent trade-offs among coupling strength, coherence, and tunability. We demonstrate that bilayer cuprate antiferromagnets, exemplified by YBa2Cu3O6+x, provide an alternative approach enabled by their unique magnon spectrum. Using a neutron-constrained bilayer spin model, we obtain the complete Gamma-point spectrum and identify an in-plane acoustic alpha mode that remains gapless and Zeeman-linear, alongside an in-plane optical beta mode stabilized by weak anisotropy whose frequency can be tuned from the gigahertz to terahertz range. When coupled to a single-mode microwave cavity, these modes create two distinct channels with a magnetically tunable alpha-photon interaction and a nearly field-independent beta-photon interaction. This asymmetric behavior enables continuous, single-parameter control spanning from dispersive to strong coupling regimes. In the dispersive limit, our analysis reveals cavity-mediated magnon-magnon coupling, while near triple resonance the normal modes reorganize into bright and dark superpositions governed by a single collective energy scale. The calculated transmission exhibits vacuum-Rabi splittings, dispersive shifts, and Fano-like lineshapes that provide concrete experimental benchmarks and suggest potential for programmable filtering and coherent state transfer across the gigahertz-terahertz frequency range if realized experimentally with suitable interfaces.