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.