# Magnon-photon coupling in antiferromagnets

## TL;DR

This paper demonstrates that antiferromagnets can exhibit strong magnon-photon coupling with unique features, enabling potential coherent information transfer between photons and magnons, similar to ferromagnets but with distinct quantum properties.

## Contribution

It reveals quantum-mechanical behavior of magnon-photon coupling in antiferromagnets, including the effects of external fields and the potential for strong coupling and coherent information transfer.

## Key findings

- One spin wave band couples with photons without external field.
- External field causes two anticrossings with photons.
- Strong coupling is achievable with damping below certain thresholds.

## Abstract

Magnon-photon coupling in antiferromagnets has many attractive features that do not exist in ferro- or ferrimagnets. We show quantum-mechanically that, in the absence of an external field, one of the two degenerated spin wave bands couples with photons while the other does not. The photon mode anticrosses with the coupled spin waves when their frequencies are close to each other. Similar to its ferromagnetic counterpart, the magnon-photon coupling strength is proportional to the square root of number of spins $\sqrt{N}$ in antiferromagnets. An external field removes the spin wave degeneracy and both spin wave bands couple to the photons, resulting in two anticrossings between the magnons and photons. Two transmission peaks were observed near the anticrossing frequency. The maximum damping that allows clear discrimination of the two transmission peaks is proportional to $\sqrt{N}$ and it's well below the damping of antiferromagnetic insulators. Therefore the strong magnon-photon coupling can be realized in antiferromagnets and the coherent information transfer between the photons and magnons is possible.