# Microwave to optical photon conversion via fully concentrated rare-earth ion crystals

## TL;DR

This paper explores microwave to optical photon conversion using fully concentrated rare-earth ion crystals, highlighting their high efficiency and large bandwidth potential due to collective coupling effects.

## Contribution

It introduces a novel approach where rare earth ions are part of the host crystal, enabling high-density, narrow-linewidth systems for quantum information transfer.

## Key findings

- High conversion efficiency demonstrated
- Large bandwidth achievable due to collective coupling
- Bandwidth limited by magnon mode spacing

## Abstract

Most investigations of rare earth ions in solids for quantum information have used rare earth ion doped crystals. Here we analyse the conversion of quantum information from microwave photons to optical frequencies using crystals where the rare earth ions, rather than being dopants, are part of the host crystal. The potential of large ion densities and small linewidths makes such systems very attractive in this application. We show that, as well as high efficiency, large bandwidth conversion is possible. In fact, the collective coupling between the rare earth ions and the optical and microwave cavities is large enough that the limitation on the bandwidth of the devices will instead be the spacing between magnon mode modes in the crystal.