Similar to many other quantum technologies, both spin and superconducting quantum computers only interact with light at microwave frequencies. This means the only way we can get quantum information out of these computers is using microwave photons, but these are poorly suited to storage or long-distance transmission due to high loss and noise. Optical photons are much better suited to these roles and so future quantum networks will transmit quantum information encoded onto optical photons

In order to connect quantum computers to these networks, we need a microwave-to-optical frequency converter that is bidirectional, 100% efficient, and has sufficient bandwidth to match the qubits converted. Because the frequency mismatch is so large, it is extremely difficult to find materials with sufficient optical non-linearity to meet these criteria, and current converters have high bandwidth or high efficiency, but not both.

This project aims to build a frequency converter that is both efficient and high-bandwidth by using a highly concentrated erbium crystal with optical and microwave transitions resonant with the two fields. When magnetically ordered, this crystal has strong and extremely narrow transitions leading to a very strong non-linearity.

The final conversion efficiency will crucially depend on the properties of the material, so the early part of the project involves experimental characterisation of the system with optical and microwave spectroscopy in both the ordered and unordered regimes. The project also involves theoretical modelling of the resonances and engineering design of the final device.