Bidirectional electro-optic conversion reaching 1% efficiency with thin-film lithium niobate.

Superconducting cavity electro-optics (EO) presents a promising route to coherently convert microwave and optical photons and distribute quantum entanglement between superconducting circuits over long-distance through an optical network. High EO conversion efficiency demands transduction materials with strong Pockels effect and excellent optical transparency. Thin-film Lithium Niobate (TFLN) offers these desired characteristics however so far has only delivered unidirectional conversion with efficiencies on the order of $10^{-5}$, largely impacted by its prominent photorefractive (PR) effect at cryogenic temperatures. Here we show that, by mitigating the PR effect and associated charge-screening effect, the device's conversion efficiency can be enhanced by orders of magnitude while maintaining stable cryogenic operation, thus allowing a demonstration of conversion bidirectionality and accurate quantification of on-chip efficiency. With the optimized monolithic integrated superconducting EO device based on TFLN-on-sapphire substrate, an on-chip conversion efficiency of 1.02% (internal efficiency, 15.2%) is realized. Our demonstration indicates that with further device improvement, it is feasible for TFLN to approach unitary internal conversion efficiency.