  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.
