Efficient bidirectional microwave optical photon conversion is a key capability for scaling superconducting quantum processors into distributed networks. However, achieving the necessaryconversion efficiency requires filling a critical knowledge gap in understanding the loss mechanisms of electro optic materials. Here, we characterize the microwave properties of single crystal bulk LiNbO3 and AlN over a broad range of powers, down to single photon levels, and spanning from millikelvin temperatures to above 1K. We demonstrate that both materials exhibit two level systems (TLS) behavior, while piezoelectric related losses are excluded. We show that TLS induced dissipation is predominantly localized on the surface rather than being an intrinsic bulk property, a result further corroborated by room temperature 3D XPS and time of flight SIMS analyses. These findings provide useful insights to engineer hybrid architectures that integrate bulk electro optic crystals within superconducting cavities, proving that microwave quality factors compatible with high efficiency microwave optical transduction are within reach.
Superconducting radio-frequency (SRF) cavities offer a promising platform for quantum computing due to their long coherence times and large accessible Hilbert spaces, yet integratingnonlinear elements like transmons for control often introduces additional loss. We report a multimode quantum system based on a 2-cell elliptical shaped SRF cavity, comprising two cavity modes weakly coupled to an ancillary transmon circuit, designed to preserve coherence while enabling efficient control of the cavity modes. We mitigate the detrimental effects of the transmon decoherence through careful design optimization that reduces transmon-cavity couplings and participation in the dielectric substrate and lossy interfaces, to achieve single-photon lifetimes of 20.6 ms and 15.6 ms for the two modes, and a pure dephasing time exceeding 40 ms. This marks an order-of-magnitude improvement over prior 3D multimode memories. Leveraging sideband interactions and novel error-resilient protocols, including measurement-based correction and post-selection, we achieve high-fidelity control over quantum states. This enables the preparation of Fock states up to N=20 with fidelities exceeding 95%, the highest reported to date to the authors‘ knowledge, as well as two-mode entanglement with coherence-limited fidelities reaching up to 99.9% after post-selection. These results establish our platform as a robust foundation for quantum information processing, allowing for future extensions to high-dimensional qudit encodings.
We propose a novel quantum transduction hybrid system based on the coupling of long-coherence time superconducting cavities with electro-optic resonators to achieve high-efficiencyand high-fidelity in quantum communication protocols and quantum sensing.