Flux-port Isolation in Radio-frequency Quantum Upconverters

The superconducting radio-frequency quantum upconverter (RQU) leverages the flux-dependent inductance of a three-junction interferometer to upconvert low-frequency signals (<300 MHz) to microwave frequencies, enabling sensitive quantum measurement protocols such as backaction evasion. The RQU design incorporates a microwave resonator terminated by a Josephson junction interferometer, which is biased via inductively coupled flux loops. However, these flux bias ports can introduce loss and decoherence through parasitic coupling between the low-frequency flux bias and high-frequency flux signal lines, as well as coupling to parasitic resonance modes. I present designs to isolate the low-frequency bias lines from the flux signal lines and the high-frequency RF circuit. The analysis includes simulation of RQUs in Sonnet with on-chip isolation and filter circuits, showing effective mitigation of losses to preserve the resonator's coherence and quality factor, and elimination of mode hybridization. I also compare to experimental data. Eliminating bias flux port loss and hybridization is essential to enable backaction-evading measurements that require long microwave coherence times.