Flux-Port Isolation in Radio-Frequency Quantum Upconverters Using Gradiometric Josephson Design and On-Chip Filtering

The superconducting radio-frequency quantum upconverter (RQU) uses a three-junction interferometer to upconvert low-frequency flux 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. To mitigate these effects, I implement a multilayer gradiometric Josephson design that enhances differential flux coupling, rejects common-mode flux noise, and stabilizes operation near the optimal bias point. In parallel, on-chip low-pass filters isolate the low-frequency bias ports from the microwave circuit, suppressing high-frequency leakage. Sonnet simulations show that these combined strategies preserve resonator coherence and quality factor while eliminating flux-port–induced loss and hybridization. These improvements are essential for achieving quantum-limited, backaction-evading measurements in future RQU architectures.