Optimized dispersion engineering of coplanar-waveguide-based Josephson traveling-wave parametric amplifiers

Readout multiplexing of superconducting qubits is a key approach to limit hardware overhead when scaling to larger quantum processors. A requirement of readout multiplexing is that downstream parametric amplifiers have sufficient bandwidth and dynamic range to handle frequency-multiplexed readout signals, thus motivating the adoption of wideband designs such as those achieved with Josephson traveling-wave parametric amplifiers (JTWPAs). Here we describe JTWPAs co-optimized to support the Hummingbird processor made available by IBM Quantum. In particular, we focus on optimizing the JTWPA dispersion engineering using coplanar waveguide resonators. While such resonators offer relaxed fabrication requirements in comparison to lumped element resonators, we discuss how higher frequency modes of coplanar waveguide resonators can lead to unintended couplings between modes that must be managed in the overall frequency allocation of the multiplexed readout. We furthermore explore device designs aimed at mitigating these spurious couplings using additional high-frequency dispersion resonators.