Design and Characterization of the Heavy Double Cooper-Pair Tunneling Element

Symmetry plays an important role in various physical systems, giving rise to parity, selection rules, and interference effects. Notably, single-Cooper-pair tunneling across a symmetric Josephson rhombus biased at its frustration point is forbidden, allowing the exploration of multi-Cooper-pair tunneling processes. Here, we report the implementation of an inductively shunted double-Cooper-pair junction in the heavy regime, with circuit parameters tailored to enhance the novel effect. Specifically, we discuss fabrication optimization, control and readout strategy, and decoherence mechanisms of the circuit, establishing the framework for building other types of superconducting circuits with similar topologies. The circuit can also be broadly adapted for potential applications in nano-fabrication testing, quantum metrology, and quantum sensing.