Characterization of topologically protected charge-parity qubits: Part 2*

Decoherence poses a major impediment to the implementation of large-scale quantum processing with superconducting qubits. There have been tremendous improvements in superconducting qubit coherence times over the past two decades, with current state-of-the-art coherence levels approaching the threshold for fault-tolerant quantum computing. Nevertheless, for scalability, high-fidelity qubit control and qubits protected from sources of noise at the hardware level are needed. We implement qubit protection with pi-periodic superconducting elements made from flux-biased plaquettes of kinetic inductors and Josephson junctions that allow only double Cooper pair tunneling. The amount of protection from noise in such plaquette-based qubits can be further increased by concatenating plaquettes. We describe the design and characterization of a plaquette-based qubit with an embedded SQUID switch.