Toward implementation of protected charge-parity qubits

Topologically protected qubits hold the potential for orders-of-magnitude improvements in coherence compared to conventional qubits. One such protected qubit is the charge-parity (C-parity) qubit, which consists of a pi-periodic Josephson element shunted by a large capacitance. Here, we implement imperfect pi-elements as "plaquettes" consisting of two arms in parallel, each arm incorporating a small-area Josephson junction in series with a large inductor. When the plaquette is biased at half a flux quantum, the first harmonic of the Josephson energy is suppressed and the second harmonic, which is proportional to cos(2φ), remains. While individual plaquettes are not protected against dephasing from flux noise in the loop, protection scales exponentially with the number of plaquettes concatenated in series. In this talk, we describe the design, fabrication, and characterization of multi-plaquette devices that aspire to protection.