Toward implementation of protected charge-parity qubits

Topologically protected qubits are an area of growing interest and active research, with 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 Φ₀/2, 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, protection scales exponentially with the number of plaquettes concatenated in series. In this talk we describe the design, fabrication, and characterization of a three-plaquette device.