Implementation of Protected Qubits with π-periodic Josephson Elements*

The coherence times of superconducting qubits have undergone tremendous growth over the past two decades through a combination of materials science efforts and improvements in device design and measurement. Moving significantly beyond the current state-of-the-art coherence achievable with transmon qubits will require new device designs. Circuits that have a natural topological protection against decoherence due to local noise are particularly attractive. The 0-π qubit was an early example of such a protected design, with a combination of Josephson junctions and superinductors resulting in an effective π-periodic Josephson element. However, the challenging requirements for extremely large inductances and tight tolerances on parameter uniformity have made the original 0-π design difficult to implement. More recently, the charge-parity qubit design, composed of chains of concatenated imperfect π-periodic Josephson elements, offers a more feasible approach to achieving protection. In this case, the level of protection against local noise increases exponentially with the number of concatenated π-periodic elements. I will present progress with the implementation and characterization of these devices and the pathway to high-fidelity protected gate operations.

In collaboration with: Kenneth Dodge, Yebin Liu, Brad Cole, Jaseung Ku, Michael Senatore (Syracuse University), Abigail Shearrow, Shaojiang Zhu, Sohair Abdullah, Andrey Klots, Lara Faoro, Robert McDermott (University of Wisconsin, Madison), Lev B Ioffe (Google).