d-Mon: A Transmon with Strong Anharmonicity Based on Planar c-Axis Tunneling Junction between d-Wave and s-Wave Superconductors

We propose a novel qubit architecture based on a planar c -axis Josephson junction between a thin flake d -wave superconductor, such as a high-Tc cuprate Bi2Sr2CaCu2O8+x, and a conventional s -wave superconductor. When operated in the transmon regime the device-that we call "d mon"-becomes insensitive to offset charge fluctuations and, importantly, exhibits at the same time energy level spectrum with strong anharmonicity that is widely tunable through the device geometry and applied magnetic flux. Crucially, unlike previous qubit designs based on d -wave superconductors the proposed device operates in a regime where quasiparticles are fully gapped and can be therefore expected to achieve long coherence times.