d-mon and the physics of d/s Superconducting Josephson Junctions

Some of the major obstacles in today's quantum computing landscape are necessarily hardware bottlenecks. Superconducting qubits have shown substantial promise in building robust quantum hardware that is insensitive to noise. The leading superconducting qubit architecture of the past decade has been the Transmon qubit, now made popular by commercial quantum computers from IBM, Google and Rigetti. Transmon uses Josephson tunneling of Cooper pairs between two s-wave superconductors to create a spectrum which can be tuned to hold qubit states. However the spectrum of Transmon is weakly anharmonic, implying that qubit control has to be slow to prevent leaking away from the computational subspace. We show that replacing the s/s junction with a d/s junction created using planar c-axis Josephson tunneling between a thin flake d-wave superconductor, such as BSCCO and a conventional s-wave superconductor produces a strongly anharmonic spectrum that is widely tunable through the device geometry and applied magnetic flux. We also show that the proposed device has fully gapped quasiparticles and can therefore be expected to achieve long coherence times.