$d$-mon: transmon with strong anharmonicity

TL;DR

The paper introduces a new transmon qubit design called '$d$-mon' using a $d$-wave superconductor junction, offering strong tunable anharmonicity and potential for long coherence times due to a fully gapped quasiparticle regime.

Contribution

It proposes a novel $d$-wave superconductor-based transmon qubit architecture with tunable anharmonicity and improved coherence prospects.

Findings

Device exhibits strong, tunable anharmonicity.

Operates in a fully gapped quasiparticle regime.

Insensitive to offset charge fluctuations.

Abstract

We propose a novel qubit architecture based on a planar $c$-axis Josephson junction between a thin flake $d$-wave superconductor ($d$SC), such as a high-$T_c$ cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$, 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.