Differential Unimon Qubit: A Distributed-Element Superconducting Qubit Based on an Odd-Mode Transmission Line Resonator

Next-generation superconducting qubits have been investigated to address the reduced anharmonicity of transmons by effectively harnessing the intrinsic nonlinearity of the Josephson junction. Most superconducting qubits are based on lumped-element circuits that employ a Josephson junction as the nonlinear element. The recently introduced Unimon qubit, the first distributed-element superconducting qubit, was implemented on a half-wavelength, short-ended coplanar-waveguide resonator and exhibits circuit topology and Hamiltonian features similar to those of the fluxonium qubit. Here we propose a differential-type variant of the Unimon by introducing an odd-mode coplanar-stripline (CPS) resonator. The structure consists of a half-wavelength short-ended CPS resonator with a Josephson junction shunting the two striplines. The junction couples exclusively to the odd modes, and the fundamental odd mode remains the lowest-energy qubit mode throughout its full tuning range. This architecture offers enhanced anharmonicity, tunability, and scalability for distributed-element superconducting qubits.