Planar Fluxonium Qubits Design with 4-way Coupling

Recent advancements in material improvement and design optimization have enabled fluxonium superconducting qubits to demonstrate millisecond-scale coherence times and 99.9% fidelity in two-qubit gates. However, unlike its counterpart the transmon qubit, a clear path to scalability has yet to be established for building an extensible architecture with fluxonium qubits. Transmon qubits leverage a shunting capacitor engineered into specific geometries to facilitate coupling between readout resonators and neighboring qubits. For fluxonium qubits, inductive coupling is a natural choice due to their large phase matrix element. However, achieving nanohenry-scale mutual coupling is generally accomplished by galvanically sharing junctions between two fluxonia, which leads to DC loops and poses challenges when scaling to more than two fluxonia. Here, we propose a novel planar fluxonium architecture that uses the inductor loop to achieve coupling in four directions, while optimizing the capacitor design to minimize the physical footprint and unwanted capacitive coupling. Finally, we explore the feasibility of nanohenry-scale wireless inductive coupling to overcome the limitations of galvanic connections. This architecture aims to address the scalability challenges of fluxonium qubits while maintaining their superior coherence properties and gate fidelities.