An Architecture for High-Fidelity, Robust Two-Qubit Fluxonium Gates with a Transmon Coupler

Qubit lifetimes and weak anharmonicities in superconducting transmon-based quantum computers are leading causes of gate infidelity. The fluxonium qubit is a promising alternative to transmons, with coherence times reaching milliseconds and anharmonicities of several gigahertz. In this work, we present an architecture consisting of fluxonium qubits coupled via a tunable-transmon coupler (FTF, for fluxonium-transmon-fluxonium). FTF provides two very important benefits: (1) it allows for stronger couplings for non-computational state gates, and (2) suppresses the static ZZ down the kHz levels without requiring strict parameter matching. We take advantage of these strong couplings by performing a microwave-activated CZ gate, achieving high-fidelities with clear paths forward toward 99.9%. Furthermore, the frequency at which the gate is driven can be tuned over a large range, corresponding to the coupler spectrum.