Fluxonium as a control qubit for bosonic quantum information

Abstract: Bosonic codes in superconducting resonators are a hardware-efficient avenue for quantum error correction and benefit from favorable error hierarchies provided by long-lived cavities compared to typical superconducting qubits. The required coupling to an ancillary control qubit, however, can negate these benefits by inducing highly detrimental effects such as excess decoherence and undesired nonlinearities. An important question is thus whether a cavity-qubit coupling can be realized that offers readout and control capabilities without spoiling the cavity. Here, motivated by its long lifetime and design flexibility of its Hamiltonian, we experimentally investigate the fluxonium as a control qubit for superconducting cavities. We couple a fluxonium qubit to a superconducting resonator in the strong-dispersive regime and use it to measure the coherence and inherited nonlinearities of the resonator. We then demonstrate universal control by preparing and characterizing resonator Fock states and their superpositions, with fidelities limited by resonator decay in our planar prototype device. Finally, we use the predictability of the resonator's inherited nonlinearities to show numerically that the fluxonium can reach cavity-coupling regimes that eliminate undesirable cavity nonlinearities. These results demonstrate the potential of the fluxonium as a high-performance bosonic control qubit for superconducting cavities.