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Fluxonium as a Control Qubit for Bosonic Quantum Information

Ke Nie1, J. Nofear Bradford1, Supriya Mandal1, Aayam Bista1, Wolfgang Pfaff1,2,3,4,*, and Angela Kou1,2,3

  • *Contact author: wpfaff@illinois.edu
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PRX Quantum 7, 010357 – Published 20 March, 2026

DOI: https://doi.org/10.1103/dgwk-w3jj

Abstract

Bosonic codes in superconducting resonators are a hardware-efficient avenue for quantum error correction and benefit from the inherent bias toward relaxation errors 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. This is exemplified by the typically employed transmon qubit that imposes limits on cavity operations due to its finite lifetime as well as the inevitably induced self-Kerr nonlinearity. 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 flux-tunable inherited nonlinearities of the resonator. We then demonstrate quantum control of the oscillator by preparing and characterizing resonator Fock states and their superpositions, with fidelities limited by resonator decay in our planar prototype device. Finally, we show that the fluxonium can realize cavity-couplings that eliminate undesirable cavity nonlinearities and reach regimes of suppressed self-Kerr that are inaccessible to transmon-based devices. Our results thus demonstrate the potential of the fluxonium as a high-performance bosonic control qubit for superconducting cavities that can outperform existing transmon-based architectures.

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Popular Summary

Quantum bits (qubits) are the fundamental building blocks of quantum computers. Their fragility is what makes the realization of useful quantum computers difficult, and finding ways to encode quantum information in more robust ways is therefore a crucial, ongoing research theme. One promising route toward a beneficial encoding is to use microwave photons stored in superconducting cavities—it turns out that in this way fewer errors might occur. The catch is that to manipulate quantum information in a cavity, the cavity needs to be coupled to a qubit and can inherit errors from it. The question is thus, can this coupling be made in way to minimize errors, but still permit the manipulation of quantum states? Our work demonstrates experimentally that the so-called fluxonium qubit can interact with cavities in a way that can suppress inherited properties that give rise to errors. This qubit can control the quantum state of the cavity photons much in the same way as the current state-of-the-art solution—the so-called transmon qubit—but additionally allows fine-tuning the details of the cavity-qubit interaction. Our work shows a detailed understanding of this interaction in a prototype device that allows both quantum control of microwave photons as well as tunable interactions. From the experiment we can predict that it should be possible to build better-performing qubit-cavity systems than are currently available, an important step toward quantum computation with low error rates.

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PRX Quantum 7, 010357– Published 20 March, 2026
  • Received 10 June 2025
  • Revised 30 December 2025
  • Accepted 17 February 2026
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