arXiv:2607.07798

Observation of coherent flux-charge interaction in a gate-tunable fluxonium

Brian D. Isakov, Shikhar Singh, Adrian Parra-Rodriguez, David Feldstein-Bofill, Zhenhai Sun, Anders Kringhøj, Svend Krøjer, Alexandre Blais, Morten Kjaergaard, András Gyenis

quant-phcond-mat.mes-hallSubmitted Jul 8, 2026• Announced Jul 10, 2026• 0 citations

arXiv HTML PDF

Abstract

Interactions that mix conjugate variables, such as the flux through a circuit element and the charge across it, lie outside the reach of the elementary couplings of superconducting circuits. Capacitors connect charge to charge, and inductors connect flux to flux, while no two-terminal element couples flux to charge directly. A native flux-charge coupling would thus serve as a circuit primitive in its own right, opening direct routes to non-reciprocity, protected modes, and unconventional readout. In this work, we demonstrate a flux-charge coupling by harnessing a voltage-tunable Josephson junction with parametrically modulated critical current, which mediates the interaction between a classical charge variable and a quantum flux operator. Relying on parity-selection rules in a hybrid superconducting-semiconductor fluxonium, we isolate the flux-charge coupling from other parasitic capacitive contributions and perform cross-quadrature-activated coherent control of states. Critically, we realize a flux-charge coupling that scales linearly with driving amplitude while keeping the transition energy first-order-insensitive to gate voltage. Such unconventional interaction broadens the toolbox of superconducting circuits with a critical missing component that enables the coherent coupling of conjugate variables.

Related papers

A Tunable, Modeless, and Hybridization-free Cross-Kerr Coupler for Miniaturized Superconducting Qubits

Gihwan Kim, Andreas Butler, Oskar Painter

Superconducting quantum circuits typically use capacitive charge-based linear coupling schemes to control interactions between elements such as qubits. While simple and effective, this coupling scheme makes it difficult to satisfy competing circuit design requirements such as maintaining large qubit anharmonicity and coherence along with a high degree of qubit connectivity and packing density. Moreover, tunable interactions using linear coupling elements produce dynamical variations in mode hybridization, which can induce non-adiabatic transitions, resulting in leakage errors and limiting gate speeds. In this work we attempt to address these challenges by proposing a junction-based coupling architecture based on SQUID (superconducting quantum interference device) couplers with relatively small Josephson energies. SQUID couplers provide intrinsic cross-Kerr interactions that can be controlled by external fluxes and that do not rely on mode hybridization. The small Josephson energies of the coupler maintain the interaction at a perturbative scale, which limits undesired higher-order mixing between coupled elements while achieving a sufficiently strong cross-Kerr interaction originating from diagonal coupling elements. Based on these properties, we show that a SQUID coupler can be used to implement a fast, adiabatic, and high-fidelity controlled-Z gate without introducing extra modes, and the operation is robust against junction asymmetry for high-frequency qubits. Although unconventional crosstalk may arise due to junction asymmetries and parasitic hybridization with spectator qubits, we show that these effects are sufficiently small for realistic circuit parameters. As an example of the utility of such junction-based coupling schemes, we present a scalable tiling strategy for a miniaturized superconducting quantum processor based on merged-element transmon qubits.

quant-pharXiv:2602.03186PRX QuantumFeb 4, 20260 citations

Tunable coupling scheme for implementing two-qubit gates on fluxonium qubits

Ilya N. Moskalenko, Ilya S. Besedin, Ilya A. Simakov +1 more

A superconducting fluxonium circuit is an RF-superconducting quantum interference device-type flux qubit that uses a large inductance built from an array of Josephson junctions or a high kinetic inductance material. This inductance suppresses charge sensitivity exponentially and flux sensitivity quadratically. In contrast to the transmon qubit, the anharmonicity of fluxonium can be large and positive, allowing for better separation between the low energy qubit manifold of the circuit and higher-lying excited states. Here, we propose a tunable coupling scheme for implementing two-qubit gates on fixed-frequency fluxonium qubits, biased at half flux quantum. In this system, both qubits and couplers are coupled capacitively and implemented as fluxonium circuits with an additional harmonic mode. We investigate the performance of the scheme by simulating a universal two-qubit fSim gate. In the proposed approach, we rely on a planar on-chip architecture for the whole device. Our design is compatible with existing hardware for transmon-based devices with the additional advantage of lower qubit frequency facilitating high-precision gating.

arXiv:2107.11550Applied Physics LettersNov 8, 202135 citations

Gate- and flux-tunable sin(2φ) Josephson element with planar-Ge junctions

Axel Leblanc, Chotivut Tangchingchai, Zahra Sadre Momtaz +11 more

Abstract Hybrid superconductor-semiconductor Josephson field-effect transistors (JoFETs) function as Josephson junctions with a gate-tunable critical current. Additionally, they can feature a non- sinusoidal current-phase relation (CPR) containing multiple harmonics of the superconducting phase difference, a so-far underutilized property. In this work, we exploit this multi-harmonicity to create a Josephson circuit element with an almost perfectly π-periodic CPR, indicative of a largely dominant charge-4e supercurrent transport. Such a Josephson element was recently proposed as the basic building block of a protected superconducting qubit. Here, it is realized using a superconducting quantum interference device (SQUID) with low-inductance aluminum arms and two nominally identical JoFETs. The latter are fabricated from a SiGe/Ge/SiGe quantum-well heterostructure embedding a high-mobility two-dimensional hole gas. By carefully adjusting the JoFET gate voltages and finely tuning the magnetic flux through the SQUID close to half a flux quantum, we achieve a regime where the sin(2φ) component accounts for more than 95 % of the total supercurrent. This result demonstrates a new promising route for the realization of superconducting qubits with enhanced coherence properties.

arXiv:2405.14695Nature CommunicationsJan 24, 202516 citations

Native Approach to Controlled- Z Gates in Inductively Coupled Fluxonium Qubits

Xizheng Ma, Gengyan Zhang, Feng Wu +27 more

The fluxonium qubits have emerged as a promising platform for gate-based quantum information processing. However, their extraordinary protection against charge fluctuations comes at a cost: when coupled capacitively, the qubit-qubit interactions are restricted to XX interactions. Consequently, effective ZZ or XZ interactions are only constructed either by temporarily populating higher-energy states, or by exploiting perturbative effects under microwave driving. Instead, we propose and demonstrate an inductive coupling scheme, which offers a wide selection of native qubit-qubit interactions for fluxonium. In particular, we leverage a built-in, flux-controlled ZZ interaction to perform qubit entanglement. To combat the increased flux-noise-induced dephasing away from the flux-insensitive position, we use a continuous version of the dynamical decoupling scheme to perform noise filtering. Combining these, we demonstrate a 20 ns controlled-z gate with a mean fidelity of 99.53%. More than confirming the efficacy of our gate scheme, this high-fidelity result also reveals a promising but rarely explored parameter space uniquely suitable for gate operations between fluxonium qubits.

arXiv:2308.16040Physical Review LettersFeb 8, 202423 citations

Quasiparticle-induced transitions in a fluxonium qubit

Maksim Litskevich, Kesavan Manivannan, Benjamin Byrd +4 more

Quasiparticles are a prominent decoherence source in superconducting qubits, but their effects are notoriously difficult to isolate in fluxonium. Unlike the transmon, fluxonium is insensitive to offset charge, precluding charge-parity detection of quasiparticle tunneling. We address this challenge by measuring the excitation and de-excitation rates in a fluxonium qubit under controlled on-chip quasiparticle injection and isolating the quasiparticle-induced contribution by subtracting the background transition rates. We show that to accurately model the external magnetic flux dependence of the quasiparticle-induced transition rates, it is necessary to account for the superconducting gap asymmetry across the Josephson junctions. A comparison between theory and experiment constrains the relative quasiparticle densities in the small junction and the junction array and helps explain previously reported discrepancies between the bounds on the quasiparticle densities inferred for these two circuit elements.

quant-pharXiv:2607.21329Jul 24, 20260 citations

Gatemonium: A Voltage-Tunable Fluxonium

William M. Strickland, Bassel Heiba Elfeky, Lukas Baker +6 more

We present a new style of fluxonium qubit, gatemonium, based on an all-superconductor-semiconductor hybrid platform. The linear inductance is achieved using 600 planar Al - In As Josephson junctions (JJs) in series. By tuning the single junction with a gate voltage, we demonstrate electrostatic control of the effective Josephson energy, tuning the weight of the fictitious phase particle. One- and two-tone spectroscopy of the gatemonium transitions further reveal details of the hybrid plasmon-fluxon spectrum. Accounting for the nonsinusoidal current-phase relation of the single junction, we fit the measured spectra to extract charging and inductive energies. We conduct time-domain characterization of the plasmon modes and find that energy-relaxation times are limited by inductive loss, possibly in the thin aluminum film. We discuss future directions for this platform in gate-voltage-tunable high-plasma-frequency enhanced-impedance junction arrays, and enhanced coherence times for voltage-tunable architectures.

arXiv:2406.09002PRX QuantumFeb 10, 202513 citations