Scaling up to a full-scale multi-qubit processor adds complexity to the control signals input requirements which can be addressed by the use of specially designed cryogenic interface circuits. In this paper, we discuss the use of cryogenic classical electronics for control of a fluxonium artificial atom. In recent years, fluxonium has gained notoriety among the superconducting qubit community for exceptionally long coherence times and large anharmonicity - both highly desirable properties when designing the next generation of qubit systems. The feasibility of controlling a fluxonium based quantum processors with a compact control system ultimately enables a highly scalable system; however, the cryogenic environment poses very strict requirements on electronics. Many trade-offs arise in terms of noise, power consumption, and circuit complexity. Addressing these requires comprehensive simulations that account for the physics of fluxonium qubits and the potential impact of control circuits on their performance. This paper focuses on the models and specification derivation for qubit control and explores constraints in fluxonium-based system design. The models and specifications derived enable the design of optimal control circuits targeted for scalable fluxonium quantum processors.
The path toward practical superconducting quantum processors requires the integration of a large number of high-performance qubits. Modular architectures could offer a way to address the scaling limitations of monolithic designs by partitioning a large quantum processor into physically separated modules, or chiplets, linked through long-range interconnects. In this context, although fluxonium qubits have emerged as a compelling platform for quantum computing due to their long coherence times and high-fidelity gates, existing coupling schemes remain restricted to qubits in close proximity on a single chip. This limitation inherently precludes the long-range interconnects essential for modular integration. In this work, we propose a long-range tunable coupler designed to interconnect fluxonium qubits separated by more than one centimeter, thereby supporting the realization of modular fluxonium quantum processors. Under realistic assumptions, the proposed coupler has the potential to achieve inter-module two-qubit gate performance, specifically sub-100-ns gates with intrinsic errors below
10−4, comparable to that of intra-module (intra-chiplet) gates, while enabling modular integration with low quantum crosstalk, a key requirement for scalable systems. We further discuss the integration of this coupler into modular fluxonium lattices and explore its feasibility for achieving the higher connectivity and longer coupling range required for complex quantum error correction codes. This work could contribute to the development of large-scale fluxonium quantum processors by bridging their demonstrated potential with modular scalability.
Fluxonium qubits combine long coherence times with strong anharmonicity, making them attractive for scalable superconducting quantum processors. Recent experiments have demonstrated high-fidelity two-qubit gates and multi-qubit entanglement in one-dimensional and connectivity-four fluxonium processors, motivating their extension to highly connected two-dimensional (2D) architectures. A central challenge in this extension is to achieve strong coupling to multiple circuit elements while preserving the finite capacitance budget of fluxonium qubits, which constrains the attainable qubit-qubit effective coupling strength and, consequently, two-qubit gate performance. Here, we establish a unified theoretical framework that identifies the connectivity-dependent upper bound on capacitive coupling and quantifies its reduction by distinct parasitic-capacitance channels, applicable across both fluxonium and transmon regimes. We show that pad-to-ground loading is primarily geometry limited and can be substantially suppressed through qubit-pad engineering, whereas inter-pad loading is ultimately constrained by parasitic capacitances of Josephson junctions and Josephson junction arrays. Building on these insights, we formulate practical design principles and numerically demonstrate two-qubit gates as fast as 27 ns and leakage-induced infidelity below
10−3 under representative fabrication variations in 2D fluxonium architectures. These results establish a quantitative framework for understanding and mitigating capacitive loading, suggesting that it does not impose a fundamental performance limit on highly connected fluxonium processors.