We present a nonperturbative tensor-network approach to the excitation spectra of superconducting circuits based on Josephson-junction arrays. These arrays provide the large lumped inductances required for qubit designs, yet their intrinsically many-body nature is typically reduced to effective single-mode descriptions. Perturbative treatments attempt to include the collective array modes neglected in these approximations, but a fully nonperturbative analysis is challenging due to the many-body structure and the collective character of these modes. We overcome this difficulty using the DMRG-X algorithm, which extends tensor-network methods to excited states. Our key advance is a construction of trial states from the linearized mode structure, enabling direct computation of excitations, even in degenerate manifolds, which was previously inaccessible. Our results reveal significant deviations from, and allow us to improve upon, previous perturbative treatments in the regime of low array junction impedance.

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• Circuit quantum electrodynamics
• Optoelectronics
• Phase slips
• Quantum circuits
• Quantum information with solid state qubits
• Superconducting devices
• Superconducting qubits
• Many-body techniques
• Tensor network methods