Measurement Induced State Transitions and Quantum Non-Demolition Readout in Fluxonium Qubits

In circuit quantum electrodynamics, the projective nature of measurements relies on the validity of the dispersive approximation between a qubit and its resonator. In practice, this approximation frequently breaks down, inducing unwanted transitions that compromise the quantum non-demolition (QND) nature of the measurement [1,2,3]. Depending on parameter regime, we find that these arise even at low powers in strongly anharmonic systems such as the fluxonium.

In this work, we investigate multi-photon absorption to non-computational states intrinsic to the fluxonium Hamiltonian, finding good agreement between Floquet-based simulations and experimental data. We further examine other proposed transition mechanisms, including fluxonium array modes [4] and inelastic scattering processes [5], and assess their impact on measurement. These results provide new insights into the limits of dispersive readout in fluxonium qubits and point to design strategies that enable robust QND measurement.

[1] D. Sank, et al., PRL, 2016

[2] M. Khezri, et al., PRA, 2023

[3] M. F. Dumas, et al., PRX, 2024

[4] S. Singh, et al., PRX Quantum, 2025

[5] T. Connolly, et al., arXiv:2506.05306, 2025