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In superconducting qubits, the standard measurement strategy is dispersive readout, where the qubit is coupled to an off-resonant cavity via a Jaynes-Cummings coupling [1]. This readout scheme works well for weak readout pulses, but its fidelity diminishes when the readout strength exceeds a critical value [2] due to measurement-induced state transitions [3].
Here, we introduce a new dispersive qubit readout scheme specially designed for strong readout pulses. By applying a specific pulse shape, we can correct non-QND features, inducing an effective longitudinal readout despite the qubit-cavity coupling being of the Jaynes-Cummings type. Based on exact stochastic numerical simulations of the qubit measurement dynamics [4], we demonstrate that the proposed protocol achieves high fidelity and QND-ness at moderate driving strengths, ultimately limited by finite qubit decay. Furthermore, we show numerically that the scheme mitigates non-dispersive errors in state-of-the-art transmon readout, such that the measurement becomes fundamentally limited by leakage to higher levels and beyond-rotating-wave effects.
[1] A. Blais, A. L. Grimsmo, S. M. Girvin, and A. Wallraff, “Circuit quantum electrodynamics”, Mod. Rev. Phys. 93, 025005 (2021). Authors, Journal, Issue (Year) page
[2] T. Walter et al., "Rapid High-Fidelity Single-Shot Dispersive Readout of Superconducting Qubits", Phys. Rev. Applied 7, 054020 (2017).
[3] M. F. Dumas et al., “Measurement-Induced Transmon Ionization”, Phys. Rev. X 14, 041023 (2024).
[4] L. Pereira, J.J. García-Ripoll, T. Ramos, “Complete physical characterization of QND measurements via tomography”, Phys. Rev. Lett. 129, 010402 (2022).