Longitudinal qubit readout from Jaynes-Cummings coupling and strong drive

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 strength exceeds a critical value [2].

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 of qubit readout and induce a longitudinal readout despite the qubit-cavity coupling being of the Jaynes-Cummings type. Based on exact stochastic numerical simulations of the measurement dynamics [3,4], we show that this readout protocol achieves a high fidelity and QND-ness at moderate driving strength, reaching infidelities of 10^-3 to 10^-4 for state-of-art parameters, only fundamentally limited by finite qubit anharmonicity and decay.

[1] A. Blais, A. L. Grimsmo, S. M. Girvin, and A. Wallraff, “Circuit quantum electrodynamics”, Mod. Rev. Phys. 93, 025005 (2021).

[2] T. Walter et al., "Rapid High-Fidelity Single-Shot Dispersive Readout of Superconducting Qubits", Phys. Rev. Applied 7, 054020 (2017).

[3] L. Pereira, J.J. García-Ripoll, T. Ramos, “Complete physical characterization of QND measurements via tomography”, Phys. Rev. Lett. 129, 010402 (2022).

[4] L. Pereira, J.J. García-Ripoll, T. Ramos, “Parallel QND measurement tomography of multi-qubit quantum devices”, npj Quantum Information 9, 22 (2023).