Measurement of the charge dependance of readout-induced transition in transmons : Theory

Improving qubit readout is essential to reach fault-tolerant quantum computation with the cQED architecture. However, this is made difficult by the degradation of the readout fidelity observed at small to moderate measurement drive amplitude [1,2]. This phenomenon is predicted to be gate-charge dependent even deep in the transmon regime [3,4]. In part 2 of this talk, we show that the theoretical framework developed in Ref. [4] accounts for the experimentally observed gate-charge dependent reduction of fidelity discussed in part 1. We show that a simple semiclassical model based on Floquet theory accurately predicts the charge dependence of ionization. Additionally, we highlight the importance of accurately modeling the higher harmonics of the transmon’s Josephson junction potential [5] to accurately predict the onset of ionization. Our results suggest that a careful modeling of the transmon, such as the inclusion of gate charge and higher harmonics, is crucial to predict and eventually improve readout in cQED.

[1] T. Walter et al., Phys. Rev. Appl. 7 (2017)

[2] M. Khezri et al., Phys. Rev. Appl. 20 (2023)

[3] J. Cohen et al., PRX Quantum 4 (2023)

[4] M.F. Dumas et al., Phys. Rev. X 14 (2024)

[5] D. Willsch et al., Nature Physics 20 (2024)