Measurement-induced transitions in a transmon qudit: Theory

Dispersive readout of the transmon qubit has been demonstrated to have high QNDness at low photon number. However, increasing the readout power, as is necessary to obtain fast readout, is observed to reduce the QNDness because of transitions of the transmon to high-energy states [1,2,3]. A recent theoretical framework explains this behavior, predicting which states will be populated by the measurement [4,5]. In part 2 of this talk, we theoretically analyze the transitions in a transmon where up to 10 levels can be dispersively measured. We show that the theory accurately predicts most of the observed transitions, including the final state and the critical photon number. The remaining transitions are attributed to residual coupling with a nearby spectator transmon, highlighting the importance of crosstalk suppression. We also highlight the importance of accounting for higher harmonics of the Josephson junction potential to obtain accurate predictions. Our work paves the way for the optimization of transmon qudit readout.

[1] D. Sank et al., Phys. Rev. Lett. 117, 190503 (2016).

[2] T. Walter et al., Phys. Rev. Applied 7, 054020 (2017).

[3] M. Khezri et al., Phys. Rev. Applied 20, 054008 (2023).

[4] R. Shillito et al., Phys. Rev. Applied 18, 034031 (2022).

[5] M. F. Dumas et al., Phys. Rev. X 14, 041023 (2024).