Measurement-induced transitions in a transmon qudit: Experiments

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 [1-3]. A recent theoretical framework explains this behavior, predicting which states will be populated by the measurement [4, 5]. In part 1 of this talk, we present our experimental investigation of measurement-induced state transitions in a deep transmon with a large EJ/EC ratio around 300 [6, 7]. By resolving in a dispersive measurement transmons levels up to 10, we are able to identify which transmon states are populated during the measurement, the threshold measurement photon number at which we begin to see these transitions, and how much populations will be affected. Moreover, by preparing the transmon in different initial states, we observe transitions at different photon numbers. In part 2 of this talk, we compare these experimental results to theory.

[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).

[6] Z. Wang et al., arXiv:2407.17407.

[7] E. Champion et al., arXiv:2405.15857.