Impact of transmon ionization in neighbouring qubits

Dispersive readout in circuit QED is known to loose its quantum non-demolition character at small to moderate measurement drive amplitudes [1,2]. Recent work provides a theoretical explanation for the degradation of the dispersive readout fidelity with increasing measurement drive amplitude [3]. This theoretical framework has been shown to be in agreement with experiments for the dispersive readout of a single transmon. The impact of ionization for readout of a transmon embedded on a multi-qubit chip, however, remains unexplored. We present a general method to characterize measurement-induced ionization when the qubit under readout is weakly coupled to other superconducting circuit elements, a ubiquitous situation in circuit QED. As an example, we focus on the case of 2 transmon qubits, and we show that (i) the spectator qubit can be impacted by the ionization of the readout qubit, and (ii) the presence of the spectator qubit can lower the ionization threshold of the readout qubit. Finally, we identify parameters sets reducing measurement-induced transitions in coupled transmon systems.

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

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

[3] MF. Dumas et al., arXiv:2402.06615v1 (2024)