Theory of measurement-induced transitions in the transmon qubit, part 1.

Dispersive readout in circuit QED enables fast and high-fidelity measurements that are essential for quantum computation. However, it is a common experimental observation that increasing the measurement drive amplitude to even moderate values leads to quantum non-demolition and low-fidelity readout. Recent work suggests that this is due to ionization of the qubit to higher-energy excited states by the drive [1,2] and that this can be understood as a signature of classical chaos in the driven transmon [3]. In this work, we present a unified theoretical picture of measurement-induced transitions in the transmon qubit. We predict the critical photon number at which deleterious transitions occur, in excellent agreement with experimental results. We also use our theoretical framework to propose avenues for mitigating measurement-induced transitions.

In part 1 of this talk, we compare the transmon-resonator energy spectrum with a semiclassical model of a classically driven transmon. We identify measurement-induced transitions as arising from anticrossings in the Floquet quasi-energy spectrum and show that the anticrossings accurately predict the onset of non-QNDness. This framework also gives insight on the sensitivity of readout to gate charge fluctuations and qubit-resonator detuning.

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

[2] M. Khezri et al., arXiv:2212.05097v1 [quant-ph]

[3] J. Cohen et al., PRX Quantum 4, 020312 (2023).