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The field of superconducting qubits is constantly evolving with new types of circuit and designs but, when it comes to qubit readout, the use of simple transverse linear coupling is overwhelmingly prevalent. This type of coupling intrinsically limits the readout mode’s dispersive shift and is known to cause Purcell effect. We propose here to overcome these limitations by engineering a non-linear cosϕ-coupling between the transmon qubit and a dedicated readout mode. This is based upon previous published work [1] on qubit readout with a non-perturbative cross-Kerr coupling engineered by a transmon molecule circuit. A new sample with optimized design and parameters shows a readout fidelity of 99.21% measured using a parametric amplifier and a high Quantum Non- Demolition (QND) fidelity of 97% [2]. Interestingly, these results have been achieved with 89 photons in the readout mode. In addition, we have observed suppression of measurement-induced state transitions (MIST) up to high photon counts above 300 [3]. This effect can be explained by the symmetry of the coupling, which is tunable with a magnetic field. All of these measurements were corroborated by a theoretical study, a numerical analysis of the spectra associated with the nonlinearly coupled circuit, and simulations of the corresponding classical dynamics [3]. In conclusion, we show that the cosϕ-coupling is more robust to measurement photons than the usual linear coupling, making it a compelling alternative for high fidelity and non-destructive transmon readout.
[1] R. Dassonneville, et al., “Fast high-fidelity quantum non-demolition qubit readout via a nonperturbative cross-Kerr coupling”, Phys. Rev. X 10, 011045 (2020).
[2] C. Mori, et al., “High-power readout of a transmon qubit using a nonlinear coupling”, arXiv 2507.03642 (2025).
[3] C. Mori, et al., “Suppression of measurement-induced state transitions in cosϕ-coupling transmon readout”, arXiv 2509.05126 (2025).