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Link visio: https://youtube.com/live/hAzkwqulPq8?feature=share
The defence will be in English.
Abstract: 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 the readout photon number and is known to cause Purcell effect. This work proposes to overcome these limitations by engineering a nonlinear coupling between the transmon qubit and a dedicated readout mode. This readout scheme is implemented by 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 Quantum Non-Demolition (QND) fidelity of 96.6%. Interestingly, these results have been achieved with 90 photons in the readout mode, which is an order of magnitude higher compared to most high-fidelity readout experiments. This underlines the promising QND robustness of the transmon molecule readout scheme in the presence of a large number of readout photons. To understand this robustness, we have studied measurement-induced state transitions (MIST) as function of the readout power. Indeed, MIST are known to cause leakage outside of the computational state which is one of the main sources of readout error at high power. For this purpose, we have implemented a multistate single-shot readout which is able to probe up to the fifth excited state of the transmon qubit in presence of readout photons and to identify when the qubit leaves the computational subspace. The measurements indicate that the MIST have a minimal effect on the qubit up to about 350 readout photons, opening the way to higher readout and QND fidelity. These experimental results are corroborated by a theoretical study through branch analyses, which accurately predict the behavior of MIST and their suppression thanks to the properties of the transmon molecule readout scheme.
