Suppression of measurement-induced state transitions in cosφ-coupling transmon readout: Part I. Experiment.

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 discussed in Part II.