High power readout and suppression of measurement-induced state transitions in cosϕ-coupling transmon readout
Olivier Buissan, NEEL Institute
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 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 200 [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].
[1] R. Dassonneville, et al., “Fast high-fidelity quantum nondemolition 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 cos-coupling transmon readout”, arXiv 2509.05126 (2025).
Bio
Dr Olivier Buisson is Directeur de Recherche in NEEL Institute in UGA-CNRS since 2008 and was director of the consortium LANEF in Grenoble. His main research areas concern mesoscopic physics and superconducting qubits. Dr Olivier Buisson obtained his PhD in 1990 working on superconducting mesoscopic disks and superconducting networks. After a Post-doc in Rio de Janeiro, he was contracted in CRTBT-CNRS in Grenoble and first studied plasma modes in thin superconducting films (thin wires, thin films and wire networks). In 1999, he initiated in Grenoble quantum dynamics in superconducting qubits and proposed in 2000 Quantum ElectroDynamics experiments using superconducting qubits through a theoretical study in collaboration with Frank Hekking. Since, he experimentally studied phase qubits, multilevel quantum dynamics, but also superconducting artificial atom with two degrees of freedom. Currently inside the Superconducting Quantum Circuit team in collaboration with Wiebke Guichard, Cécile Naud, Quentin Ficheux and Nicolas Roch, he is studying superconducting qubits and high fidelity transmon readout, propagating modes and quantum phase slip in long Josephson junctions chain, many body physics in a superconducting qubit plateform, Josephson quantum amplifier, and quantum dot in hybrid Al/Ge/Al nanowire heterostructures.
Location
QNC 0101
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