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QI: Fachverband Quanteninformation

QI 19: Superconducting Electronics: Qubits I (joint session TT/QI)

QI 19.7: Vortrag

Mittwoch, 20. März 2024, 16:30–16:45, H 0104

Measuring and understanding quasiparticle effects in magnetic-field-resilient 3D transmons (Experiment) — Jonas Krause¹, Christian Dickel¹, Giampiero Marchegiani², •Lucas Janssen¹, Gianluigi Catelani^2,3, and Yoichi Ando¹ — ¹University of Cologne — ²Technology Innovation Institute Abu Dhabi — ³Forschungszentrum Juelich

Recent research shows quasiparticle-induced decoherence of superconducting qubits depends on the superconducting gap asymmetry due to the different thickness of the top and bottom films in Al-AlOx-Al junctions [1]. With magnetic-field-resilient transmons [2] we investigate this from a new angle. We present spectroscopy and parity-switching-time (τ_p) measurements of a 3D transmon up to 400 mT in-plane field. The magnetic field tunes the transmon frequency f₀₁ without a strong reduction in T₂^*. The gap asymmetry, initially close to hf₀₁, causes a non-monotonic evolution of τ_p. After an increase with in-plane field up to 150 mT, τ_p decreases at higher fields. Higher Josephson harmonics are needed to accurately model the spectrum [3]. At low fields, small parity splitting requires qutrit pulse sequences for parity measurements. Magnetic fields are an interesting tuning knob to study quasiparticle loss and gap engineering because they allow changing both the gap and gap difference. Charge-parity measurements are also a readout mechanism for topological qubits which often require high fields.

[1] G. Marchegiani et al., RX Quantum 3 (2022) 040338

[2] J. Krause et al., Phys. Rev. Applied 17 (2022) 034032

[3] D. Willsch et al., arXiv:2302.0919

Keywords: circuit QED; superconducting qubits; quasiparticle loss; gap engineering; charge-parity switching