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QI: Fachverband Quanteninformation

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

QI 19.8: Talk

Wednesday, March 20, 2024, 16:45–17:00, H 0104

Measuring and understanding quasiparticle effects in magnetic-field-resilient 3D transmons (Theory) — Jonas Krause¹, Christian Dickel¹, Giampiero Marchegiani², •Luc Janssen¹, Gianluigi Catelani^2,3, and Yoichi Ando¹ — ¹Physics Institute II, University of Cologne, Germany — ²Quantum Research Center, Technology Innovation Institute, UAE — ³JARA Institute for Quantum Information (PGI-11), Forschungszentrum Juelich, Germany

In this talk, we present the modeling of the charge-parity lifetime (τ_p) of a magnetic-field resilient 3D transmon [1]. Experimentally, the lifetime τ_p depends non-monotonically on the in-plane magnetic field. We explain this unexpected behavior within a generalized approach to quasiparticle decoherence. The model accounts for the transmon being a SQUID measured mainly at the bottom sweet spot and for the magnetic field tuning (Fraunhofer effect). It also incorporates effects of temperatures on the order of the transmon frequency. At zero field, the qubit frequency f₀₁ is nearly resonant with the superconducting gap difference [2], so quasiparticle tunneling gives a sizable contribution to the parity-switching rate 1/τ_p. Increasing the in-plane field, f₀₁ decreases and becomes detuned from the gap difference, causing the initial growth in τ_p, while photon-assisted qubit transitions increase producing the subsequent decay at higher fields. We show that τ_p and the qubit lifetime T₁ can be consistently described by the model.

[1] J. Krause et al., Phys. Rev. Appl. 17 (2022) 034032

[2] G. Marchegiani et al., PRX Quantum 3 (2022) 040338

Keywords: quasiparticles; Fraunhofer effect; transmon