Quasiparticle effects in magnetic-field-resilient 3D transmons (Part 2 - Theory)

In part two of two, we present the modeling of the charge-parity lifetime (τ_p) of a magnetic-field resilient 3D transmon [1].

We measure a non-monotonic evolution of τ_p with in-plane magnetic field, increasing up to 150 mT, followed by a decrease at higher fields (see Part 1). We understand this exotic behavior with a generalized approach to quasiparticle decoherence. The model accounts for the transmon being a SQUID measured mainly at the bottom sweetspot 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 resonant quasiparticle tunneling gives a sizable contribution to the parity-switching rate τ_p^-1. With a higher magnetic field, f₀₁ decreases and gets detuned from the gap difference, causing the initial increase of τ_p, while photon-assisted qubit transitions increase producing the subsequent decay at higher fields. At the highest field (300-400 mT) τ_p decays faster than the model predicts. We show that τ_p , the qubit lifetime T₁, as well as residual transmon excitation due to quasiparticles can all be consistently described.

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

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