Quasiparticle effects in magnetic-field-resilient 3D transmons (Part 1 - Experiment)

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-AlO_x-Al junctions [1]. With magnetic field resilient transmons [2] we investigate this from a new angle. We present spectroscopy and parity lifetime (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 f₀₁, 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.

In part one of two, we present the setup, the magnetic field dependent spectrum, the parity measurement schemes and other measurements relevant for the modeling (Part 2).

[1] G. Marchegiani et al., PRX Quantum (2022)

[2] J Krause et al., PR Appl. (2022)

[3] Willsch et al. arXiv:2302.0919