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TT: Fachverband Tiefe Temperaturen

TT 47: Superconducting Electronics: Qubits I (joint session TT/QI)

TT 47.2: Talk

Wednesday, March 20, 2024, 15:15–15:30, H 0104

Superconducting flux qubits with stacked Josephson junctions — •Alex Kreuzer¹, Hossam Tohamy¹, Thilo Krumrey¹, Alexandru Ionita¹, Hannes Rotzinger^{1, 2}, and Alexey V. Ustinov^{1, 2} — ¹Physikalisches Institut (PHI), Karlsruher Institut für Technologie (KIT) — ²Institut für Quantenmaterialien und -technologien (IQMT), Karlsruher Institut für Technologie (KIT)

Josephson junctions are commonly employed as nonlinear inductive components in superconducting qubits, allowing to tailor specific circuit properties. The promising flux qubit types like fluxonium or quarton qubits require compact inductances, often implemented as arrays of Josephson junctions. Challenges arise due to stray capacitance, originating from the capacitive coupling of an array island to the ground, leading to parasitic resonances at GHz frequencies that can degrade or compromise qubit performance. To address this limitation, we investigate an alternative approach: implementing qubit inductances by stacking Josephson junctions vertically. Junction stacks help to minimize the parasitic capacitance of their electrodes to the ground. We present transport characteristics of the stacks as well as microwave loss measurement data using a quarton-type flux qubit with stacked Josephson junctions. The experimental data are compared to results of numerical simulations.

Keywords: flux qubit; Quarton qubit; Josephson junction array; stacked Josephson junctions; stray capacitance