Dresden 2026 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 22: Superconductivity – Poster I

TT 22.14: Poster

Monday, March 9, 2026, 18:00–20:00, P1

Superconducting quarton qubits with stacked Josephson junctions — •Alex Kreuzer¹, Hossam Tohamy¹, Thilo Krumrey¹, Jürgen Lisenfeld¹, 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)

Realizing compact, low-loss inductive elements is a key challenge for scalable superconducting qubits. Although planar Josephson junction arrays offer large inductances, they also have a substantial footprint, which causes additional surface loss and increases the stray capacitance. We implement quarton flux qubits with a large positive anharmonicity using vertically stacked Josephson junctions, which offer compact inductance, low stray capacitance, and reduced surface participation.

We present DC transport characteristics of stacked junctions and microwave-loss measurements of quarton qubits. Qubit spectroscopy reveals numerous avoided crossings induced by two-level systems (TLS), suggesting the TLS in the array junctions as the dominant loss channel. We discuss how qubit coherence and the TLS density are influenced by stack geometry.

Keywords: impedance; Josephson junction arrays; flux qubit; superconducting qubits; fluxonium