Regensburg 2025 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 55: Superconducting Electronics: SQUIDs, Qubits, Circuit QED III

TT 55.10: Talk

Friday, March 21, 2025, 12:00–12:15, H36

Investigation of parasitic two-level systems in merged-element Transmon qubits — •Etienne Daum¹, Benedikt Berlitz¹, Alexey V. Ustinov^1,2, and Jürgen Lisenfeld¹ — ¹PHI, Karlsruher Institut für Technologie, 76131 Karlsruhe, Deutschland — ²IQMT, Karlsruher Institut für Technologie, 76131 Karlsruhe, Deutschland

In conventional transmon qubits, decoherence is dominated by large numbers of parasitic two-level systems (TLS) residing at the edges of its large area coplanar shunt capacitor. Avoiding these defects by improvements in design, fabrication and materials proved to be a significant challenge that so far led to limited progress. The merged-element transmon qubit, a recently proposed paradigm shift in transmon design, attempts to address these issues by engineering the Josephson junction to act as its own parallel shunt capacitor. Incorporating an additional aluminium deposition and oxidation into the in-situ bandaged Niemeyer-Dolan technique, we were able to fabricate flux-tunable mergemon qubits in the low transmon regime (E_J/E_C ≈ 34). A mean T₁ relaxation time of about 20µs (Q ≈ 5.4 × 10⁵) has been observed over a six hour time period. TLS spectroscopy under applied strain and electric fields revealed that the majority of coherence limiting TLS (∼ 60%) still reside on the interfaces of exposed qubit electrodes, despite their drastically reduced surface area. This indicates that material and fabrication improvements, in combination with optimized electrode geometries, are still necessary before the advantages of the "mergemon" approach can be exploited.

Keywords: Two Level System; Merged-Element Transmon; Qubit; Quantum Computing