A phonon-engineered merged-element transmon qubit: Part I

Improving superconducting qubit coherence often requires reducing dielectric losses associated with large shunt capacitors, either through advanced fabrication or by engineering a lower surface energy participation ratio. In particular, two-level-system (TLS) defects residing in amorphous oxides can relax by emitting phonons, creating a decoherence channel for a qubit when coupled to such defects. In this work, we investigate a phonon-engineered “merged-element transmon” (MET) architecture, where a large-area Al/AlOx/Al Josephson junction (≈ 2 µm²) is embedded in phononic bandgap crystals.

In Part I, we describe the operation of the MET, where a single Josephson junction provides both the nonlinearity and the capacitance required for transmon operation. We discuss design strategies that optimize the device geometry to tune the energy participation ratio between the junction barrier and native oxides on the electrodes, thereby ensuring that the qubit’s performance is governed primarily by the intrinsic quality of the junction. Finally, we present the fabrication and characterization of MET qubits on both silicon and silicon-on-insulator (SOI) substrates, observing comparable energy-relaxation times exceeding 30 µs. These results demonstrate that self-shunted, large-junction transmons can achieve long coherence across different substrates while substantially simplifying circuit layout and fabrication.