Merged-Element Transmons: Design and Qubit Performance

We demonstrate a superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon potentially offers a smaller footprint than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces the relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations, we produce functional devices with EJ/EC in the low-transmon regime (EJ/EC≲30). Cryogenic I-V measurements show a sharp dI/dV structure with low subgap conduction. Qubit spectroscopy of tunable versions shows a small number of avoided level crossings, suggesting the presence of two-level systems. We observe mean T1 times typically in the range of 10-90μs, with some annealed devices exhibiting T1>100μs over several hours. The results suggest that energy relaxation in conventional small-junction transmons is not limited by junction loss.