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 $\mathrm{Al}$/$\mathrm{Al}\mathrm{O}$${}_x$/$\mathrm{Al}$ junctions with long oxidations, we produce functional devices with $E_J/E_C$ in the low-transmon regime ($E_J/E_C\lesssim 30$). 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 $T_1$ times typically in the range of $10–90\mu \mathrm{s}$, with some annealed devices exhibiting $T_1>100\mu \mathrm{s}$ over several hours. The results suggest that energy relaxation in conventional small-junction transmons is not limited by junction loss.