Merged-Element Transmons: Design and Qubit Performance

TL;DR

This paper introduces a new superconducting transmon qubit design called merged-element transmons (MET), which simplifies fabrication, reduces electric field participation, and achieves competitive coherence times, potentially improving quantum computing hardware.

Contribution

The paper presents the design, fabrication, and characterization of merged-element transmons, demonstrating their potential advantages over conventional transmons in terms of size, fabrication, and performance.

Findings

Achieved T1 times of 10-90 microseconds, with some exceeding 100 microseconds.

Demonstrated low sub-gap conduction and sharp dI/dV features in measurements.

Observed few avoided level crossings, indicating limited TLS interactions.

Abstract

We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with $E_{J}$/$E_{C}$ in the low transmon regime ($E_{J}$/$E_{C}$ $\lesssim$30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed mean T1 times typically in the range of 10-90 microseconds, with some annealed devices exhibiting T1 > 100 microseconds over several hours. The results suggest that energy relaxation in conventional, small-junction transmons is not limited by junction loss.