Design and Characterization of a Functional Merged Element Transmon

A merged element transmon (MET) is a superconducting qubit with a Josephson junction that has been engineered to act as its own parallel shunt capacitor [1]. Because it eliminates the need for a separate capacitor, it allows for a significantly more compact qubit. Furthermore, because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/Al₂O₃/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 <~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 T₁ times typically in the range of 30-50 μs, with some devices exhibiting T₁>100 μs over several hours.

[1] R. Zhao et al, https://arxiv.org/pdf/2008.07652.pdf