Investigation of geometrical effects on merged-element transmon.

Recently, we have designed and fabricated a prototype merged-element transmon qubit (mergemon), [1] which may potentially bring several advantages compared to the conventional transmons, such as increased coherence time using low-loss epitaxial substrates, qubit frequency less affected by junction variation, and increased scalability by reduced qubit footprint. One of the major challenges in further improving the design is the loss from the interfaces associated with the tunnel barrier edges.
Electromagnetic simulations were carried out to investigate how the different geometrical parameters such as radius and thickness of the tunnel barrier affect the loss and the T₁ of the mergemon (participation ratio model). The loss comes primarily from the tunnel barrier and interfaces near the edge of the electrode, due to the fringing fields. We found that T₁ increases as the radius and thickness of the tunnel barrier. With typical numbers expected for loss tangents, interface thicknesses, and dimensions of tunnel junction fabricated by optical lithography, we give simple estimates of T₁ for the mergemon qubit. Our result can provide a useful guidance to the development of the quantum processor based on merged element transmons.

[1] R. Zhao et al., arXiv:2008.07652v4