Epitaxial Merged-Element Transmon

The merged-element transmon (MET), in which a Josephson junction with sufficiently large self-capacitance eliminates the need for a shunt capacitor, has recently gained much attention thanks to its compact design [1, 2]. Another potential benefit of MET is that the electric field lies predominantly inside the Josephson tunnel barrier, thus greatly reducing the effect of lossy interfaces which typically limit the standard transmon lifetime. In fact, the coherence lifetime of MET should primarily depend on the defect density within the tunnel barrier, meaning any improvement in the barrier quality can have a significant effect on qubit performance. For example, replacing the amorphous AlOx (the usual choice of tunnel junction in transmons) by an epitaxial barrier in a MET geometry should in principle reduce the relevant defect density and substantially improve qubit lifetime. Here we report preliminary results on epitaxially grown MET. We focus on qubit spectroscopy measurement with flux tunable METs to estimate the defect density within the epitaxial barrier. [1]. R. Zhao et al., Phys. Rev. Applied 14, 064006 (2020), [2] H. J. Mamin et al., Phys. Rev. Applied 16, 024023 (2021).