Flux-tunable transmon with an Al/AlO x /4Hb-TaS2 Josephson junction

Abstract

Incorporating van der Waals (vdW) superconductors into Josephson elements extends circuit-QED beyond conventional Al/AlOx/Al junctions, enabling microwave probes of unconventional condensates and subgap excitations. We realize a flux-tunable transmon whose nonlinear inductive element is an Al/AlOx/4Hb-TaS2 Josephson junction, with a tunnel barrier formed by full in situ oxidation of ultrathin Al layers on an exfoliated 4Hb-TaS2 flake in a process compatible with standard transmon fabrication. In a three-dimensional copper cavity, the device shows a SQUID-like flux-dependent spectrum that is well reproduced by a dressed transmon–cavity Hamiltonian in the transmon regime. We measure sub-microsecond energy relaxation (T1=0.08–0.69 μs), while Ramsey measurements indicate dephasing faster than our 16 ns resolution. A naive Ambegaokar–Baratoff comparison using room-temperature junction resistances gives a suppressed effective energy scale; however, cross-sectional TEM reveals an amorphous region at the TaS2 interface, most likely produced by the pre-deposition ion-milling step. We therefore attribute the suppressed EJRn product primarily to a non-ideal hybrid interface. This work demonstrates the integration of 4Hb-TaS2 into a circuit-QED device and establishes an interface-optimization baseline for future edge-sensitive designs targeting boundary and subgap degrees of freedom in vdW superconductors.