Flux-tunable transmon incorporating a van der Waals superconductor via an Al/AlO$_x$/4Hb-TaS$_2$ Josephson junction

TL;DR

This work addresses the challenge of bringing van der Waals superconductors into coherent quantum circuits by implementing a flux-tunable transmon whose Josephson element is an Al/AlO_x/4Hb-TaS2 junction. The authors develop a repeatable, in situ oxidation-based fabrication process and integrate the hybrid junction into a 3D cQED cavity, validating a SQUID-like, flux-controlled spectrum described by a standard transmon–cavity model. They report sub-microsecond energy relaxation and rapid dephasing, along with a notable Ambegaokar-Baratoff discrepancy, highlighting nontrivial junction physics and potential material-specific loss channels. The results establish a practical route to study boundary and subgap physics in vdW superconductors within coherent circuits and lay the groundwork for edge-sensitive designs that could couple to unconventional excitations in 4Hb-TaS2-based junctions.

Abstract

Incorporating van der Waals (vdW) superconductors into Josephson elements extends circuit-QED beyond conventional Al/AlO$_x$/Al tunnel junctions and enables microwave probes of unconventional condensates and subgap excitations. In this work, we realize a flux-tunable transmon whose nonlinear inductive element is an Al/AlO$_x$/4Hb-TaS$_2$ Josephson junction. The tunnel barrier is formed by sequential deposition and full in-situ oxidation of ultrathin Al layers on an exfoliated 4Hb-TaS$_2$ flake, followed by deposition of a top Al electrode, yielding a robust, repeatable hybrid junction process compatible with standard transmon fabrication. Embedding the device in a three-dimensional copper cavity, we observe a SQUID-like flux-dependent spectrum that is quantitatively reproduced by a standard dressed transmon--cavity Hamiltonian, from which we extract parameters in the transmon regime. Across measured devices we obtain sub-microsecond energy relaxation ($T_1$ from $0.08$ to $0.69~μ$s), while Ramsey measurements indicate dephasing faster than our $16$ ns time resolution. We also find a pronounced discrepancy between the Josephson energy inferred from spectroscopy and that expected from the Ambegaokar--Baratoff relation using room-temperature junction resistances, pointing to nontrivial junction physics in the hybrid Al/AlO$_x$/4Hb-TaS$_2$ system. Although we do not resolve material-specific subgap modes in the present geometry, this work establishes a practical route to integrating 4Hb-TaS$_2$ into coherent quantum circuits and provides a baseline for future edge-sensitive designs aimed at enhancing coupling to boundary and subgap degrees of freedom in vdW superconductors.

Figures (3)

• Figure 1: (a) Photograph of the 3D copper cavity package and optical micrographs of a device 1B. (b,c) Schematic layouts of SQUID-transmon design 1 and 2. Design 1 routes supercurrent through the flake between the capacitor pads; Design 2 uses an asymmetric SQUID to reduce sensitivity to bulk transport through the flake.
• Figure 2: Two-tone spectroscopy as a function of coil current (external flux). Dashed curves: transition frequencies obtained from diagonalizing the dressed Hamiltonian in Eq. \ref{['eq:H']} using extracted parameters. Weak additional features at high drive are consistent with cavity-assisted and multi-photon transitions within the dressed spectrum. The lines marked by A, B and C represent a Raman transition at a frequency of $(f_{1,0}-f_{0,0})+(f_{3,0}-f_{0,1})$, $(f_{1,0}-f_{0,0})+(f_{4,0}-f_{1,1})$ and $(f_{1,0}-f_{0,0})+(f_{5,0}-f_{2,1})$, respectively. The 01 transition of the transmon is power-broadened.
• Figure 3: Energy relaxation of devices 1A ($T_1 = 0.08 \pm 0.01~\mu$s) and 2A ($T_1 = 0.69 \pm 0.03~\mu$s).