Flux-Tunable Transmon Achieves Robust Performance with 4hb-Tas Josephson Junctions

Researchers are pioneering a new avenue in superconducting quantum circuits by integrating two-dimensional van der Waals materials into Josephson junctions. Eliya Blumenthal, Ilay Mangel, and Amit Kanigel, all from the Department of Physics at the Technion , Israel Institute of Technology, alongside Shay Hacohen-Gourgy, demonstrate a flux-tunable transmon qubit utilising a novel Al/AlO/4Hb-TaS Josephson junction , a significant step beyond traditional aluminium-based designs. This work establishes a repeatable fabrication process for hybrid superconducting circuits and reveals intriguing discrepancies between expected and measured Josephson energies, suggesting complex physics within the van der Waals material. By successfully embedding this device within a 3D cavity and achieving sub-microsecond energy relaxation times, the team provides a crucial foundation for future exploration of unconventional superconductivity and enhanced qubit coupling in two-dimensional materials.

The team achieved this by developing a robust and repeatable hybrid junction process, sequentially depositing and fully oxidising ultrathin aluminium layers onto exfoliated 4Hb-TaS₂ flakes, followed by a top aluminium electrode deposition, a process compatible with standard transmon fabrication techniques. This innovative approach allows for the creation of devices that can explore condensate properties and subgap excitations in vdW materials with unprecedented control. Although material-specific subgap modes were not resolved in this initial geometry, this work firmly establishes a practical pathway for integrating 4Hb-TaS₂ into coherent quantum circuits, providing a crucial baseline for future designs. The demonstrated fabrication process and characterisation techniques pave the way for exploring a wider range of vdW materials and superconducting heterostructures within the circuit-QED platform, promising advancements in quantum computing and materials science.

Van der Waals Josephson Junction Fabrication Details

Researchers realized a flux-tunable transmon featuring an Al/AlOₓ/4Hb-TaS₂ Josephson junction, meticulously fabricating devices on Si/SiO₂ substrates pre-patterned with either Pt/Cr or Nb alignment markers, substrates were solvent-cleaned via sequential sonication in acetone, ethanol, and IPA, then baked at 180°C for 2 minutes. Flakes of 4Hb-TaS₂ were exfoliated using a dry-transfer method and registered to alignment markers, subsequently coated with a PMMA trilayer resist stack (two layers of 495 A4 followed by 950 A2) and patterned using electron-beam lithography with a 10kV beam. Following development in a 1:3 MIBK:IPA solution for 90 seconds and rinsing in IPA, a 30-second oxygen-plasma ashing step removed resist residues. Metal deposition occurred in an e-beam evaporator, employing a multi-step process to create a controlled tunnel barrier on the exfoliated vdW crystal, an in-situ Ar ion mill first removed surface contamination and milled a few nanometers into the 4Hb-TaS₂ at a 45° incidence to promote robust side-contact.

A Ti gettering evaporation achieved base pressure before sequentially depositing 2, 3 thin Al layers (approximately 5Å each at 0.5Å/s) at a 10° tilt, fully oxidizing each layer in-situ (80 Torr, 5% O₂ in Ar for 40 minutes) prior to the next deposition. A top Al layer (approximately 200nm) was then deposited at the same tilt angle, followed by lift-off in NMP at 80°C for 3 hours, IPA rinsing, and N₂ drying. The study pioneered two SQUID-based transmon layouts; Design 1 positioned the SQUID loop between one capacitor pad and the 4Hb-TaS₂ flake, while Design 2 implemented an asymmetric SQUID geometry with one branch containing the hybrid Al/AlOₓ/4Hb-TaS₂ junction and the other a conventional Al/AlOₓ/Al junction, two samples of Design 1 and three samples of Design 2 were fabricated and cleaved before mounting within a 3D OFHC copper cavity cooled to approximately 10 mK in a dilution refrigerator. Standard filtered microwave lines provided cavity readout and qubit control, and a superconducting coil applied a DC magnetic field to tune the Josephson energy.

TaS2 junction yields coherent tunable superconducting qubit performance

From this analysis, they extracted key parameters characterizing the transmon regime, confirming the device’s coherent behaviour. Experiments revealed sub-microsecond energy relaxation times, specifically measuring T₁ values of 0.08 ±0.01μs for device 1A and 0.69 ±0.03μs for device 2A. Notably, two samples of Design 1 and three samples of Design 2 were fabricated and tested, all cooled to a base temperature of approximately 10 mK within a dilution refrigerator. The observed transitions were accurately modelled using the Hamiltonian H = ħωca†a + 4ECn² − EJ(Φ) cos φ + ħg(a† + a)n, where parameters were extracted to define the device characteristics.

For device 1B, a total Josephson energy of EJ,Σ/h ≈ 13.7GHz was extracted, while room-temperature junction resistances measured 2.4 kΩ and 1.9 kΩ for devices 1A and 1B respectively. Applying these values to the Ambegaokar, Baratoff relation yielded an inferred superconducting gap of only 33, 35 μV, inconsistent with the known aluminum gap of ∼162 μV and the reported 4Hb-TaS₂ gap of ∼390 μV. Measurements confirm that despite relaxation times being below those of optimized Al/AlOₓ/Al transmons, this first-generation hybrid platform opens avenues for exploring unconventional superconductivity. Researchers postulate that the high dephasing rate may stem from flux noise or intrinsic material properties, such as quasiparticle-induced dephasing linked to enhanced subgap density of states in 4Hb-TaS₂. This work paves the way for utilising circuit-QED as a spectroscopic probe of this intriguing material class.

4Hb-TaS2 integrated with transmon qubit device shows promising

This achievement demonstrates a viable method for combining these materials with established qubit technology, opening avenues for exploring unconventional superconductivity using circuit-QED techniques. The fabricated device exhibited a SQUID-like spectrum and energy relaxation times (T₁) ranging from 0.08 to 0.69 microseconds, confirming coherent circuit operation. Acknowledging limitations, the authors note the need for further work to determine whether the observed dephasing originates from intrinsic material properties or extrinsic factors like fabrication quality or flux noise. Future research directions include etching the 4Hb-TaS₂ flake into a narrower strip to improve junction size control and edge mode coupling, potentially revealing Andreev bound states. Additionally, a more detailed study of the relationship between junction resistance and Josephson energy is planned, promising a deeper understanding of these hybrid superconducting junctions and paving the way for advanced circuit-QED studies of unconventional superconductivity.