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arXiv:2302.12280 · IEEE Transactions on Applied Superconductivity

Characterization of Asymmetric Gap-Engineered Josephson Junctions and 3D Transmon Qubits

Zach Steffen, S. K. Dutta, H. Wang, Kungang Li, Yizhou Huang, Yi-Hsiang Huang, Advait Mathur, F. C. Wellstood, B. S. Palmer

Published Feb 24, 2023• 4 citations
arXivHTMLPDF

Abstract

We have fabricated and characterized asymmetric gap-engineered junctions and transmon devices. To create Josephson junctions with asymmetric gaps, Ti was used to proximitize and lower the superconducting gap of the Al counter-electrode. DC IV measurements of these small, proximitized Josephson junctions show a reduced gap and larger excess current for voltage biases below the superconducting gap when compared to standard Al/AlO$\mathrm{_{x}}$/Al junctions. The energy relaxation time constant for an Al/AlO$\mathrm{_{x}}$/Al/Ti 3D transmon was$T_{1}= 1\ \mu$s, over two orders of magnitude shorter than the measured$T_{1} = 134\ \mu$s of a standard Al/AlO$\mathrm{_{x}}$/Al 3D transmon. Intentionally adding disorder between the Al and Ti layers reduces the proximity effect and subgap current while increasing the relaxation time to$T_{1} = 32\ \mu$s.

DOI
10.1109/tasc.2023.3247987

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We have fabricated and characterized asymmetric gap-engineered junctions and transmon devices. To create Josephson junctions with asymmetric gaps, Ti was used to proximitize and lower the superconducting gap of the Al counter-electrode. DC IV measurements of these small, proximitized Josephson junctions show a reduced gap and larger excess current for voltage biases below the superconducting gap when compared to standard Al/AlOx\mathrm{_{x}}x​/Al junctions. The energy relaxation time constant for an Al/AlOx\mathrm{_{x}}x​/Al/Ti 3D transmon wasT1=1 μT_{1}= 1\ \muT1​=1 μs, over two orders of magnitude shorter than the measuredT1=134 μT_{1} = 134\ \muT1​=134 μs of a standard Al/AlOx\mathrm{_{x}}x​/Al 3D transmon. Intentionally adding disorder between the Al and Ti layers reduces the proximity effect and subgap current while increasing the relaxation time toT1=32 μT_{1} = 32\ \muT1​=32 μs.
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Superconducting transmon qubits based on hybrid superconductor-semiconductor Josephson junctions (gatemons) offer gate tunability, but their relaxation times remain well below those of state-of-the-art transmons, and the origin of this discrepancy is not fully understood. Here, we co-fabricate gatemons and SIS-junction transmons with nominally identical circuit layouts, gate dielectrics, and control lines, so that the Josephson element is the only intentional distinction. Across multiple chips, transmons in this architecture reach relaxation times in the tens of microseconds, whereas gatemons saturate in the few-microsecond range. Using the transmons as on-chip references, we construct a loss budget including Purcell decay, spontaneous emission through the control line, and internal dielectric loss, and find that the corresponding T1 limits exceed all measured gatemon values by more than an order of magnitude. Temperature-dependent T1 measurements follow a common quasiparticle-activation model and yield similar superconducting gaps for S-Sm-S and SIS junctions, indicating that the reduced gatemon coherence is dominated by additional temperature-independent, junction-intrinsic dissipation.
We have engineered the band gap profile of transmon qubits by combining oxygen-doped Al for tunnel junction electrodes and clean Al as quasiparticle traps to investigate energy relaxation due to quasiparticle tunneling. The relaxation time T1 of the qubits is shown to be insensitive to this band gap engineering. Operating at relatively low-E(J)/E(C) makes the transmon transition frequency distinctly dependent on the charge parity, allowing us to detect the quasiparticles tunneling across the qubit junction. Quasiparticle kinetics have been studied by monitoring the frequency switching due to even-odd parity change in real time. It shows the switching time is faster than 10 μs, indicating quasiparticle-induced relaxation has to be reduced to achieve T1 much longer than 100 μs.
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.