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Bulletin of the American Physical Society

Transport and spectroscopy of junction Andreev bound states in half-shell nanowire transmon devices

Alisa Danilenko, Deividas Sabonis, Georg W. Winkler, Oscar Erlandsson, Anders Kringhøj, Bernard van Heck, Peter Krogstrup, Charles M. Marcus

Published Mar 15, 2021• 0 citations
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Abstract Josephson junctions based on semiconductor nanowires are potential building blocks for electrically tunable qubit structures, e.g., the gatemon or the Andreev qubit. However, an actual realization requires the thorough investigation of the intrinsic excitation spectrum. Here, we demonstrate the fabrication of low-loss superconducting microwave circuits that combine high quality factors with a well-controlled gate architecture by utilizing a flip-chip approach. This platform is then used to perform single-tone and two-tone experiments on Andreev states in in-situ grown InAs/Al core/half-shell nanowires with shadow mask defined Josephson junctions. In gate-controlled and flux-biased spectroscopic measurements we find clear signatures of single quasiparticle as well as quasiparticle pair transitions between discrete Andreev bound states mediated by photon-absorption. Our experimental findings are supported by simulations that show that the junction resides in the intermediate channel length regime.
Josephson junctions formed in semiconductor nanowires host Andreev bound states and serve as a physical platform to realize Andreev qubits tuned by electrostatic gating. With the Andreev bound state being confined to the nanoscale weak link, it couples to a circuit-QED architecture via the state-dependent supercurrent flowing through the weak link. Thus, increasing this coupling strength is a crucial challenge for this architecture. Here, we demonstrate the fabrication and microwave characterization of a weak link which is defined in an In As - Al (core-half shell) nanowire and embedded in a superconducting loop with a lumped-element resonator patterned from a thin Nb Ti N film with high kinetic inductance. We investigate several devices with various weak-link lengths and performed spectroscopy that revealed pair transitions and single-quasiparticle transitions arising from spin-orbit-split Andreev bound states. Our approach offers a compact geometry and a large resonator impedance above 12 k Ω at a resonator frequency of 8 GHz, which facilitates large coupling in the system. For the pair transitions, the experimentally observed energy level splitting demonstrates the coupling to an Andreev level of 490 MHz. We apply a perturbative model that shows good agreement with the experimental data and extract the maximum coupling of 968 MHz. Moreover, we show that the coupling is even stronger to an Andreev level with a higher transmission. In addition, spectroscopy of single-quasiparticle transitions reveals spin-orbit-split Andreev bound states with the extracted spin-photon coupling of 77 MHz.