Approaching the ultrastrong-coupling regime between an Andreev level and a microwave resonator

Journal article, 2025

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 InAs-Al (core-half shell) nanowire and embedded in a superconducting loop with a lumped-element resonator patterned from a thin NbTiN 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 kQ 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.