Multimode RF reflectometry for spin qubit readout and device characterization

We introduce a multimode superconducting inductor architecture that enables radio frequency reflectometry at multiple discrete frequencies up to 2 GHz, addressing limitations of conventional single-mode designs. The spiral inductor's distributed capacitance yields distinct resonant modes with varied impedance-matching conditions. By probing a quantum dot across several modes, we extract tunneling rates over a broad frequency range and identify signatures of nearby charge defects. Using a higher-order mode of the distributed inductor, rather than the fundamental mode, we demonstrate single-shot spin readout via a radio frequency single-electron transistor (RF-SET), achieving singlet-triplet readout with an integration time of 8 μs and a readout fidelity of 98%. These results establish multimode inductance as a scalable and flexible component for fast spin qubit readout and device-quality characterization.