Raising the Cavity Frequency in cQED

We demonstrate dispersive readout of a standard transmon qubit using a 3D cavity operated at a much higher frequency, 21 GHz, than previously studied. By coupling a 4–7 GHz transmon to the cavity, we realize a strongly detuned regime that preserves robust coherence while enabling high-contrast single-shot readout without parametric amplification. This setup also allows us to study the quantum nondemolition (QND) nature related to the offset charge of the qubit and the influence of the environmental temperature on the system. Our results establish high-frequency cavities as a powerful and scalable platform for superconducting qubit readout, opening new opportunities for suppressing measurement-induced errors and for operating quantum processors under less stringent thermal constraints.