High frequency dispersive measurement of a transmon qubit with a 20 GHz readout resonator

Error correction is one of the most critical challenges in the scaling of quantum processors. Achieving high-fidelity and quantum non-demolition (QND) readout of the qubit state is a key step toward that goal. In superconducting qubits, QND measurement is enacted using dispersive readout. However, when the frequency of the readout resonator is similar to that of the qubit, this approach suffers from measurement-induced transitions that eject the qubit from the computational subspace. This problem can be ameliorated by using a readout resonator with frequency much larger than that of the qubit [1,2]. At the same time, increasing the overall frequency scale of the system helps both mitigate the problem of thermal excitations and speeds up operations. In this experiment, we demonstrate single-shot dispersive measurement with a 20 GHz readout cavity and a 2 GHz transmon qubit, more than doubling the overall frequency scale compared to the previously demonstrated high frequency readout [1]. We anticipate that this may lead to a reduction of the qubit thermal population, less dephasing from photons in the readout resonator, and better reset of leakage states.

[1] Kurilovich, P. D., Connolly, T. et al. "High-frequency readout free from transmon multi-excitation resonances." https://arxiv.org/abs/2501.09161. 2025.

[2] Connolly, T., Kurilovich P.D. et al. "Full characterization of measurement-induced transitions of a superconducting qubit." https://arxiv.org/abs/2506.05306. 2025.