Quantum computation will rely on quantum error correction to counteract decoherence. Successfully implementing an error correction protocol requires the fidelity of qubit operations to be well-above error correction thresholds. In superconducting quantum computers, measurement of the qubit state remains the lowest-fidelity operation. For the transmon, a prototypical superconducting qubit, measurement is carried out by scattering a microwave tone off the qubit. Conventionally, the frequency of this tone is of the same order as the transmon frequency. The measurement fidelity in this approach is limited by multi-excitation resonances in the transmon spectrum which are activated at high readout power. These resonances excite the qubit outside of the computational basis, violating the desired quantum non-demolition character of the measurement. Here, we find that strongly detuning the readout frequency from that of the transmon exponentially suppresses the strength of spurious multi-excitation resonances. By increasing the readout frequency up to twelve times the transmon frequency, we achieve a quantum non-demolition measurement fidelity of 99.93% with a residual probability of leakage to non-computational states of only 0.02%.
@article{arxiv.2501.09161,
title = {High-frequency readout free from transmon multi-excitation resonances},
author = {Pavel D. Kurilovich and Thomas Connolly and Charlotte G. L. Bøttcher and Daniel K. Weiss and Sumeru Hazra and Vidul R. Joshi and Andy Z. Ding and Heekun Nho and Spencer Diamond and Vladislav D. Kurilovich and Wei Dai and Valla Fatemi and Luigi Frunzio and Leonid I. Glazman and Michel H. Devoret},
journal= {arXiv preprint arXiv:2501.09161},
year = {2025}
}