Fast and accurate qubit measurement remains a critical challenge on the path to fault-tolerant quantum computing. In superconducting quantum circuits, fast qubit measurement has been achieved using a dispersively coupled resonator with a large external linewidth. This necessitates the use of a Purcell filter that protects the qubit from relaxation through the readout channel. Here we show that a readout resonator and filter resonator, coupled to each other both capacitively and inductively, can produce a compact notch-filter circuit that effectively eliminates the Purcell decay channel through destructive interference. By utilizing linewidths as large as 42 MHz, we perform 56-ns simultaneous readout of four qubits and benchmark an average assignment fidelity of 99.77%, with the highest qubit assignment fidelity exceeding 99.9%. These results demonstrate a significant advancement in speed and fidelity for multiplexed superconducting qubit readout.
@article{arxiv.2409.04967,
title = {Fast multiplexed superconducting qubit readout with intrinsic Purcell filtering},
author = {Peter A. Spring and Luka Milanovic and Yoshiki Sunada and Shiyu Wang and Arjan F. van Loo and Shuhei Tamate and Yasunobu Nakamura},
journal= {arXiv preprint arXiv:2409.04967},
year = {2025}
}