High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that effectively increases the energy-relaxation time, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout - without using a quantum-limited amplifier.
@article{arxiv.2208.05879,
title = {Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier},
author = {Liangyu Chen and Hang-Xi Li and Yong Lu and Christopher W. Warren and Christian J. Križan and Sandoko Kosen and Marcus Rommel and Shahnawaz Ahmed and Amr Osman and Janka Biznárová and Anita Fadavi Roudsari and Benjamin Lienhard and Marco Caputo and Kestutis Grigoras and Leif Grönberg and Joonas Govenius and Anton Frisk Kockum and Per Delsing and Jonas Bylander and Giovanna Tancredi},
journal= {arXiv preprint arXiv:2208.05879},
year = {2025}
}