We demonstrate a qubit readout scheme that exploits the Jaynes-Cummings nonlinearity of a superconducting cavity coupled to transmon qubits. We find that in the strongly-driven dispersive regime of this system, there is the unexpected onset of a high-transmission "bright" state at a critical power which depends sensitively on the initial qubit state. A simple and robust measurement protocol exploiting this effect achieves a single-shot fidelity of 87% using a conventional sample design and experimental setup, and at least 61% fidelity to joint correlations of three qubits.
@article{arxiv.1004.4323,
title = {High-Fidelity Readout in Circuit Quantum Electrodynamics Using the Jaynes-Cummings Nonlinearity},
author = {M. D. Reed and L. DiCarlo and B. R. Johnson and L. Sun and D. I. Schuster and L. Frunzio and R. J. Schoelkopf},
journal= {arXiv preprint arXiv:1004.4323},
year = {2010}
}