Qubit readout is an indispensable element of any quantum information processor. In this work, we experimentally demonstrate a non-perturbative cross-Kerr coupling between a transmon and a polariton mode which enables an improved quantum non-demolition (QND) readout for superconducting qubits. The new mechanism uses the same experimental techniques as the standard QND qubit readout in the dispersive approximation, but due to its non-perturbative nature, it maximizes the speed, the single-shot fidelity and the QND properties of the readout. In addition, it minimizes the effect of unwanted decay channels such as the Purcell effect. We observed a single-shot readout fidelity of 97.4% for short 50 ns pulses, and we quantified a QND-ness of 99% for long measurement pulses with repeated single-shot readouts.
@article{arxiv.1905.00271,
title = {Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling},
author = {R. Dassonneville and T. Ramos and V. Milchakov and L. Planat and É. Dumur and F. Foroughi and J. Puertas and S. Leger and K. Bharadwaj and J. Delaforce and C. Naud and W. Hasch-Guichard and J. J. García-Ripoll and N. Roch and O. Buisson},
journal= {arXiv preprint arXiv:1905.00271},
year = {2020}
}