Many superconducting qubit systems use the dispersive interaction between the qubit and a coupled harmonic resonator to perform quantum state measurement. Previous works have found that such measurements can induce state transitions in the qubit if the number of photons in the resonator is too high. We investigate these transitions and find that they can push the qubit out of the two-level subspace, and that they show resonant behavior as a function of photon number. We develop a theory for these observations based on level crossings within the Jaynes-Cummings ladder, with transitions mediated by terms in the Hamiltonian that are typically ignored by the rotating wave approximation. We find that the most important of these terms comes from an unexpected broken symmetry in the qubit potential. We confirm the theory by measuring the photon occupation of the resonator when transitions occur while varying the detuning between the qubit and resonator.
@article{arxiv.1606.05721,
title = {Measurement-induced state transitions in a superconducting qubit: Beyond the rotating wave approximation},
author = {Daniel Sank and Zijun Chen and Mostafa Khezri and J. Kelly and R. Barends and B. Campbell and Y. Chen and B. Chiaro and A. Dunsworth and A. Fowler and E. Jeffrey and E. Lucero and A. Megrant and J. Mutus and M. Neeley and C. Neill and P. J. J. O'Malley and C. Quintana and P. Roushan and A. Vainsencher and J. Wenner and T. White and Alexander N. Korotkov and John M. Martinis},
journal= {arXiv preprint arXiv:1606.05721},
year = {2016}
}