We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ~ 1 GHz and find agreement with values extrapolated from a 1/fα fit to the measured flux noise spectral density below 1 Hz.
@article{arxiv.1206.6946,
title = {Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise},
author = {D. H. Slichter and R. Vijay and S. J. Weber and S. Boutin and M. Boissonneault and J. M. Gambetta and A. Blais and I. Siddiqi},
journal= {arXiv preprint arXiv:1206.6946},
year = {2012}
}