Measurement-Induced Qubit State Mixing in Circuit QED from Up-Converted Dephasing Noise
D. H. Slichter, Rajamani Vijay, Steven J. Weber, Samuel Boutin, Maxime Boissonneault, Jay Gambetta, Alexandre Blais, Irfan A. Siddiqi · Physical Review Letters · 2012
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/f(α) fit to the measured flux noise spectral density below 1 Hz.