Measurement-induced qubit state mixing in circuit QED from Up-converted dephasing noise

Abstract

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∼1GHz and find agreement with values extrapolated from a 1/fα fit to the measured flux noise spectral density below 1Hz. © 2012 American Physical Society.