title: 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~1 GHz and find agreement with values extrapolated from a 1/f(α) fit to the measured flux noise spectral density below 1 Hz.
authors.affiliation: Quantum Nanoelectronics Laboratory, Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: D H
authors.initials: DH
authors.lastname: Slichter
authors.forename: R
authors.initials: R
authors.lastname: Vijay
authors.forename: S J
authors.initials: SJ
authors.lastname: Weber
authors.forename: S
authors.initials: S
authors.lastname: Boutin
authors.forename: M
authors.initials: M
authors.lastname: Boissonneault
authors.forename: J M
authors.initials: JM
authors.lastname: Gambetta
authors.forename: A
authors.initials: A
authors.lastname: Blais
authors.forename: I
authors.initials: I
authors.lastname: Siddiqi