title: Weak qubit measurement with a nonlinear cavity: beyond perturbation theory.
abstract: We analyze the use of a driven nonlinear cavity to make a weak continuous measurement of a dispersively coupled qubit. We calculate the backaction dephasing rate and measurement rate beyond leading-order perturbation theory using a phase-space approach which accounts for cavity noise squeezing. Surprisingly, we find that increasing the coupling strength beyond the regime describable by leading-order perturbation theory (i.e., linear response) allows one to come significantly closer to the quantum limit on the measurement efficiency. We interpret this behavior in terms of the non-Gaussian photon number fluctuations of the nonlinear cavity. Our results are relevant to recent experiments using superconducting microwave circuits to study quantum measurement.
authors.affiliation: Physics Department, McGill University, Montreal, Quebec, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Laflamme
authors.forename: A A
authors.initials: AA
authors.lastname: Clerk