Renormalized dynamics in charge qubit measurements by a single electron transistor

We investigate charge qubit measurements using a single electron transistor, with focus on the backaction-induced renormalization of qubit parameters. It is revealed the renormalized dynamics leads to a number of intriguing features in the detector's noise spectra, and therefore needs to be accounted for to properly understand the measurement result. Noticeably, the level renormalization gives rise to a strongly enhanced signal-to-noise ratio, which can even exceed the universal upper bound imposed quantum mechanically on linear-response detectors.