Short-time quantum detection: Probing quantum fluctuations

We study the information provided by a detector click about the state of an initially excited two-level system. By computing the time evolution of the corresponding conditioned probability beyond the rotating-wave approximation, we show that a click in the detector is related to the decay of the source only for long interaction times. For short times, non-rotating-wave approximation effects such as self-excitations of the detector forbid a naive interpretation of the detector readings. These effects might appear in cQED experiments.