Sudden Replacement of a Mirror by a Detector in Cavity QED: Are Photons Counted Immediately?

We consider an excited atom in a cavity such that spontaneous emission is inhibited, and address the question of whether a sudden replacement of one of the cavity mirrors by a detector can result in a photon count immediately or only after some retardation time. The feasibility of an experiment of this type has led to considerable discussion as to its outcome. Following a brief summary of the conflicting arguments, we show that it is possible to count a photon immediately following the substitution of a photodetector for a mirror.