I study the response of a detector that is coupled non-linearly to a quantized complex scalar field in different types of classical electromagnetic backgrounds. Assuming that the quantum field is in the vacuum state, I show that, when in { \it inertial} motion, the detector responds { \it only} when the electromagnetic background produces particles. However, I find that the response of the detector is { \it not} proportional to the number of particles produced by the background.
Categories: hep-th gr-qc