  We highlight fundamental differences in the models of light-matter
interaction between the behaviour of Fock state detection in free space versus
optical cavities. To do so, we study the phenomenon of resonance of detectors
with Fock wavepackets as a function of their degree of monochromaticity, the
number of spatial dimensions, the linear or quadratic nature of the
light-matter coupling, and the presence (or absence) of cavity walls in space.
In doing so we show that intuition coming from quantum optics in cavities does
not straightforwardly carry to the free space case. For example, in $(3+1)$
dimensions the detector response to a Fock wavepacket will go to zero as the
wavepacket is made more and more monochromatic and in coincidence with the
detector's resonant frequency. This is so even though the energy of the
free-space wavepacket goes to the expected finite value of $\hbar\Omega$ in the
monochromatic limit. This is in contrast to the behaviour of the light-matter
interaction in a cavity (even a large one) where the probability of absorbing a
Fock quantum is maximized when the quantum is more monochromatic at the
detector's resonance frequency. We trace this crucial difference to the fact
that monochromatic Fock states are not normalizable in the continuum, thus
physical Fock states need to be constructed out of normalizable wavepackets
whose energy density goes to zero in the monochromatic limit as they get
spatially delocalized.
