  We construct a class of Hamiltonians that describe the photodetection process
from beginning to end. Our Hamiltonians describe the creation of a photon, how
the photon travels to an absorber (such as a molecule), how the molecule
absorbs the photon, and how the molecule after irreversibly changing its
configuration triggers an amplification process---at a wavelength that may be
very different from the photon's wavelength---thus producing a macroscopic
signal. We use a simple prototype Hamiltonian to describe the single-photon
detection process analytically in the Heisenberg picture, which neatly
separates desirable from undesirable effects. Extensions to more complicated
Hamiltonians are pointed out.
