  A single atom in free space can have a strong influence on a light beam and a
single photon can have a strong effect on a single atom in free space.
Regarding this interaction, two conceptually different questions can be asked:
can a single atom fully absorb a single photon and can a single atom fully
reflect a light beam. The conditions for achieving the full effect in either
case are different. Here we discuss related questions in the context of an
optical resonator. When shaping a laser pulse properly it will be fully
absorbed by an optical resonator, i.e., no light will be reflected and all the
pulse energy will accumulate inside the resonator before it starts leaking out.
We show in detail that in this case the temporal pulse shape has to match the
time-reversed pulse obtained by the cavity's free decay. On the other hand a
resonator, made of highly reflecting mirrors which normally reflect a large
portion of any incident light, may fully transmit the light, as long as the
light is narrow band and resonant with the cavity. The analogy is the single
atom - normally letting most of the light pass - which under special conditions
may fully reflect the incident light beam. Using this analogy we are able to
study the effects of practical experimental limitations in the atom-photon
coupling, such as finite pulses, bandwidths, and solid angle coverage, and to
use the optical resonator as a test bed for the implementation of the quantum
experiment.
