When two light beams cross each other, neither of the beams is affected
by the presence of the other. This fundamental property of light is
often very convenient, for example when observing light which
traveled large distances like the emission of distant stars.  However,
it is believed that novel computing and sensing technologies could
greatly benefit from light-light interaction. These devices can
potentially outperform their conventional counterparts by exploiting
quantum mechanical properties when the light-light interaction is
strong enough that an individual light particle, aka photon,
significantly affects another one, for example by changing the
propagation direction of the second photon.

The trick to make photons interact is to send them through a medium
which is extremely light sensitive. In our work we show that a single
atom can act as such a medium. To couple the photons to the atom, we
adapt a focusing configuration which is well-known from high-
resolution imaging called 4Pi microscopy. In this configuration the
light impinges onto the atom from two opposing directions.
Our main results are the observations of a large photon-atom
scattering probability, and of a change of the photon statistics
induced by the atom. The latter is a signature of effective photon-
photon interaction.  With some technical improvements, the results from
our proof-of-concept experiment can lead to a strong interaction between
photons.