  Implementing nonlinear interactions between single photons and single atoms
is at the forefront of optical physics. Motivated by the prospects of
deterministic all-optical quantum logic, many efforts are currently underway to
find suitable experimental techniques. Focusing the incident photons onto the
atom with a lens yielded promising results, but is limited by diffraction to
moderate interaction strengths. However, techniques to exceed the diffraction
limit are known from high-resolution imaging. In this work, we adapt a
super-resolution imaging technique, 4Pi microscopy, to efficiently couple light
to a single atom. We observe 36.6(3)% extinction of the incident field, and a
modified photon statistics of the transmitted field -- indicating nonlinear
interaction at the single-photon level.
