  In three dimensions, dipole-dipole interactions which alter atomic level
shifts and spontaneous decay rates only persist over distances comparable to
the wavelength of the emitted light. In this paper we show that it is possible
to significantly extend the range of these interactions with the help of a
partially transparent asymmetric mirror interface. Suppose two two-level atoms
are placed on opposite sides of the interface, each at the position of the
mirror image of the other. In this case, their emitted light interferes almost
exactly as it would when the atoms are right next to each other. Hence their
dipole-dipole interaction assumes an additional maximum, even when the actual
distance of the atoms is several orders of magnitude larger than the transition
wavelength. Although the resulting ultralong-range interactions are in general
relatively weak, we expect them to find applications in quantum technology,
like non-invasive quantum sensing.
