  Controlling molecular binding at the level of single atoms is one of the holy
grails of quantum chemistry. Rydberg macrodimers -- bound states between highly
excited Rydberg atoms -- provide a novel perspective in this direction.
Resulting from binding potentials formed by the strong, long-range interactions
of Rydberg states, Rydberg macrodimers feature bond lengths in the micrometer
regime, exceeding those of conventional molecules by orders of magnitude. Using
single-atom control in quantum gas microscopes, the unique properties of these
exotic states can be studied with unprecedented control, including the response
to magnetic fields or the polarization of light in their photoassociation. The
high accuracy achieved in spectroscopic studies of macrodimers makes them an
ideal testbed to benchmark Rydberg interactions, with direct relevance to
quantum computing and information protocols where these are employed. This
review provides a historic overview and summarizes the recent findings in the
field of Rydberg macrodimers. Furthermore, it presents new data on interactions
between macrodimers, leading to a phenomenon analogous to Rydberg blockade at
the level of molecules, opening the path towards studying many-body systems of
ultralong-range Rydberg molecules.
