  We investigate the interaction between two rubidium atoms in highly excited
Rydberg states, and find that very long-range potential wells exist. These
wells are shown to support many bound states. We calculate the properties of
the wells and bound levels, and show that their lifetimes are limited by that
of the constituent Rydberg atoms. We also show how these $\mu$m-size bound
states can be populated {\it via} photoassociation (PA), and how the signature
of the ad-mixing of various $\ell$-character producing the potential wells
could be probed. We discuss how sharp variations of the PA rate %to macrodimer
bound levels could act as a switching mechanism with potential application to
quantum information processing.
