  We report the observation of resolved atomic interaction sidebands (ISB) in
the ${}^{87}$Sr optical clock transition when atoms at microkelvin temperatures
are confined in a two-dimensional (2D) optical lattice. The ISB are a
manifestation of the strong interactions that occur between atoms confined in a
quasi-one-dimensional geometry and disappear when the confinement is relaxed
along one dimension. The emergence of ISB is linked to the recently observed
suppression of collisional frequency shifts in [1]. At the current
temperatures, the ISB can be resolved but are broad. At lower temperatures, ISB
are predicted to be substantially narrower and usable as powerful spectroscopic
tools in strongly interacting alkaline-earth gases.
