The 2012-2013 lecture described experiments using Rydberg atoms to measure and manipulate photons trapped in a cavity. In these studies, highly excited atoms are used as probes and tools to study the field. The coupling of the atoms to the cavity is exploited not only to control the field it contains, but also to entangle the atoms successively passing through it. The unusual orders of magnitude of Rydberg atoms (large size and large induced electric dipoles in particular) play an essential role in these studies.
For the past ten years or so, Rydberg atoms have been studied in a different context for fundamental physics experiments. The aim is no longer to couple these atoms to cavities, but to directly exploit the very strong dipole-dipole coupling between Rydberg atoms to study atom-atom or atom-photon interaction under highly unusual conditions, and to demonstrate quantum information operations (realization of logic gates coupling atoms together). These experiments use lasers to control atomic position and velocity (cooling and optical trapping of atoms). This year's lecture will describe these experiments on "cold Rydberg atoms", which represent a new and very promising chapter in the physics of controlling isolated quantum particles and ultra-cold atomic gases.