This review summarizes experimental works performed over the last decade by several groups on the manipulation of a few individual interacting Rydberg atoms. These studies establish arrays of single Rydberg atoms as a promising platform for quantum-state engineering, with potential applications to quantum metrology, quantum simulation and quantum information.
Rydberg atoms are highly excited atoms, where a valence electron has a large principal quantum number n > 1. They have exaggerated properties, and they interact very strongly with each other via the dipole–dipole interaction. This is the basis for the Rydberg blockade, i.e. the inhibition of the excitation of ground-state atoms to the Rydberg state by the presence of a nearby Rydberg atom. Over the last decade, theoretical proposals suggesting the use of the blockade to create entangled states of neutral atoms and quantum gates triggered a lot of experimental activity to observe the blockade in ensembles of laser cooled atoms. The field is now evolving in many directions, from quantum optics, with the promise of realizing single-photon nonlinearities, to many-body physics in large ensembles. This paper reviews recent experimental work on the Rydberg blockade and its application to the entanglement of two atoms as well as on the measurement of interactions between Rydberg atoms. We focus on small, well-controlled systems of a few individual atoms trapped in arrays of addressable optical tweezers. We will only briefly mention recent works based on individual atoms held in optical lattices that use quantum gas microscopes. This review is organized as follows. We first recall the motivation behind those studies, and in particular the principles of the quantum gates based on the blockade mechanism. Then, after a theoretical reminder about interactions between Rydberg atoms, we introduce the basic experimental techniques used to manipulate individual Rydberg atoms. We then review experiments that demonstrated the Rydberg blockade, quantum gates and entanglement of two atoms, and the direct measurements of the interactions between Rydberg atoms in various regimes. Finally, we discuss the current efforts aimed at extending those studies to larger numbers of atoms.
