Observation of coherent many-body Rabi oscillations

A two-level quantum system coherently driven by a resonant electromagnetic field oscillates sinusoidally between the two levels at frequency Omega (refs 1,2). In dilute gases, the inhomogeneous distributions of both the coupling strength to the field and the interactions between individual atoms reduce the visibility of these so-called Rabi oscillations and may even suppress them completely. However, in the limit where only a single excitation is present, a collective, many-body Rabi oscillation at a frequency root N Omega arises that involves all N >> 1 atoms, even in inhomogeneous systems(3,4). When one of the two levels is a strongly interacting Rydberg level, many-body Rabi oscillations emerge as a consequence of a phenomenon known as Rydberg excitation blockade(5). Here we report initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms, with a 0.67(10) preparation efficiency of the singly-excited many-body state. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.