| Rydberg atoms are well know for years to exhibit very large cross sections in collisions with various atoms or molecules, due to the large excursion of the Rydberg electron. The energy transfer between Rydberg atoms present even much larger cross sections, by several orders of magnitude, due to long-range dipole-dipole interactions. For a few years now, these interacting systems have been receiving a great interest, due to some important perspectives they offer, for instance in quantum information. Here we report on a new experimental method for accurate density measurement of a Rydberg gas, based on controled dipole-dipole induced Landau-Zener transitions in pairs of Rydberg atoms. First we present a study of Landau-Zener transitions in a supersonic beam of sodium atoms. The atoms are excited in a Rydberg state of principal quantum number around 50, and subsequently experience a homogeneous, time-dependent electric field. This field crosses an accidental energy degeneracy of a system made of two Rydberg atoms. This energy degeneracy is removed by the dipole-dipole interaction and the gap of the induced avoided crossing depends parametrically on the distance between the atoms. Since the atom pair is frozen on the time-scale of a Landau-Zener transition, it follows that the adiabaticity of the transition depends on only two parameters: the interatomic distance and the sweep rate of the electric field. Experimentally, a detection based on pulsed-field ionisation allows us to discriminate between the possible final states of a pair. The experimental results fully agree with a Landau-Zener model in the two-atom system. Then we present an experimental, very accurate density measurement of the Rydberg gas which follows from the aforementioned study. Indeed, the binary, non-collisional Landau-Zener transitions allows us to probe the nearest neighbour distribution in the Rydberg gas, which links to the density of the gas. Such measurements bring also a very accurate calibration of the Rydberg detection process. We discuss how such measurements could be extended to atomic or molecular Rydberg in supersonic beams, but also to ultracold atomic samples in the regimes of dipole blockade or anti-blockade. |
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