• laser
• ottica quantistica
• Rydberg
• van der Waals
• atomi freddi

The van der Waals force is the sum of all attractive or repulsive forces between atoms or molecules due to the interactions between permanent or induced dipole moments; it is a topic of widespread interest in many fields of science, such as atomic and condensed matter. physics, chemistry and biology as it is at the basis of very different phenomena, from the complex nature of interatomic and intermolecular interactions, to the characteristic three-dimensional shape of biological macromolecules, to effects in the macroscopic domain such as the capability of geckos to stick on walls without falling.
Even if van der Waals force has been known for over a century, it is only in the last few years that experiments targeted to measure the force between two isolated atoms have been carried out: these experiments focus mainly on measuring the effect of the force on internal degrees of freedom of the system, that is the displacement of atomic energy levels due to the interaction.
A more direct approach to studying the effect of van der Waals force is to perform measurements on the external degrees of freedom instead, observing the spatial dynamics of the atoms involved in the interaction.
However, traditional techniques that have been employed in a lot of experiments in atomic physics through the years, as such those based on fluorescence, cannot be applied in this kind of experiment: in fact, we must deal with small atom numbers, distributed within a large volume of around a cubic millimeter, which the above imaging techniques are not able to detect.
Our experiment is performed using an ultracold gas of rubidium atoms trapped in a magneto-optical trap, and we observe the van der Waals force between atoms excited to the 70S Rydberg state, for which the interaction is repulsive.
We use Rydberg atoms because, thanks to their larger electrical dipole moments with respect to their ground state counterparts, they lead to larger values of the van der Waals interaction coefficient and thus facilitate the observation of the resulting force.
In order to measure the effect of the van der Waals force, we excite about ten atoms in the atomic cloud to the Rydberg state and study their expansion over time by field ionizing them at different moments and gathering information about the arrival times of the corresponding ions to the detector.
Our detection technique is based on the analysis of the arrival times of the ions, in a similar way to the study of Coulomb explosions; in order to use this tool we need to make a detailed characterization of our detection apparatus and a calibration of the arrival times. Through this technique we can monitor the spatial expansion of the cloud, as each time we ionize the atoms we take a snapshot of the cloud as it is right before ionization.
We also compare the results of the expansions with a numerical simulation without using any free parameter, and a good agreement with the experimental data is achieved.
Our experiment represents an innovative approach to the measurement of the van der Waals force using real-space measurements of its mechanical effects, that lead to what may be called a "van der Waals explosion".