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Tesi etd-07022015-061437

Thesis type
Tesi di laurea magistrale
Real-space measurement of the mechanical effect of the van der Waals-force on Rydberg atoms.
Corso di studi
relatore Prof. Morsch, Oliver
Parole chiave
  • laser
  • atomi freddi
  • van der Waals
  • Rydberg
  • ottica quantistica
Data inizio appello
Data di rilascio
Riassunto analitico
The van der Waals force is the sum of all attractive or repulsive forces between atoms or<br>molecules due to the interactions between permanent or induced dipole moments; it is a topic<br>of widespread interest in many fields of science, such as atomic and condensed matter<br>physics, chemistry and biology as it is at the basis of very different phenomena, from the<br>complex nature of interatomic and intermolecular interactions, to the characteristic three-<br>dimensional shape of biological macromolecules, to effects in the macroscopic domain such<br>as the capability of geckos to stick on walls without falling.<br>Even if van der Waals force has been known for over a century, it is only in the last few years<br>that experiments targeted to measure the force between two isolated atoms have been<br>carried out: these experiments focus mainly on measuring the effect of the force on internal<br>degrees of freedom of the system, that is the displacement of atomic energy levels due to the<br>interaction.<br>A more direct approach to studying the effect of van der Waals force is to perform<br>measurements on the external degrees of freedom instead, observing the spatial dynamics of<br>the atoms involved in the interaction.<br>However, traditional techniques that have been employed in a lot of experiments in atomic<br>physics through the years, as such those based on fluorescence, cannot be applied in this<br>kind of experiment: in fact, we must deal with small atom numbers, distributed within a large<br>volume of around a cubic millimeter, which the above imaging techniques are not able to<br>detect.<br>Our experiment is performed using an ultracold gas of rubidium atoms trapped in a magneto-<br>optical trap, and we observe the van der Waals force between atoms excited to the 70S<br>Rydberg state, for which the interaction is repulsive.<br>We use Rydberg atoms because, thanks to their larger electrical dipole moments with<br>respect to their ground state counterparts, they lead to larger values of the van der Waals<br>interaction coefficient and thus facilitate the observation of the resulting force.<br>In order to measure the effect of the van der Waals force, we excite about ten atoms in the<br>atomic cloud to the Rydberg state and study their expansion over time by field ionizing them<br>at different moments and gathering information about the arrival times of the corresponding<br>ions to the detector.<br>Our detection technique is based on the analysis of the arrival times of the ions, in a similar<br>way to the study of Coulomb explosions; in order to use this tool we need to make a detailed<br>characterization of our detection apparatus and a calibration of the arrival times. Through this<br>technique we can monitor the spatial expansion of the cloud, as each time we ionize the<br>atoms we take a snapshot of the cloud as it is right before ionization.<br>We also compare the results of the expansions with a numerical simulation without using any<br>free parameter, and a good agreement with the experimental data is achieved.<br>Our experiment represents an innovative approach to the measurement of the van der Waals<br>force using real-space measurements of its mechanical effects, that lead to what may be<br>called a &#34;van der Waals explosion&#34;.