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Non-classical states in mesoscopic Bose-Einstein condensates

Final Activity Report Summary - MESBEC (Non-classical states in mesoscopic Bose-Einstein condensates)

The scope of this project was the investigation of non-classical states of matter. Our experimental studies are focused on ultra-cold gases of rubidium atoms at temperatures of less than a billionth of a degree above absolute zero temperature. Fundamentally, quantum statistical studies of matter rely on the classification of particles as being either bosons or fermions. Our samples consist of weakly interacting bosons that undergo a phase transition to a Bose-Einstein condensate (BEC) at very low temperatures in regular three dimensional environments.

Using optical lattices, we are able to drastically alter the behaviour of the cold gas by confining it to a flat two-dimensional world. In this situation the quantum many body physics is fundamentally changed and a different type of phase transition occurs, a so called Berizinskii-Kosterlitz-Thouless (BKT) transition. In the framework of this project we have found first-time experimental evidence for the microscopic driving mechanism of this transition: thermally activated quantized vortices. When the temperature is increased, the number of vortices proliferates, alongside with a loss of order in the quantum phase of the gas, as predicted by the BKT theory. These results were obtained by preparing two independent two-dimensional samples that were left to overlap, so that the matter-waves interfered. The interference patterns could be used to derive all relevant information.

Further studies included quantitative measurements of the critical point of the transition in a two-dimensional Bose gas. At various temperatures we could observe such a critical point at which the atomic density profile changed its shape dramatically when the atom number was increased beyond a critical value. Coinciding with this point, matter-wave interference becomes observable in our experiment. Subsequent theoretical work helped us to fully understand our quantitative findings in the light of the BKT theory in combination with conventional mathematical tools used for studies of weakly interacting bosonic gases (so called mean field theory). This interpretation allowed us to contrast BEC-driven behaviour and BKT physics, i.e. a quantum statistical phenomenon and an effect that is caused by interactions in the system. We conclude that for our experimental parameters, both phenomena are equally relevant and future experiments could explore the crossover between BEC and BKT physics.
% Work still in progress addresses the question of how information contained in matter-wave interference patterns can be exploited to infer even more complete characteristic properties of non-trivial states in multi-particle mesoscopic quantum gases.

This project has led to publications in highly ranked scientific journals, such as Nature and Physical Review Letters. It also triggered significant media coverage, in particular in Nature News and Views, Physics Today, Nature Physics News and Views, and it was selected as a Physical Review Letters editor's suggestion.