Thesis defense of Gaétan HERCE

Thesis defense of Gaétan HERCE, PhD student in the Quantum gas group of the Laboratoire Charles Fabry, on 17 January 2023 at 10am in the Auditorium of the Institut d'Optique in Palaiseau, on the topic: "Study of quantum depletion in highly depleted Bose-Einstein condensates."

Abstract: "This defense will present my thesis work on the study of interacting bosonic systems through measurements of momentum correlations between individual atoms. The objective of this work is to link macroscopic collective phenomena emerging in interacting quantum systems, such as the superfluidity of a Bose gas, to microscopic processes of interaction between particles.  To this end, our research team is studying metastable Helium-4 Bose-Einstein condensates whose time-of-flight detection allows the measurement of momentum correlations between individual atoms. The interaction regime of the trapped gas is controlled by means of an optical lattice. This allows us to explore the weakly depleted regime, where the condensed fraction remains close to 1 and the gas can be described by the perturbative Bogoliubov treatment of the interactions. In this regime, we demonstrate the existence of pairs of correlated atoms with opposite impulses in the depletion of the condensate, thus validating the prediction made by Bogoliubov over 70 years ago. In a second step, we focus on the strongly interaction-depleted regime, beyond the Bogoliubov approximation. The extension of momentum correlation measurements between atoms in this regime allows us to explore the evolution of the N-body ground state with the strength of the interactions."