Soutenance de thèse de Maxime ALLEMAND

Soutenance de thèse de Maxime ALLEMAND, doctorant dans le groupe Gaz quantiques du Laboratoire Charles Fabry, le 19 décembre 2025 à 14h00 dans l'Auditorium de l'Institut d'Optique Graduate School à Palaiseau, sur le thème : "Probing non-Gaussian features of lattice Bose gases via atom correlations and universal order-parameter statistics."

Abstract : "My PhD thesis focuses on studying quantum many-body systems experimentally using quantum gases of metastable Helium-4 atoms in optical lattices. The latter are used to tune the effect of atomic interactions and provide a realisation of an inhomogeneous Bose-Hubbard Hamiltonian which features a superfluid and a Mott insulating phases separated by a continuous phase transition. The many-body state is probed from measuring repeatedly the single-atom-resolved momentum distribution of the atomic cloud to access atom number statistics in momentum space – including full distributions and atom correlations. First, we perform a detector tomography to characterise the effect of the detection on the measured atom number distributions. We identify the states that are well-suited to perform such a characterisation and introduce a method to include shot-to-shot number fluctuations associated to the preparation of the atomic gas. We find that our detection is well-described by a binomial process. Then, we study the many-body state in the weakly- to strongly-interacting superfluid regime from monitoring two-body connected correlations at opposite momenta. Our measurements show the appearance and increase of such correlations at weak interactions as predicted by Bogoliubov theory. At larger interactions, the correlations decrease and become negative which translates to a breakdown of Bogoliubov theory followed by the emergence of non-Gaussian correlations. These results probe the increasing complexity of atomic correlations at increasing interactions. Finally, we increase further the interactions and cross the superfluid-to-Mott insulator phase transition. We study the critical behaviour from measuring the full distribution of the BEC order parameter amplitude across the phase transition. We observe the universal non-Gaussian statistics of the order parameter at criticality and oscillations of the corresponding high-order cumulants."