Thèse Effets des Interactions à Trois Corps dans des Condensats de Bose-Einstein de 39K à Deux Composantes Couplées de Manière Cohérente H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Physique École doctorale : Ondes et Matière Laboratoire de recherche : Laboratoire Charles Fabry Direction de la thèse : Thomas BOURDEL ORCID 0000000287843865 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-04-30T23:59:59 Our group has expertise in producing potassium-39 Bose-Einstein condensates within a few seconds and in controlling two-body interactions via magnetically tunable scattering resonances. Recently, we have discovered an alternative method to control interparticle interactions. More precisely, this method is based on preparing the condensate in a dressed state, i.e., a superposition of two spin states coupled by a radio-frequency (RF) field. Such a system not only allows for the control of two-body interactions but also uniquely introduces effective three-body interactions (PRL 128, 083401 (2022)). Three-body interactions can even dominate the condensate dynamics: for example, we have observed the collapse of a Bose-Einstein condensate induced by these interactions. Recent progress includes the implementation of a digital micromirror device enabling the imprinting of arbitrary potentials onto the atoms (PRA 110, 043316 (2024)), an ultra-precise magnetic field stabilization system for improved RF-dressing control (Rev. Sci. Instrum. 96, 063201 (2025)), and a theoretical understanding of three-body collision processes within perturbation theory (PRA 111, 053319 (2025)).In the future, we plan to further study the effects of the emerging three-body interactions. A first idea is the study of phonons, sound density ripples that propagates in the condensate. More precisely, we expect that the shape of the phonon dispersion curve remains the same (linear at low k and quadratic at high k) although a speed of sound that will be modified by three-body interactions. In particular we plan to explore the unstable regime of attractive interaction where modulation instabilities occur. The quantitative determination of the most instable wavevector will give us direct measurement of the three-body interactions.

A second idea is to create quantum droplets stabilized by repulsive three-body interactions. More precisely, attractive two-body interactions will tend to collapse the condensate that will be stabilize by repulsive three-body interactions. A difficulty is that the three-body repulsive interaction is a minute third order effect in the interaction and is typically very small. As a consequence, one should accurately prepare and control the system in order to be sensitive to such small effects.
Mixtures of quantum gases are studied since many years with focus on phase separation, on interacting fermions or on physics of impurities. Radio-frequency transfer and spectroscopy between spin states is a well-established technique for the characterization several properties of quantum gases. In this context, we plan to further study the consequence of three-body interactions. The objectives of the PhD thesis is the study of Bose-Einstein condensate physics in the presence of three-body interactions, in particular in quasi-1D geometries. We will use standard methods of optical trapping and imaging of ultracold atoms. In order to best use dressing as a mean to control interactions, a crucial point is the magnetic field stability. In contrast to most precision measurement experiments, we need a large (typ.100 G) and stable (typ. 10^-6) magnetic field, which requires specific stabilization techniques. We will also implement a Raman laser system as another way to coupled the different spin states.

Master de physique fondamentale, notamment physique quantique avancée, physique atomique, physique de la matière condensée.