Thanks to light cooling, supplemented by evaporative cooling, we know how to lower the temperature of atomic gases to below the microkelvin. When atoms are whole-spin particles, i.e. "bosons", this cooling can give rise to a Bose-Einstein condensate. This phenomenon, first observed some twenty years ago, has given rise to a vast field of research, ranging from N-body physics to metrology.
This year's lecture was devoted to the study of the coherence and superfluidity properties of these gases of atoms. Coherence appears to be a direct consequence of the condensation phenomenon predicted by Albert Einstein in 1925, based on the work of Satyendranath Bose. The accumulation of atoms in a particular state gives rise to a macroscopic matter wave whose coherence can be assessed using interference experiments. In parallel with macroscopic coherence, these gases exhibit superfluid behavior, meaning that they do not heat up when an impurity passes through them. Superfluidity also manifests itself in the form of permanent currents, long-lived metastable macroscopic excitations. There is therefore a close link between the physics of cold atomic gases and that of liquid helium, despite a difference of 8 to 10 orders of magnitude between their densities.
In this lecture, we set out to establish links between the different properties of atomic quantum fluids. Using relatively simple theoretical models (Gross-Pitaevskii equation, Bogoliubov method, Gutzwiller ansatz), we studied the case of homogeneous systems as well as gases confined in optical lattices, including the transition between a superfluid state and a "Mott insulator" state. We also described a series of recent experiments carried out on these systems, highlighting different facets of macroscopic coherence and superfluidity.