Quantum fluids are those many-particle systems in whose behaviour the effects of both the quantum mechanics and quantum statistics are important. They range from atoms and molecules, such as liquid Helium 4He and 3He and dilute atomic alkali gases, photons interacting via coupling to some matter component to electrons in metals and other solid state quasiparticles such as excitons, polaritons and magnons. In these lectures we explore collective properties of such systems. We begin by discussing the principal quantum collective phenomenon, which lies at the heart of many related concepts, that of a Bose-Einstein condensation in bosonic systems. We then progress to look at how fermions can cooperate to also “bose-condense” and how it is possible to cross from “fermionic” to “bosonic” condensates by changing particle density and/or interaction strength – the BCS-BEC crossover. We then discuss one of the most exciting manifestations of many particle quantum collective behaviour that of superfluidity. We also review physical experimental systems focusing on ultra-cold atomic gases, excitons and polaritons, and interacting photons in various settings. We closes the course with a short discussion of strongly interacting quantum systems such as atoms in optical lattices and coupled cavity lattices.
Marzena is Professor of Physics at University College London