Bose Einstein condensation: Its role in the excitations of liquid helium and in trapped Bose gases

V. N. Minasyan, V.N. Samoilov

S. Stringari

Frédéric Chevy, Jean Dalibard

This chapter presents an overview of the specific experimental tools developed in the field of ultra-cold gases to achieve and probe superfluidity in vapours of bosonic atoms. First, it describes the main cooling techniques: the magneto-optical trap, which brings an atomic vapour from room temperature down to the sub-mK range, and the evaporative cooling scheme, which bridges the gap to the superfluid regime. It then proceeds to the study of interaction effects. It shows that these play a central role in the understanding of both static and dynamic properties of trapped Bose–Einstein condensates (BECs), despite the dilute character of these gases. As a consequence, gaseous BECs and superfluid helium obey the same laws of hydrodynamics and exhibit similar dynamical properties, although their densities differ by several orders of magnitude. The third section is devoted to the coherence properties of gaseous BECs. It shows that, by contrast with liquid helium, specific features of ultra-cold gases make them suitable for a direct probing of the quantum coherence associated with Bose–Einstein condensation. Finally, the possibility of tailoring trapping potentials and realizing low-dimensional systems is discussed, where one or two directions of motion are frozen.

Chandra Raman, Roberto Onofrio, Michael Köhl +4 more

Summary form only given. A distinguishing feature of Bose-Einstein condensates in trapped atomic gases is the ability to manipulate them with laser light. This is a tool which is not available for the study of liquid helium. In this work, we use the mechanical forces exerted by light on atoms to create and study collective excitations. We can apply either repulsive or attractive forces locally to a Bose-Einstein condensed gas with scanning, focused laser beams tuned to the blue or to the red of the principal atomic resonance, respectively. With spatio-temporal control of the forces, we have studied dissipation in the condensate as well as creating collective excitations with a controllable symmetry and time dependence.

Yvan Castin

L. Madeira, A. Cidrim, M. Hemmerling +3 more

The field of quantum turbulence is related to the manifestation of turbulence in quantum fluids, such as liquid helium and ultracold gases. The concept of turbulence in quantum systems was conceived more than 70 years ago by Onsager and Feynman, but the study of turbulent ultracold gases is very recent. Although it is a young field, it already provides new approaches to the problem of turbulence. The authors review the advances and present status, of both theory and experiments, concerning atomic Bose–Einstein condensates (BECs). The authors present the difficulties in characterizing turbulence in trapped BECs, if compared to classical turbulence or turbulence in liquid helium. The authors summarize the challenges ahead, mostly related to the understanding of fundamental properties of quantum turbulence, including what is being done to investigate them.