Experiment Backs Up Bogoliubov Theory

Cool a dilute gas of bosonic atoms to ultralow temperatures, and the atoms will form a Bose-Einstein condensate (BEC). That’s the simplest picture of Bose-Einstein condensation. In practice, however, the atoms repel each other, and as a result, some of them get kicked out of the BEC. Back in 1947, Soviet theoretical physicist Nikolay Bogoliubov came up with a theory to describe such interacting BECs, and he predicted the exact fraction of the atoms that remains in the BEC as a function of the so-called gas parameter, which is a measure of the strength of the interactions between the atoms. But in most cold-atom experiments, the interactions are too weak for the effect to be observable, and an experimental confirmation of Bogoliubov’s prediction has been lacking. Raphael Lopes from the University of Cambridge, UK, and colleagues have now accomplished this feat.