EVOLUTION OF THE BOSON GAS AT ZERO TEMPERATURE: MEAN-FIELD LIMIT AND SECOND-ORDER CORRECTION

A large system of N integer-spin atoms, called Bosons, manifests one of the most coherent macroscopic quantum states known to date, the "Bose-Einstein condensate", at extremely low temperatures. As N →∞, this system is usually described by a mean-field limit: a single-particle wave function, the condensate wave function, that satisfies a nonlinear Schrödinger-type equation. In this expository paper, we review kinetic aspects of the mean-field Boson evolution. Furthermore, we discuss recent advances in the rigorous study of second-order corrections to this mean-field limit. These corrections originate from the quantum-kinetic mechanism of pair excitation, which lies at the core of pioneering works in theoretical physics including ideas of Bogoliubov, Lee, Huang, Yang and Wu. In the course of our exposition, we revisit the formalism of Fock space, which is indispensable for the analysis of pair excitation.