A model for the coherent output coupler of the Bose-Einstein condensed atoms from a trap in the recent MIT experiment (Phys. Rev. Lett., 78 (1997) 582) is established with a simple many-boson system of two states with linear coupling. Its exact solution for the many-body problem shows a factorization of dynamical evolution process, i.e., the wave function initially prepared in a direct product of a vacuum state and a coherent state remains in a direct product of two coherent states at any instance in the evolution of the total system. This conclusion always holds even for a system with a finite average particle number in the initial state. Its thermodynamical limit can be directly dealt with in the Bogoliubov approximation and manifests that an ideal condensate in the trap will remain in a coherent state after the r.f. interaction while the output-coupler pulse of atoms is also in a coherent state, which means a coherent output of atomic beam to form a macroscopic quantum state in a propagating mode.
Effects of Interactions on Bose-Einstein Condensation
玻色-爱因斯坦凝聚是一种独特的相变，因为它不受粒子间相互作用的驱动，但理论上可以发生在理想气体中，纯粹是量子统计的结果。本章讨论的问题是\emph{`How is this ideal Bose gas condensation modified in the presence of interactions between the particles?' }这个看似简单的问题，结果却出奇地难回答。在这里，我们概述了这个问题的理论背景，并讨论了一些最近的测量超冷原子玻色气体，试图提供一些答案。
