  Quantum phenomena appear in a macroscopic scale in Bose-Einstein condensates.
The Gross-Pitaevskii (GP) equation describes the dynamics of the weakly
interacting Bose-Einstein condensates. The GP equation has a form of the
Schroedinger equation with self-interaction. The coupled Gross-Pitaevskii
equations are used to describe some mixtures of Bose-Einstein condensates. In
this paper, we will show some numerical results of coupled Gross-Pitaevskii
equations without self-interaction, which has a form of nonlinearly-coupled
Schroedinger equations. We demonstrate that the macroscopic oscillation and the
interference of two quantum wave packets decay in time owing to mutual
interaction, which is analogous to the decoherence in quantum mechanics of many
particles.
