Quantum decoherence from adiabatic entanglement with external one or a fewdegrees of freedom

Based on the Born-Oppenhemer approximation, the concept of adiabatic quantu m entanglement is introduced to account for quantum decoherence of a quantu m system due to its interaction with a large system of one or a few degrees of freedom. In the adiabatic limit, it is shown that the wave function of the total system formed by the quantum system plus the large system can be factorized as an entangled state with correlation between adiabatic quantum states and quasi-classical motion configurations of the large system. In a ssociation with a novel viewpoint about quantum measurement, which has been directly verified by most recent experiments [e.g., S. Durr et al., Nature 33, 359 (1998)], it is shown that the adiabatic entanglement is indeed res ponsible for the quantum decoherence and thus can be regarded as a "clean" quantum measurement when the large system behaves as a classical object. By baking the large system respectively to be a macroscopically distinguishab le spatial variable, a high spin system and a harmonic oscillator with a co herent initial state, three illustrations are presented with their explicit solutions in this paper.