NONLINEAR SCHRODINGER-EQUATION AND WAVE-FUNCTION COLLAPSE - AN UNRELIABLE CONSEQUENCE OF THE SEMICLASSICAL APPROXIMATION

We consider the spin-boson model as a measurement device, with the 1/2 -spin system acting as a pointer and the influence of the environment simulated by means of the interaction between the 1/2-spin system and a dissipative oscillator. In accordance with the recent finding of Bon illa and Guinea [L. L. Bonilla and F. Guinea, Phys. Rev. A 45, 7718 (1 992)] the assumption that the oscillator is classical results, under s pecific conditions, in the occurrence of a wave-function collapse. How ever, our theoretical analysis shows that the wave-function collapse i s provoked by the action exerted by the 1/2-spin system on the oscilla tor (reaction field) and that the reaction field is as intense as the quantum-mechanical fluctuation neglected by the semiclassical approxim ation. A numerical treatment shows that no collapse occurs within a fu lly-quantum-mechanical treatment.