Following Scully et al.’s study on the mechanism of complementarity, we further investigate the role of detector in which-way experiment. We will show that the initial quantum pure state of particle will reduce to a mixture state because of the inevitable interaction between particle and detector, then the coherence of wavefunction for the particle falling on the screen will be destroyed, which leads to the disappearance of interference fringes in which-way experiment.
 M. S. Chapman, T. D. Hammond, A. Lenel, J. Schmiedmeyer, R. A. Rubinstein, E. Smith and D. E. Pritchard, “Photon Scattering from Atoms in an Atom Interferometer: Coherence Lost and Regained,” Physical Review Letters, Vol. 75, No. 21, 1995, pp. 3783-3787. doi:10.1103/PhysRevLett.75.3783