Information gain versus interference in Bohr's principle of complementarity.

We study the wave and particle nature in a symmetric Mach-Zehnder interferometer from the viewpoint of quantum information theory. By introducing either the von Neumann or Zurek's model of quantum measurement, we find that the classical correlation can be used to quantify the particle nature since its monotonicity is similar to the path distinguishability. The environment in Zurek's model induces the emergence of the optimal measuring basis, and reduces the classical and quantum correlation comparing to the von Neumann's model. A way is presented analytically to calculate the quantum correlation of a two-qubit separable state other than X-type.