  Based on modern quantum measurement theory, we use Zurek's "triple model" to
study, from the viewpoint of quantum information theory, the wave and particle
nature of a photon in a symmetric Mach-Zehnder interferometer. In the process
of quantum measurement, the state of both the system and the detector is not an
entangled state but a correlated state. We find that the information gain about
the photon is related to the correlations (including classical and quantum
correlations) between the photon and the detector. We also derive the
relationship between the information gain and the fringe visibility. We find
that the classical correlations remain consistent with the path
distinguishability and can be used to describe the particle-like property of
the photon. Quantum correlations are not exactly the same as fringe visibility,
but both can represent the quantum coherence of the photon. Finally, we provide
an analytical expression for quantum correlations of one type of two-qubit
separable states.
