We propose schemes for preparing W and GHZ photonic states entangled in polarization
via quantum walks, where the trajectory exchanging of identical photons plus
postselection induces the coupling between their polarization degrees of freedom.
Being different from the polarization beam splitter-based approach, the indistinguishability
of identical photons provides an alternative for coupling photons, which is more
phase stable and may induce richer actions than the traditional polarization beam splitter
on input photons. Our schemes demonstrate that this new coupling mechanism for
photons is originated from the fact that trajectory-exchanging operation plus postselection
can extract entangled states from the initially product states, and this new
coupling mechanism may find considerable potential applications in both quantum
communication and quantum computation.