  In this work we suggest an original, realistic and simple, form of the
experiment of interference of single photon at beam splitter. Here quantum
entanglement (Schrodinger cat effect) between photon and photon trajectories
detector (which includes two same simple two-level systems) is studied. It
admits a consistent model of the detection or measurement procedure. Concretely
here (self-)collapse at detector is consistently treated as the spontaneous
(non-dynamical) breaking (effective hiding) of the unitary symmetry
(superposition or entanglement) of quantum mechanical dynamics. It can be
considered as an especial case of the general formalism of the spontaneous
symmetry breaking.
