  The concept of the Lorentz-invariant mass of a group of particles is shown to
be applicable to biphoton states formed in the process of spontaneous
parametric down conversion. The conditions are found when the Lorentz-invariant
mass is related directly with (proportional to) the Schmidt parameter $K\gg 1$
determining a high degree of entanglement of a biphoton state with respect to
transverse wave vectors of emitted photons.
