  We report a scalable linear optical scheme for generating entangled states of
multiple ququarts in which the individual single-ququart state is prepared with
the biphoton polarization state of frequency-nondegenerate spontaneous
parametric down-conversion. The output state is calculated with the full
consideration of the higher order effect (double-pair events) of spontaneous
parametric down-conversion. Scalability to multiple-ququart entanglement is
demonstrated with examples: linear optical entanglement of three and four
individual biphoton ququarts.
