Entanglement of biphoton-based qutrits and ququarts

In this paper we discuss physical aspects of entanglement in biphoton-based qutrits and ququarts, taking into account the bosonic correlations due to the indistinguishability of photons. We show that two apparently different types of entanglement in these systems, 'entanglement of modes' and 'entanglement of photons', are in fact closely related to each other. The latter is to a remarkable extent connected with the degree of polarization of two-photon light. Specifically, we demonstrate that a beamsplitter and subsequent post-selection using a coincidence circuit converts 'entanglement of photons' of a qutrit into 'entanglement of modes' of a ququart.