Quantum key distribution on biphotons-ququarts with test states

The operational inclusion of the subclass of entangled states in a quantum key distribution protocol based on biphoton-ququarts is analyzed. Four Bell states are proposed to be used as test states to estimate the error level, leaving the subclass of 12 factorized polarization states of biphotons as information states. The elementary analysis of two strategies for an attack on a quantum communication channel, as well as of the key generation rate, has been performed.